Standards and Measurements

Astronomy's Inventory of the Universe

With bigger and bigger telescopes, astronomers cold view further into the universe - as it turned out beyond our home galaxy. But it was not yet clear, how all these new discoveries were tied together.

In Astronomy, the time from the late 18th to the mid 20th Century was mainly a time of catalogues, maps and standards. The most important catalogues are listed here in chronological order; for an extensive list, see Wikipedia, NASA and David Darling.

In addition to all of the inventory, new breakthroughs in physics, most notably the the Theory of Relativity and the Theory of Quantum Mechanics provided a better understanding of the complex dynamics of the universe.

The resolution of telescopes grew dramatically, allowing the discoveries of fain object like Ceres and Pluto. With the growing number of catalogued stars, the number of constellations grew - at one point (in 1801) to over one hundred. At the end of the 19th century, astronomers started to work out a standard, which resulted in 88 acknowledged constellations in 1928.

The primary focus of this site is not astronomy, but Star Lore, which is folklore based upon stars and star patterns. We try to create a collection of mythical stories about stars and constellations from all over the world. However, to better understand the myths and legends of stars and constellations, a brief history of the development of our modern constellations might be helpful. This is by no means a scientific paper on the history of astronomy, but merely an illustrated collection of highlights of that history, along with some links to what we think are reliable sources on the subject.

Bits of the history of Astronomy from the late 18th to the mid 20th Century

The Board of Longitude and the Nautical Almanach (1737, 1767)

In 1707, a fleet of four British warships ran aground off the Scilly Islands. The disaster cost the lives of nearly 2,000 sailors.

One of the reasons for the disaster was the navigators' inability to accurately calculate their positions due to inadequate compasses and chronometers. This caused the British government to form a commission and to offer a scheme of prizes for a solution to the problem of finding longitude at sea.

The Board of Longitude was founded by the Longitude Act of 1714, though it did not meet for a first session until 1737.

In 1767, the board published the first issue of the Nautical Almanac, the first nautical almanac to contain data dedicated to the convenient determination of longitude at sea.

Sources: Wikipedia, Her Majesty's Nautical Almanac Office

Nautical Almanach of 1767
Nautical Almanac Office

Looking beyond stars and constellations - The Messier Catalogue (1781)

In 1781, French astronomer and comet hunter Charles Messier published a list of diffuse objects that were not comets, to help comet hunters to distinguish between permanent and transient visually diffuse objects.

His first list contained 45 objects; the final version, published in 1781 and known as the Messier Catalogue listed 103 objects. Messier called it Catalogue des Nébuleuses et des Amas d'Étoiles (Catalogue of Nebulae and Star Clusters).

When telescopes improved, many of Messier's "Nebulae" were identified as star clusters or galaxies. But in spite of the limitations of its time, the Messier Catalogue remains the first list of objects beyond our galaxy.

Sources: Wikipedia

Messier Catalogue
Source: SEDS

The Flamsted Numbers - first used by Jérôme Lalande (1783)

In 1783, French astronomer Jérôme Lalande published Éphémérides des mouvemens célestes, a revised edition of Flamsteed’s catalogue in French language.

Lalande numbered Flamsteed’s stars consecutively by constellation, thus inventing the numbering system we now know as Flamsted Designation.

Sources: Wikipedia, Ian Ridpath

Éphémérides des mouvemens célestes

A Theory about Black Holes - 150 years ahead if its time (1783)

On November 27, 1783 English natural philosopher John Michell proposed a theory of "dark stars" in a paper written for the Philosophical Transactions of the Royal Society of London.

Newton's corpuscular theory of light, states that light consists of minuscule particles. Mitchell concluded that such particles, when emanated by a star, would be slowed down by its gravitational pull. He calculated that for a star more than 500 times the size of the Sun, the star's gravitational pull might be so strong that the escape velocity needed for light particles would exceed the speed of light.

Black hole Cygnus X-1 pulls matter from the super giant blue star near it.
Source: NASA-CXC-M.Weiss/
Mitchell reasoned that there might be many "dark stars" in the universe. He also proposed that "dark stars" could be detected by their effects on other stars. This was an amzingly correct prediction as shown in the picture above, where gas from a blue supergiant is spiraling into a companion black hole.

The Reverend John Mitchell did groundbreaking work in astronomy, geology, optics, and gravitation. The American Physical Society described Michell as being so far ahead of his scientific contemporaries that his ideas languished in obscurity, until they were re-invented more than a century later.

Source: Wikipedia

The Great Jupiter–Saturn Inequality (1784)

In three sections, in 1784, 1785, and 1786, French scholar and polymath Pierre-Simon, marquis de Laplace presented a memoir on planetary inequalities, now known as the "great Jupiter–Saturn inequality". Laplace solved a longstanding problem in the study and prediction of the movements of these planets by identifying the perturbations, which is the complex motion of a massive body that is subject to forces of an additional body other than the one on the focus of its orbit.

Source: Wikipedia

Connecting the Dots - Nouvelle Uranographie (1786)

Today, the most common depiction of a constellation is a diagram of the brightest stars, connected by thin lines. One could easily assume that this has been the common wayto depict constellations for a long time, but up until the the late 18th century, all constellations were described by rather elaborate pictures. The geometrical rendering of the constellations was invented by an almost forgotten French astronomer.

Ian Ridpath writes in "Pictures of Matchstick Men":

Ruelle's renderings of Ursa Major and Ursa Minor
Source: Teca Digitale Viewer
"Alexandre Ruelle (1756–18??) [was] an assistant at Paris Observatory in pre-revolutionary France. Extending the growing revolutionary fervour to the sky, Ruelle swept away the conventional constellation shapes of the astronomical ancien régime and substituted a sparer, more proletarian style. He presented his results in 1786 on a chart titled Nouvelle uranographie, consisting of northern and southern hemispheres with an equatorial strip below. In the accompanying booklet he explained his rationale:

I thought there could be no simpler and easier way of teaching the knowledge of the sky than to substitute for these fantastic figures triangles, squares, polygons or other geometric figures which really make visible the various groups of stars by supposing the brightest stars of each constellation linked together by lines."

Ruelle's concept took on only slowly. Two of the most popular presentatations of constellations as pictures (Bode's Uranographia and Urania's Mirror) were published 15 and 38 years respectively, after Ruelle's publication. Today, however, the "Matchstick Men" are the most common way to depict constellations.

Ruelle's renderings of Orion
Source: Teca Digitale Viewer

There is no official rule of how to draw the lines between the stars of a constellation, leaving some room for creative interpretations.

Sources: Ian Ridpath: Pictures of Matchstick Men, Anthony Auerbach: Joining the Dots

Catalogue of Nebulae and Clusters of Stars (1786)

In 1786, German-born British astronomer William Herschel and his sister Caroline Herschel published the Catalogue of Nebulae and Clusters of Stars which originally had 1,000 entries. Two additions in 1789 and 1802 brought it to 2,500 objects - a huge step up from Messier's catalogue just 20 years earlier.

Source: Wikipedia

New Tables for Planetary Movement (1792)

In 1782, while recording a transit of Mercury across the Sun, French astronomer Jean Baptiste Joseph Delambre noticed, that the existing tables on planetary movement were inaccurate. He then devoted much effort into producing new ones.

After ten years of observation, in 1792, Delambre published Tables du Soleil, de Jupiter, de Saturne, d'Uranus et des satellites de Jupiter, which were calculated taking de Laplace's perturbations into account.

Source: University of St Andrews

Exposition du Système du Monde (1796)

In 1796, de Laplace published his findings on the Great Jupiter–Saturn Inequality in Exposition du système du monde (Exhibition of the Wold System).

Mécanique Céleste (1799 - 1825)

In 1799, the first two volumes of de Laplace's Mécanique céleste (Celestial Mechanics) are published. They contained methods for calculating the motions of the planets, determining their figures, and resolving tidal problems.

Système du monde

Mécanique céleste

The third and fourth volume were published in 1802, 1805. They contained mathematical applications to solve astronomical problems and and several astronomical tables.

The last volume was published in 1825. It was mainly historical, but also had appendices containing the results of Laplace's latest researches, especially his Nebular hypothesis.

Based on the findings of Emanuel Swedenborg and Immanuel Kant, de Laplace concluded that the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust which later collapsed into a new star (the Sun) with some of the material forming a protoplanetary disc from which the planets evolved.

Sources: A.Pannekoek, The planetary theory of Laplace
Wikipedia, Universe Today,

Artist’s impression of the early Solar System

The Beginning of Astronomical Spectroscopy (1800, 1814)

In 1800, William Herschel split sunlight through a prism and measured the energy given out by different colors. His experiments laid the foundations of astronomical spectroscopy.

Herschel noticed a sudden increase in energy beyond the red end of the spectrum, discovering invisible infrared. In years to come, infrared astronomy became extremely important as it peers through clouds of gas and dust in our galaxy.

Source: Wikipedia

In 1814, Bavarian physicist Joseph von Fraunhofer build the first accurate spectrometer and used it to study the spectrum of the Sun's light. He discovered and studied the dark absorption lines in the spectrum of the sun now known as Fraunhofer Lines.

In 1815, Fraunhofer compared the Sun's spectrum with that of Venus and Sirius, noticing significant differences.

Source: Wikipedia

NASA infrared image of the
center of the Milky Way

Illustration of solar spectrum
drawn and colored by Fraunhofer
Source: Wikipedia

Ceres and the Asteroid Belt (1801)

As early as 1772, Johann Elert Bode, director of the Berlin Observatory suggested that an undiscovered planet could exist between the orbits of Mars and Jupiter.

On January 1, 1801, a small object, first thought to be a comet, was discovered by Giuseppe Piazzi, a Catholic priest at the Academy of Palermo, Sicily.

In December 1801, the observation was confirmed and Piazzi named the object, then considered a planet, Ceres Ferdinandea, after the Roman goddess of agriculture and King Ferdinand IV of Naples and Sicily. It later became Ceres.

Piazzi's observation "the new star", published in Monatliche Correspondenz

When a second object (Pallas) was discovered in 1802, William Herschel suggested the term asteroid (little star), which prevailed over Piazzi's more precise sugestion planetaroid (little planet).

By 1860, more than 60 asteroids had been discovered and the area between the orbits of Mars and Jupiter became known as the Asteroid Belt.

Source: Wikipedia, David Darling

100 Constellations and more - Bode's Uranographia (1801)

In 1801, Bode published a star catalogue called Allgemeine Beschreibung und Nachweisung der Gestirne (Description and Calculation of Celestial Bodies). It listed 17,240 stars and came with a large star atlas illustrated with twenty copper plates, called Uranographia sive astrorum.

Uranographia marked the climax of an epoch of artistic representation of the constellations. As there was still no governing body, many astronomers tried to get their names (or their ideas) written in the sky and so, Bode's catalogue contained over 100 constellations, among them innovative ideas like Bode's creation Officina Typographica, commemorating Johannes Gutenberg’s printing press, but also an entire zoo with slugs, leeches, sea horses and even a flying squirrel.

Sources: Wikipedia, Ian Ridpath

Uranographia title page
Source: Linda Hall Library

47,390 stars - Histoire Céleste Française (1801)

Bode's Uranographia listed 17,240 stars. At the same time as Bode published his catalogue in Berlin, his French colleague Lalande, after a decade of astronomical recordings taken at the Paris Observatory put together a catalogue listing almost three times as many stars.

The Histoire Céleste Française listed the locations and apparent magnitudes of 47,390 stars, up to magnitude 9. Stars were identified by their common name and, when available, by their Bayer or Flamsteed designation.

Similar to the listing of Uranus in the Flamsted Catalogue, one of the "stars" listed by Lalande was most likely the planet Neptune, which wasn't confirmed until 1846.

Source: Wikipedia

Histoire Céleste
Source: Wikipedia

Praecipuarum Sellarum Inerrantium (1803)

Starting in 1792, Sicilian astronomer Giuseppe Piazzi, the discoverer of the first asteroid, started overseeing the compilation of the Palermo Catalogue of stars, containing 7,646 star entries.

Praecipuarum Sellarum Inerrantium (The positions of the fixed stars) did not contain as many stars as Bode's and Lalande's catalogs, but at a time when the field was dominated by English, French and German astronomers, it played an important role in the internationalization of astronomy.

Source: Harvard University

Praecipuarum Sellarum Inerrantium
Source: Google Books

Royal Astronomical Society (1820)

On March 10, 1820, the world's first society for the the study of astronomy, the Astronomical Society of London was founded. In 1831, it became the Royal Astronomical Society.

Today, 200 years later, the society still promotes the study of astronomy, solar-system science, geophysics and closely related branches of science.

150th anniversary stamp
Postal Museum, Ian Ridpath
Currently, the society has over 4,000 members, most of them professional researchers or postgraduate students.

Sources: Royal Astronomical Society, Wikipedia

A Celestial Atlas (1822)

Celestial Atlas was a star atlas by British author Alexander Jamieson, published in 1822. It was inspired by Bode's Uranographia, but limited to stars visible with the naked eye, making it less cluttered.

Jamieson allowed himself greater artistic expression to draw more realistically looking figures dor the constellations.

His drawings became world famous, when they were plagiarized in Urania's Mirror in 1824.

Sources: Ian Ridpath, Wikipedia

A Celestial Atlas; Wikipedia

Urania's Mirror - the most popular depictions of the constellations (1824)

Two years after Jamieson's Celestial Atlas, Urania's Mirror, featuring a set of 32 astronomical star chart cards, was published. The brightly colored cards were inspired (or plagiarized) by Jamieson's drawings, but in addition, they had holes punched in them allow them to be held up to a light to see a depiction of the constellation's stars.

The cards were designed by the Reverend Richard Rouse Bloxam and engraved by British engraver and cartographer Sidney Hall. Until this day, they are one of the most popular creations of star charts.

You can see all images in our Star Lore Art section.

Sources: Ian Ridpath, Wikipedia

Scorpius in Urania's Mirror
Source: Wikipedia
Urania's Mirror marked the end of a thousand year-long marriage between astronomy and art. From now on, astronomy was presented in tables and diagrams rather than artistic renderings. However, a growing awareness of national identities led to a rennaicance of Astro-Art in the early 21st century. In our Star Lore Art section, we provided some examples of artistic interpretations of Native American and Australian Aboriginal constellations.

Geography of the Heavens (1833)

US-American astronomer Elijah Hinsdale Burritt is often called the "Forgotten Astronomer", as most astronomy history books don't even mention his name.

Burritt had the rare gift to explain mathematics and astronomy in terms that untrained people could understand. His Geography of the Heavens, published in 1833, was extremely popular and could be considered the first star catalogue not written for scientists but for ordinary people.

Burritt's Geography of the Heavens;
Sources: Albert J. Brooks: The forgotten Astronomer,,,
Star Names coined in the Geography of the Heavens

A number of star names were first listed in Burritt's catalogue and map. Some of them were his own creations, others were already in use but were first listed by him.


β Carinae
α Centauri
α Crucis
β Crucis
γ Crucis
υ Orionis


From "miyāh" (Arabic for water) and "placidus" (Latin for placid); Burritt's creation
Possibly from "β" and "ungula" (Latin for hoof); Burritt's creation
From "Alpha" and "Crux"; used by navigators and picked up by Burritt
From "Beta" and "Crux"; used by navigators and picked up by Burritt
From "Gamma" and "Crux"; likely Burritt's creation
From "al-thabit" (Arabic for endurer) likely Burritt's creation

Double Stars (1837)

Baltic German astronomer Friedrich Georg Wilhelm von Struve had been observing double stars at the Dorpat Observatory in what is now Estonia.

In 1827, he published Catalogus novus stellarum duplicium, a double star catalogue far surpassing all previous efforts.

Source: Wikipedia

XXXXXXXXXXXXXXXXXXXXXXXCatalogus novus stellarum duplicium, source: ETH-Bibliothek

Measuring the Distance of a Star (1838)

In 1838, German astronomer and mathematician Friedrich Bessel conducted the first successful measurement of the didstance of a star (other than than the sun). Using the method of stellar parallax, the effect of Earth's annual movement around the Sun, he calculated the distance to 61 Cygni at about 10.4 light-years, very close to the actual value of about 11.4 light-years.

61 Cygni was selected because in 1804, Giuseppe Piazzi observed and recorded the star's proper motion, a change in the apparent places of the star the sky, compared to the abstract background of more distant stars.

Source: Wikipedia

The Beginning of Astrophotography (1840)

In 1837, French artist Louis Daguerre invented the first publicly available photographic process, later known as Daguerreotype. In 1839, his attempt to take a picture of the Moon fails due to overexposure.

In 1840, New York scientist John William Draper took the first succesful image of an astronomical object - the Moon.

Draper's Moon photograph was followed by the first photograph of a partial solar eclipse (1842 by Austrian astronomer G.A. Majocchi) and the first photograph of a star (Vega, 1850 by Harvard astronomers J. A. Whipple and W.C. Bond).

Source: Pedro Ré: History of Astrophotography

First Photograph of the Moon
Source: Wikipedia

The Solar Cycle (1843)

For 17 years, German astronomer Heinrich Schwabe observed the sun trying to detect the passage of a hypothetical planet inside the orbit of Mercury. He didn't discover a new planet, but instead noticed the Solar cycle, a regular variation in the number of sunspots.

Schwabe published his findings in a short article entitled "Solar Observations during 1843."

Source: Wikipedia

Sunspots; NASA

The Largest Telescope of the 19th Century (1845)

From 1842 to 1845, Anglo-Irish astronomer William Parsons, 3rd Earl of Rosse had a large reflecting telescope of 72 inches built at Birr Castle in Ireland.

From 1845 until 1917, the telescope, called Leviathan was the largest telescope in the world.

Lord Rosse's main focus were nebulae. In 1878, his son published his father's findings, including the discovery of 226 new objects added to the NGC Catalogue.

Leviathan; Wikipedia
The title of above mentioned publication was Observations of Nebulae and Clusters of Stars Made With the Six-foot and Three-foot Reflectors at Birr Castle From the Year 1848 up to the Year 1878.

Source: Wikipedia

The "Spiral Nebula" (1845)

Parson's perhaps most important discovery using the "Leviathan" was the observation of Messier object M51. This was thought to be a nebula, but Rosse's observation revealed its spiral character. Today, this nebula is known as the Whirlpool Galaxy.

Parson's precise drawing (see right) was the first closeup image of a galaxy.

Source: Wikipedia

Whirlpool Galaxy; Wikipedia

Neptune (1846)

In 1846, French astronomer and mathematician Urbain Le Verrier predicting the existence and position of an outer planet based on the orbit of Uranus, based on discrepancies with Uranus's orbit and the laws of Kepler and Newton. He sent the coordinates to Johann Gottfried Galle in Berlin, who found Neptune in the same night he received Le Verrier's letter, within 1° of the predicted position. The discovery of Neptune is widely regarded as a dramatic validation of celestial mechanics, and is one of the most remarkable moments of 19th-century science.

Source: Wikipedia

Counting the Northern Stars (1859)

In 1859, Friedrich Wilhelm Argelander at the Bonn Observatory in Germany started compiling the positions and apparent magnitudes of all known stars. Over the course of 44 years Argelander and his assistants collected the data of approximately 325,000 stars to apparent magnitude 9–10.

In 1863, Argelander was one of the founders of the Astronomische Gesellschaft (Astronomical Society) and his collection, called the Bonner Durchmusterung (Bonn run-through examination) became the base for the main star catalogues of the 20th century, most notably the Astronomische Gesellschaft Katalog and the Smithsonian Astrophysical Observatory Star Catalog.

Sources: Wikipedia, Astronomical Catalogs, Charts, and Surveys

Spectral Classes (1866)

Italian priest and astronomer Angelo Secchi was one of the pioneers of astronomical spectroscopy.

in 1863, Secchi began collecting the spectra of stars. Still without the aid of a camera, he accumulating about 4,000 stellar spectrograms. Through analysis of this data, he discovered that the stars come in a limited number of distinct types and subtypes, which could be distinguished by their different spectral patterns. From this concept, he developed the first system of stellar classification: the five Secchi classes, introduced in 1866.

While his system was soon to be superseded by the more detailed Harvard system, Secchi still stands as discoverer of the principle of stellar classification.

Comparing the Sun's spectrum with the spectrograms of other stars, Secci was one of the first scientists to state authoritatively that the Sun is a star.

Source: Wikipedia

Secci Classes
Source: Wikipedia

The Toronto Astronomical Club (1868)

In the mid-1800s, small high resolution telescopes became available to the public and soon star gazers all over the world gathered in clubs and observed the night sky.

In 1868, a group of Canadian space enthusiasts founded the Toronto Astronomical Club, the first organization of amateur astronomers.

In 1890, the club was incorporated in the The Astronomical and Physical Society of Toronto. In 1903, it was renamed the Royal Astronomical Society of Canada.

Other early aamateur astronomer associations were the Liverpool Astronomical Society (1881), the British Astronomical Association (1890), the Astronomical Society of South Australia (1892) and the American Association of Variable Star Observers (1911).

Today, there is a global network of amateur astronomy organizations contributing to astronomical science.

Source: Wikipedia

Royal Astronomical Society of Canada

Liverpool Astronomical Society

Spectral Photography (1872)

The first records of solar and stellar spectra were taken by William Herschel and Joseph von Fraunhofer in the early 1800s (see above) and then in 1863 by Angelo Secchi.

However, without a camera, it was difficult to record those observations. In 1872, New York's Henry Draper, son of John William Draper, took the first photograph of the stellar spectrum of Vega, showing absorption lines.

He also took a photograph of a complete solar spectrum in 1872. In 1876, he took simultaneous photographs of the solar spectrum and the spectrum of atmospheric air, showing that the lines of oxygen present in the atmosphere are also present in the sun.

Sun Spectrum photograhed by Henry Draper in 1876
Source: Linda Hall Library

In 1880, Henry Draper photographed the spectrum of Jupiter. Before his death in 1882 he took over a hundred more photographs of stellar spectra.

Source: Wikipedia, Linda Hall Library

Uranometria Argentina (1874)

In 1868, US-American astronomer Benjamin Gould was invited by the Argentinian government to organize the Argentine National Observatory in Córdoba. From 1871 until 1885, Gould was the Observatory's director.

In Córdoba, in 1874, Gould published the Uranometria Argentina, a catalogue listing all bright stars within 100 degrees of the south celestial pole similar to the bright stars in the northern hemispehere listed in Flamsteed's Catalogue.

Sources: Wikipedia,

Uranometria Argentina
Source: Paolantonio, Garcia

Drawing Lines between Constellations (1874)

While working on his Uranometria Argentina, US-American astronomer Benjamin Gould tackled a problem that arose with the invention of the telescope:

Since the times of Ptolemy, there had been a consensus about which bright stars belonged to which constellation. However, the patterns of the traditional constellations were based on the 6,000 stars visible to the naked eye.

The number of known stars grew with every new stronger telescope - in the mid-19th century it had risen to 325,000 - and most of them were orphaned between the traditional constellations.

Gould's Border between
Lupus and Centaurus
Source: Paolantonio, Garcia
Gould was the first one to draw border lines between constellations using arcs of right ascension circles and parallels of declination. With that approach, every star could be assigned to one of the traditional constellations.

In 1928, when the International Astronomical Union standardized the star maps, Gould's boundaries of the southern constellations was addopted almost without change and his concept was extended to the constellations of the northern hemisphere.

Source: Paolantonio, Garcia: Uranometria Argentina and the constellation boundaries

The Harvard Computers (1875)

The Harvard Computers were a group of skilled women processing astronomical data at the Harvard Observatory.

The first three computers - R.T. Rogers, R.G. Saunders, and Anna Winlock - were hired in 1875 by Edward Charles Pickering, who in 1877 became the director of the Harvard Observatory.

Sources: Wikipedia, astrobites, Smithsonian Magazine,

Harvard Computers
Source: Wikipedia

The Moons of Mars (1877)

US-American astronomer Asaph Hall III had calculated the probability of a small moon in a close orbit around Mars. On August 12 and August 18, 1877, respectively, using the telescope at the US Naval Observatory (USNO) in Washington, DC, Hall discovered first Deimos, then Phobos.

Asaph Hall also determined the orbits of satellites of other planets and of double stars, the rotation of Saturn, and the mass of Mars.

Source: Wikipedia

USNO Telescope
Source: Wikipedia

Harvard Photometry (1884)

Supervised by US-American astronomer Edward C. Pickering, The the Harvard College Observatory compiled a catalogue listing about 4,000 stars. The Harvard Photometry was first published in 1884.

Stars of the southern hemisphere were added by US-American astronomer Solon I. Bailey, leading to a Revised Catalogue in 1908.

Source: Wikipedia, NASA, astronomical catalogs, charts, and surveys

Dark Bodies (1884)

Dark Matter seems to be a fairly new concept, but it has been discussen in the scientific community for over a hundred years.

As early as 1844, British mathematical physicist and engineer Lord Kelvin estimated the mass of the galaxy from the mass of visible stars and notoced that the result was different from the mass calculated by their observed velocity in orbit of the center of the galaxy. He concluded that "many of our stars, perhaps a great majority of them, may be dark bodies."

In 1906, French mathematician and theoretical physicist Henri Poincaré came to the same conclusion. In his book The Milky Way and Theory of Gases he discussed Kelvin's finding, calling it matière obscure, which is French for "dark matter." Asaph Hall also determined the orbits of satellites of other planets and of double stars, the rotation of Saturn, and the mass of Mars.

Dutch astronomers Jacobus Kapteyn and Jan Oort discussed the phenomenon in 1922 and 1932, respectively.

In 1933, the term finally stuck, when Swiss astronomer Fritz Zwicky delivered theoretical proof of the concept and called it Dunkle Materie, which is German for - again - dark matter.

Sources: Wikipedia, Lord Kelvin: Baltimore Lectures on Molecular Dynamics and the Wave Theory of Light

The Andromeda Supernova (1885)

Between August 17 and August 20, 1885 several astronomers discovered a light burst in the "Andromeda Nebula." German astronomer Ernst Hartwig at the Dorpat Observatory in what is now Estonia was the first one to identify it as a Nova.

In 1917 (see below), US-American astronomer Heber Curtis, examined photographs of the novae he concluded that the object was too far away to be part of the Milky Way galaxy - making the event the first observed supernova outside our home galaxy.

Sources: Wikipedia Supernova 1885, Wikipedia: Timeline of white dwarfs, neutron stars, and supernovae,

First Picture of another Galaxy (1888)

Welsh hobby astronomer and astrophotographer Isaac Roberts developed a methode to take long exposure photographs of faint object by mounting his camera on a larger equatorial mounted telescope that would constantly adjust to the Earth's rotation and would keep the camera aimed accurately in the same direction for the required time, which could be an hour or more. Orion.

On December 29, 1888, Roberts took a picture of the "Andromed Nebula" in unpecedented detail.

Roberts' photograph of the Andromeda Galaxy Wikipedia

The photograph revealed that the "nebula" had a spiral structure, which was quite unexpected at the time. The Photograph caused a "quantum leap" in astronomy as it made clear that this was not a nebula.

The confirmation that the "Andromeda Nebula" was a galaxy was made by Edwin Hubble in 1924.

Source: Wikipedia

The Horsehead Nebula (1888)

While analyzing a telescope-photogrammetry plate made by astronomer W. H. Pickering, brother of E.C. Pickering, Harvard Computer Williamina Fleming discovered a small but distinct dark nebula in the constellation Orion.

Its iconic shape made the photograph one of the most popular images in astronomy books of the 19th and early 20th century.

Throughout her professional career, Williamina Fleming discovered a total of 59 gaseous nebulae, over 310 variable stars, and 10 novae and identified the first White dwarf.

Source: Wikipedia

Horsehead Nebula; Wikipedia

NGC - The New General Catalogue (1888)

In 1878, Danish astronomer John Louis Emil Dreyer published a supplement to William Herschel's Catalogue of Nebulae and Clusters of Stars.

During the 1880s, Dreyer compiled more information on Nebulae, but instead of publishing another supplement, the Royal Astronomical Society asked him the produce a completely new version.

In 1888, the New General Catalogue of Nebulae and Clusters of Stars was published.

Including two supplement in 1895 and 1908 and a revision in 1973, the New General Catalogue became the standard resource for galaxies and nebulae. Most galaxies today are best known by their NGC designation.

Sources: Wikipedia,

New General Catalogue

Draper Catalogue of Stellar Spectra (1890)

In the 1880s, Harvard astronomer Edward Charles Pickering started a survey of stellar spectra. Harvard computer Williamina Fleming was credited with classifying over 10,000 featured stars and discovering 10 novae and more than 200 variable stars.

Her work resulted in the Draper Catalogue of Stellar Spectra, published in 1890.

The Harvard system of spectral clasification would soon replace the spectral classes developed earlier by Angelo Secchi.

Source: Wikipedia

Draper Catalogue of Stellar Spectra

Astronomische Gesellschaft Katalog (1890)

From the day it was founded in 1863, the Astronomische Gesellschaft started working on a star catalogue based on the data compiled by Based on Friedrich Wilhelm Argelander.

In 1890 the first version of the Astronomische Gesellschaft Katalog (Catalogue of the Atronomical Society), listing 200 000 stars was published. A second and third version followed in 1951 and 1975.

Source: Wikipedia, Astronomical Catalogs, Charts, and Surveys

1904 edition of the Katalog

Counting the southern Stars (1892-1900)

The original Astronomische Gesellschaft Katalog listed only stars visible from Europe. In order to create a comprehensive catalogue, the Astronomische Gesellschaft conducted two additional surveys, one from Capetown, South Africa and one from Córdoba, Argentina.

The Cordoba Durchmusterung, published in 1892 included 613,959 stars down to about tenth magnitude, south of declination -22°.

The Cape Photographic Durchmusterung, published between 1896 and 1900 was the first star catalogue produced from photographic measurements of the sky. It listed 54,877 stars down to tenth magnitude, between declination -19° and -90°.

Sources: Wikipedia, Astronomical Catalogs, Charts, and Surveys

Cordoba Durchmusterung

Cape Durchmusterung
Source: Springer Link

Yerkes Observatory (1892)

Founded in 1892 by astronomer George E. Hale and financed by businessman Charles T.Yerkes, the Yerkes Observatory in Williams Bay, Wisconsin represented a shift in the thinking about observatories, from their being mere housing for telescopes and observers, to a concept of observation equipment integrated with laboratory space.

Operated by the University of Chicago for 126 years, the observatory was praised as the "birthplace of modern astrophysics."

Early instruments included a 40-inch refractor and the Snow Solar Telescope, named after the father of its benefactor, Helen Snow.

The observatory was operational until October 2018.

Source: Wikipedia

40-inch refractor in 1897
Source: Wikipedia

Spectra of Bright Stars (1897)

In 1897, Harvard computer Antonia Maury published a stellar classification catalogue called Spectra of Bright Stars Photographed with the 11-inch Draper Telescope as Part of the Henry Draper Memorial.

It included 4,800 photographs and Maury's analyses of 681 bright northern stars. This was the first instance in which a woman was credited for an observatory publication.

Sources: Wikipedia, Linda Hall Library

Spectra of Bright Stars
Source: Linda Hall Library

A Standard Candle for the Milky Way (1899)

In 1899, Harvard computer Williamina Fleming discovered the star RR Lyrae, the brightest star of a class called RR Lyrae variable.

RR Lyrae variables serve as important standard candles that are used to measure astronomical distances. The period of pulsation of an RR Lyrae variable depends on its mass, luminosity and temperature, while the difference between the measured luminosity and the actual luminosity allows its distance to be determined via the inverse-square law.

RR Lyrae variable stars are especially useful for the measurement of distances within the Milky Way. They were used by Harlow Shapley to determine the distance between our Sun and the center of the galaxy.

Source: Wikipedia

A Book about Star Lore (1899)

Richard Hinckley Allen, born in 1838, Buffalo, New York wanted to become an astronomer, but his poor eyesight crashed his dream. Instead of discovering new stars, he collected stories about the existing ones. His book Star Names: Their Lore and Meaning , published in 1899 was the first comprehensive collection of astronomical information about stars and constellations, paired with the myth and legend behind them. He collected Greco-Roman stories as well as Arabic, Babylonian, Indian and Chinese myth.

His book met criticism regarding its reliability, but it still remains the first large collection of star lore in printed form.

Source: Wikipedia

Star Names & their Meanings

Quantum Mechanics (1900)

At the turn of the century, theoretical physicists started rethinking the microworld. German physicist Max Planck found energy in packets he called quanta. This was the beginning of Quantum Mechanics.

Quantum Mechanics describe the laws of energy on the scale of atoms, which is essential to describe the physics in stars.

Planck's Quantum Theory and Einstein's Theory of Relativity became the base of many of the astronomical discoveries in the 20th century.

Source: Wikipedia,

Artist's visualization

Mount Wilson Observatory (1904)

In 1904, George E. Hale moved the Snow Sun Telescope from the Yerkes Observatory in Wisconsin to a new location at Mount Wilson in Southern California.

It was the first of a number of instruments that made the Mount Wilson Observatory one of the leading astronomical research facilities of the 20th century.

Sources: Wikipedia,

Snow Solar Telescope in 1912

E = mc2 (1905)

On September 26, 1905, German born physicist Albert Einstein published his Theory of Special Relativity regarding the relationship between space and time.

Part of the theory was the concept of the equivalence and transmutability of mass and energy, resulting in "the world's most famous formula", E = mc

Source: Wikipedia

Einstein's Formula
Altes Museum, Berlin

The Burnham Double Star Catalogue (1906)

Designed by US-American astronomer Burnham Wesley and published by the Carnegie Institution for Science, this catalogue listed 13,665 pairs of double stars.

Its official title was A General Catalogue of Double Stars Within 121° of the North Pole

Sources: Wikipedia, Astronomical Catalogs, Charts, and Surveys

Burnham Double Star Catalogue
Source: Abe Books

Hertzsprung–Russell Diagram (1906, 1912)

In 1906, Danish chemist and astronomer Ejnar Hertzsprung established a standard for measuring the true brightness of a star by showing a relationship between color and absolute magnitude for 90% of the stars in the Milky Way.

In 1912, New York's astronomer Henry Norris Russell, colaborating with Hertzsprung, presented a diagram, later known as the Hertzsprung–Russell Diagram to the Royal Astronomical Society.

Source: Wikipedia

H–R Diagram
Source: Wikipedia

Harvard Revised Photometry (1908)

The Harvard Photometry of 1884 covered only the northern hemisphere.

Between 1889 and 1891, US-American astronomers Solon I. Bailey and Edward C. Pickering collected data for the southern hemisphere. First published in 1908, the Harvard Revised Photometry listed 9,096 stars brighter than magnitude 6.5. leading to a Revised Catalogue in 1908.

Source: Wikipedia, Harvard University Ian Ridpath, astronomical catalogs, charts, and surveys

The Tunguska Event (1908)

On the morning of June 30, 1908, a massive explosion occurred near the Podkamennaya Tunguska River in the Eastern Siberian Taiga.

The explosion flattened an estimated 80 million trees over an area of 830 square miles or 2,150 square kilometers.

The event is generally attributed to the air burst of a small icy comet or a stony meteoroid about 330 feet or 100 metres in size that moved at about 10 miles (15 km) per second and collided with the Earth's atmosphere and exploded three to six miles (five to ten km) above ground.

It was the largest meteor impact in recorded human history.

Sources: Wikipedia,,

Artist's concept of the Event

Mount Wilson's 60-inch Telescope (1908)

While astronomer George E. Hale is credited with the foundation of the Mount Wilson Observatory, almost all of the early instruments were designed and built by astronomer and engineer George W. Ritchey.

In 1908, Ritchey delivered his master piece, the 60-inch Telescope, until the completion of the 100-inch Hooker telescope in 1917 the largest operational telescope in the world (the Leviathan in Ireland still held the record for the largest telescope ever built, but it had seized operation in the early 1890s).

The telescope became operational on December 8, 1908, and on December 24, 1908, Ritchey took the fist picture using the new instrument - the Orion Nebula.

Ritchey's photograph
of the Orion Nebula
In 1919, George Ritchey used the telescope to to locate faint light from erupting novae stars in spiral nebulae, suggesting that they were at extreme distances away from Earth and thus fueling the debate about galaxies other than the Milky Way.

Sources: Wikipedia,, space Today

Norton's Star Atlas (1910)

In 1910, British schoolmaster Arthur Philip Norton created a new kind of star atlas. His star charts divided the sky into six vertical slices, or gores, like portions of a globe. Each gore covered 4 hours of right ascension, from declination 60 degrees north to 60 degrees south. The north and south polar regions of the sky were covered by separate charts. Harvard Computer The atlas came with observing handbooks written by British astronomers William Henry Smyth and Thomas William Webb.

Norton's Star Atlas is one of the most popular collections of star charts, corrently in its 20th edition, published in 2003 under the editorship of Ian Ridpath.

Sources: Wikipedia, Stephen James: Arthur Philip Norton: The man and his star atlas

Norton's Star Atlas

White Dwarfs (1910)

The triple star system 40 Eridani was discovered in 1783. In 1910, Harvard Computer Williamina Fleming, together with Henry Norris Russell and Edward Charles Pickering had a second look at the system and discovered that one of the stars 40 Eridani B, despite being a dim star, was of spectral type A, or white.

This led to the clasification of a new type of stars, too faint to be obsrved with the naked eye. The discovery was important because it was somewhat outside of the model of star types accepted at the time.

Artist's concept of a white dwarf
The physics of a White Dwarf were explained in 1926 by British physicist and astronomer Ralph H. Fowler, using Quantum statistics. He described White Dwarfs as stellar core remnants composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth.

In 1930, Indian-American astrophysicist Subrahmanyan Chandrasekhar calculated the maximum mass of a stable white dwarf star. This value is now called the Chandrasekhar Limit.

Source: Wikipedia

Martians! (1911)

In 1877, Italian Astronomer Giovanni Schiaparelli observed the surface of Mars and named some of its features. Among them was a dense network of linear structures, which he called canali, which is Italian for channels, meaning features of natural origin.

In the English translation, they were erroneously named canals, meaning an artificial construction. This gave rise to decades of speculation and folklore about intelligent life on Mars.

One of the strongest supporters of the Martian Canal hypothesis was American astronomer Percival Lowell, who spent a considerable part of his life observing the "canals."

The hype about our "planetary neighbors" peaked in 1911. Lowell's drawings of the "canals" led to a New York Times article, that - in our opinion - made it onto the pages of Star Lore (as perhaps the first star legend created by the use of a telescope):

On August 27, 1911, the paper reported:

Vast Engineering Works Accomplished in an Incredibly Short Time by Our Planetary Neighbors.

In Percival Lowell's defense, it should be mentioned that he also initiated the calculations leading to a real discovery: Pluto.

Source: Wikipedia

Schiaparelli's surface map of Mars.
Source: Wikipedia

New York Times, August 27, 1911

The Harvard System (1912)

In 1901, Harvard Computer Annie Jump Cannon started working on the still unfinished spectral classification of stars.

In 1912, she introduced a system of seven categories, This became the Harvard System replacing the earlyer Secchi classes.

Source: Wikipedia

Spectral classes
on the H–R Diagram
Source: Wikipedia

A Standard Candle for long Distances (1912)

Cepheid variable are pulsating stars producing changes in brightness with a well-defined stable period and amplitude. The first one, η Aquilae was discovered in 1784 by English astronomer Edward Pigott.

In 1912, while tasked with examining photographic plates in order to measure and catalog the brightness of stars, Harvard Computer Henrietta Swan Leavitt discovered the relation between the luminosity and the period of Cepheid variables which can be used to calculate distances.

luminosity and period
Source: Universe Today
Since their discovery in 1899, RR Lyrae Variable stars were used as a standard candle for distances within the Milky Way. But these stars appeared mostly in clusters within our galaxy. Leavitt's discovery provided astronomers with the first standard candle with which to measure the distance to faraway objects.

Source: Wikipedia

Ptolemy's Almagest after almost two Millennia (1915)

English astronomer Edward Ball Knobel started out in 1872 as an amateur with a 8.5 inch reflecting telescope and eventually, in 1900 became president of the Royal Astronomical Society.

In 1875, Knobel started working on a publication about the chronology of star catalogues. This led him to studying Arabic and Persian.

In 1879 he published a translation of Ulugh Beg's Sultan's Tables, the most accurate and extensive star catalogue of the Islamic Golden Age. In 1895, he re-discovered Muḥammad al-Akhṣāṣī al-Muwaqqit, an almost forgotten Egyptian astronomer who published a star catalogue in 1650.

Over many years, Knobel and German astronomer C. H. F. Peters worked on a revision of the classic Almagest, written by Claudius Ptolemy in 147 AD.

Peters died in 1890, but Knobel continued, useing all available sources in Greek, Arabic and Latin. In 1915, Ptolemy's catalogue of stars: a revision of the Almagest was published.

Source: Wikipedia

A Revision of the Almagest

Spacetime (1915)

In 1915, Albert Einstein published the Theory of General Relativity, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime.

In 1916, German physicist Karl Schwarzschild used Einstein's theory to lay the groundwork for a theory on black holes, suggesting that if any star collapse to a certain size or smaller, its gravity will be so strong that no form of radiation will escape from it.

Source: Wikipedia

Spacetime; Source:

The nearest Star (1915)

In 1915, Scottish astronomer Robert Innes, director of the Union Observatory in Johannesburg, South Africa discovered a red dwarf belonging to the the Alpha Centauri system. With a distance of 4.244 light years, Proxima Centauri is the closest star to our sun.

Source: Wikipedia

Hubble Telescope image
of Proxima Centauri
Source: ESA
The second nearest Star (1916)

Shortly after the discovery of Proxima Centauri, US-American astronomer Edward Emerson Barnard discovered another Red Dwarf with a very large proper motion, relative to other stars.

The star, now known as Barnard's Star is six lightyears away from our sun, making it the second closest to our sun, next to the tripple system of Alpha Centauri.

Barnard's Star is one of very few stars named after an actual person.

Source: Wikipedia

Island Universes (1917)

In 1917, US-American astronomer Heber Curtis, examining photographs of a novae in the "Andromeda Nebula", concluded that the object was too far away to be part of the Milky Way galaxy. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.

In 1924, (See below), US-American astronomer Edwin Hubble discovered a Cepheid variable in the "Andromeda Nebula." Calculating its distance, he confirmed Curtis' theory.

Source: Wikipedia

Andromeda Galaxy
Source: Wikipedia

The Hooker telescope (1917)

Funded by amateur astronomer and philanthropist John Daggett Hooker, the world's first 100-inch telescope was completed in 1917 at the Mount Wilson Observatory in southern California.

This was the telescope Edwin Hubble used in 1923/24 to settle the Great Debate concerning the size of the universe and in 1928, when he made what is considered the most important astrinomical discovery of the 20th century - the expanding universe.

It was also the telescope used by Fritz Zwicky in 1933, when he observed the first evidence for the existence of Dark Matter.

Hooker Telescope at Mt. Wilson Observatory
Source: National Public Radio
Until the completion of the Hale Telescope in 1949, the Hooker Telescope was the largest telescope in the world.

Sources: Wikipedia,,

Henry Draper Catalogue (1918)

The Henry Draper Catalogue is named after Henry Draper, who in 1872 made the first photograph of a star's spectrum showing distinct spectral lines and pioneered the classification of stellar spectra.

The catalog was based on the earlier Draper Catalogue of Stellar Spectra.

The first issue, giving spectroscopic classifications for 225,300 stars was published between 1918 and 1924. 46,850 more stars were added between 1925 and 1936 and 86,933 more stars were added between 1937 to 1949.

Star Spectra; Source: Astro Index

Today, the catalogue contains 359,083 stars and its catalogue numbers are commonly used as a way of identifying stars.

Sources: Wikipedia, Astro Index

Gravitational Lense (1919)

On May 29, 1919, British astronomer Arthur Eddington observed a total eclipse of the Sun in order to confirm Einstein's theory of general relativity. According to Einstein's theory, stars with light rays that passed near the Sun would appear to have been slightly shifted because their light had been curved by the Sun's gravitational field - an effect now known as gravitational lensing.

Observing the eclipse from the island of Príncipe in West Africa, Eddington and Frank W. Dyson took pictures of the Hyades Cluster in the region around the Sun, confirming the gravitational lensing effect.

Sources: Wikipedia

Eclipse of 1919
Source: Wikipedia

Barnard Catalogue (1919)

In 1895, Edward Emerson Barnard began taking detailed pictures of the Milky Way. Barnard became especially interested in Dark nebulae obscuring the view of the Milky Way.

In 1919, Barnard published the first version of the Barnard Catalogue of Dark Markings in the Sky, which listed 182 nebulae.

The final version, listing 369 nebulae was publishe posthumously in 1927 under the title Photographic Atlas of Selected Regions of the Milky Way.

Sources: Wikipedia, The Making of

Nebula Barnard 68
Source: Wikipedia

Finding our place in the Galaxy (1919)

Ever since Copernicus had placed the Sun in the center of our solar system, it was assumed that the Sun was close to the center of the Galaxy, thus at least all the other stars would still rotate around us.

In 1914, Harvard astronomer Harlow Shapley started observing globular clusters within the Milky Way. He found that of the 100 clusters known at the time, one-third lay within the boundary of the constellation Sagittarius.

Shapley used the newly developed Standard Candle concept, using the period of variation of RR Lyrae variable stars to calculate their distance.

He found that the clusters were distributed roughly in a sphere whose centre lay in Sagittarius. Since the clusters assumed a spherical arrangement, it was logical to conclude that they would cluster around the centre of the Galaxy.

Sun's orbit in the Milky Way
Source: Wikipedia

From this conclusion, Shapley deduced that the Sun lies at a distance of about 50,000 light-years from the centre of the Galaxy; the number was later corrected to 30,000 light-years.

Shapleys observations led to the first realistic estimate for the actual size of the Milky Way. Today, we know (to the best of our knowledge) that Milky Way is about 150,000 to 200,000 light-years across, about 2,000 light-years deep, and has 100 to 400 billion stars.

Our Sun is located in the Orion-Cygnus Arm of the Milky Way, some 27,000 light years away from the center. We travel at a speed of 141.5 miles per second (828,000 km/h) around the center, taking 230 million years to complete one circle.

Sources: Encyclopedia Britannica, Wikipedia, NASA

The Measure of a Star (1920)

In 1920, US-American astronomers Francis G. Pease, Albert A. Michelson and John A. Anderson fitted Michelson's stellar interferometer to the 100-Inch-Telescope at Mt. Wilson to to measure the angular diameter of the star Betelgeuse.

This was the first time the diameter of a star other than our Sun was measured.

Source: Wikipedia

Betelgeuse; Source: Astro Bob
Saha Ionization Equation (1920)

Combining ideas of quantum mechanics and statistical mechanics, Indian physicist Meghnad Saha developed an expression that relates the ionisation state of a gas in thermal equilibrium to the temperature and pressure. In years to follow, the Saha Ionization Equation became essential for the spectral classification of stars.

Source: Wikipedia

The Great Debate (1920)

The Great Debate was a discussion between two US-American astronomers concerning the size of the universe and the nature of so-called spiral nebulae. It was held at the Smithsonian Museum of Natural History on April 26, 1920.

Harlow Shapley believed that there was only one galaxy in the universe and that distant nebulae were relatively small and lay within the outskirts of that galaxy.

Heber Curtis argued that these "nebulae" were exceedingly large and distant independent galaxies.

The debate was settled four years later by Edwin Hubble (see below).

Sources: Wikipedia,, NASA, BBC

© Dennys Bilylski

Setting final Boundaries (1922 - 1928)

At the end of the 19th century, the international astronomic community had worked out an understanding on which constellations should remain and which should be scraped. Above mentioned R.H. Allen wrote, From 80 to 90 constellations may be considered as now more or less acknowledged.

In 1922, the International Astronomical Union was founded. At its first General Assembly, the IAU officially adopted a list of 86 constellations covering the entire sky. One final adjustment to the selection of constellations was made in 1928, when the IAU adopted de Lacaille's suggestion to divide Ptolemy's large constellation Argo Navis (The Ship) into three smaller constellations.

Sources: Wikipedia, Ian Ridpath

The Breaking up of Argo Navis



Poop Deck


Constellations with modern boundaries

Wikipedia and Ian Ridpath provide detailed tables of the 88 IAU designated constellations and their history.

One last issue needed to be solved: Traditional star maps displayed constellations with selected stars that were either defined by pictures or connected by lines. That system left a lot of orphaned stars between the constellations and with ever improving telescopes, the number of orphans kept rising.

To fix the border dispute, Belgian astronomer Eugène Delporte was contracted to draw border lines between the constellations along lines of right ascension and declination, so that every point in the sky would belong to one constellation.

Delporte's work was officially adopted in 1928 and published in 1930, resulting in the 88 modern constellations we still use today.

Sources: Wikipedia, Ian Ridpath

Eugène Joseph Delporte

The Andromeda Galaxy (1924)

In 1923, Edwin Hubble discovered a Cepheid variable, a so-called standard candle in the "Andromeda Nebula." The star with the prosaic name Cepheid Variable Star V1 allowed Hubble to calculate the distance of the "nebula.

By the end of 1924 Hubble had found 12 Cepheid variables. Using their period and amplitude, he obtained a distance of 900,000 light-years - three times the distance Shapley had estimated as the diameter of the Milky Way.

Later, more precise measurements revealed that Shapley's estimate overshot the size of the Milky Way by at least 100,000 light years and that the Andromeda Galaxy is in fact 2.5 million light years away. But nevertheless, Hubbles calculations eliminated any doubt that the "Nebula" was extragalactic.

Hubble published his findings in an article entitled "Extragalactic Nature of Spiral Nebulae." 1925, he developed a system for the classification of galaxies, based on their photographic images.

Sources: Wikipedia, NASA/ESA,

Cepheid variable V1 in a picture taken by the Hubble Telescope
© NASA, ESA, Hubble Heritage

Stellar Atmospheres (1925)

In her doctoral dissertation at Harvard University Cecilia Payne-Gaposchkin relate the spectral classes of stars to their actual temperatures by applying the Saha ionization theory. Her thesis concluded that hydrogen was the overwhelming constituent of stars, making it the most abundant element in the Universe.

Source: Wikipedia

Solar flare in the Sun's atmosphere

Rotating Milky Way (1927)

In 1927, observing the apparent motions of stars, Swedish astronomer Bertil Lindblad suggested that the Milky Way rotated around its Center.

Dutch astronomer Jan Oort mathematically confirmed Lindblad's theory and calculated that our sun was about 30,000 light-years away from the center of the Galaxy and that it took the sun about 225 million years to complete its orbit.

Source: Wikipedia

Artist's concept of the spiral arms of the rotating milky way
© Xing-Wu Zheng & Mark Reid BeSSeL/NJU/CFA

The Expanding Universe (1927, 1929)

In 1927, applying Einstein's theory of general relativity to cosmology, Belgian priest, mathematician and astronomer Georges Lemaître developed a first theorie about an expanding universe.

In 1929, Edwin Hubble came to the same conclusion, examining the relation between distances and radial velocities of 24 extra-galactic nebulae as determined from their redshifts and published his findings in an article entitled A relation between distance and radial velocity among extra-galactic nebulae.

Today, the observation that galaxies are moving away from the Earth at speeds proportional to their distance is called the Hubble-Lemaître Law.

Sources: Wikipedia, NBC News

Artist's concept of an
expanding universe
Source: Pinterest

The Ninth Planet - at least for a while (1930)

After comparing photographs for nine month, on February 18, 1930 US-American astronomer Clyde Tombaugh discovered a faint, object outside the orbit of Neptune. The object was named Pluto and was declared the ninth planet.

Starting in 1992, other objects in the area of the Solar System, now called the Kuiper Belt, were discovered, leading the IAU in 2006 to rewrite the definition of a planet and declaring Pluto a Kuiper-belt object and a Dwarf Planet.

Source: Wikipedia

The Brightest Stars (1930)

In 1924,US-American astronomer Frank Schlesinger started working on a star catalogue listing the brightest stars with unprecedented accuracy. In 1930, the first edition of the Yale Bright Star Catalogue was published.

The catalogue is limited to star visible with the naked eye, but it lists these stars with a precision of about one hundredth of an arc second.

Sources: Wikipedia, David Darling: Astronomical catalogs, charts, and surveys,

Bright Star Catalogue

Radio Waves from the Center of the Milky Way (1931)

In August 1931, US-American radio engineer Karl Jansky at Bell Telephone Laboratories registered a faint steady hiss of unknown origin at a frequency of 20.5 MHz. He spent over a year investigating the source of the static and concluded that the radiation was coming from the Milky Way and was strongest in the direction of the Galactic Center, in the constellation of Sagittarius.

The detection the first radio signal detected from outer space was the beginning of Radio astronomy.

Replica of Jansky's antenna
Source: Wikipedia

The radio source was called Sagittarius A* and is now believed to be a supermassive black hole at the center of our galaxy.

Source: Wikipedia

The Aitken Double Star Catalogue (1932)

Designed by US-American astronomer Robert G. Aitken, this catalogue was the successor to the Burnham Double Star Catalogue.

It listed 17,180 pairs of double stars north of −30° declination and its official title was New general catalogue of double stars within 120° of the North Pole.

Sources: Wikipedia, Astronomical Catalogs, Charts, and Surveys

Aitken Double Star Catalogue
Source: WorthPoint

Dark Matter (1933)

In 1933, using the Hooker Telescope in California, Swiss astronomer Fritz Zwicky observed the Coma Galaxy Cluster.

When he compared the rotational velocity of luminous matter to the calculated gravitational attraction within the cluster he discovered a massive gravitational inconsistency and concluded that most of the mass was not accounted for.

(Similar observations had been made by Lord Kelvin in 1884 and by Henri Poincaré in 1906).

Artist's concept (with a lot of creative freedom)
of dark matter; Source: Science Alert

Poincaré was the first to use the the term Dark Matter. Now, Zwicky's calculations provided theoretical proof. When Zwicky called it dunkle Materie, the term Dark Matter was widely accepted as a description of an invisible mass that is thought to be responsible for adding gravity to galaxies and other bodies.

Sources: Wikipedia, Science Alert

Supernovae and Neutron Stars (1934)

In 1934, Fritz Zwicky and his colleague, German astronomer Walter Baade coined the term Supernova for the powerful and luminous stellar explosions that can be observed all over the universe from time to time.

They hypothesized that supernovae were the transition of normal stars into neutron stars, metal rich extremely dense objects, that no longer actively generate heat. They also suggested that these neutron stars were the origin of cosmic rays.

In support of this hypothesis, Zwicky spent the next 52 years searching for supernovae and discovered no less than 120 of them.

Source: Wikipedia

Artist's concept of a supernova Source: NASA

The Boss General Catalogue (1936)

In 1910, US-American astronomer Lewis Boss, director of the Dudley Observatory in Schenectady, New York published a catalogue containing 6,188 Stars.

In 1936, his son Benjamin Boss, who had followed him as director of the Dudley Observatory, published an extended issue, entitled General Catalogue of 33,342 Stars, that became known as the Boss General Catalogue.

Sources: Wikipedia, David Darling: Astronomical catalogs, charts, and surveys,

The Astronomical Navigation Tables and the stars of the Royal Air Force (1937)

In the tradition of the Nautical Almanach, at the end of the year 1937, Her Majesty's Nautical Almanac Office selected 57 stars for marine and air navigation.

All of these Selected Stars were of first or second magnitude and were selected in a way that at any given time and at any given location, there were seven stars with altitudes most suitable for astronomical navigation.

With the exception of α Pavonis and ε Carinae, all of the selected stars were well known by traditional names. Since the Royal Air Force insisted in given names for all of the selected stars, the Nautical Almanac Office had to "invent" two new names.

α Pavonis was named Peacock, which is the English name of the constellation Pavo. ε Carinae received the name Avior. No official reason for that name was given, but considering the connection to the Royal Air Force in the naming process, it is very likely that the name derived from avis, which is Latin for "bird" and is now also used for "aircraft".

Starting in 1958, the yearly Nautical Almanac was published jointly by H.M. Nautical Almanac Office and the US Naval Observatory.

Sources: Wikipedia, eSky, Donald H. Sadler: A Personal History of H.M. Nautical Almanac Office,

The first Radio Telescope (1937)

US-American radio amateur Grote Reber closely followed Karl Jansky's pioneering work on radio astronomy. In 1933, he applied for a job at Bell Telephone Laboratories, where Jansky received his famous signal from the Center of the Milky Way. Since there were no jobs available on account of the Great Depression, he decided to build his own receiver.

In 1937, built from scratch in his backyard, Reber presented the world's first parabolic radio telescope.

The first large dish radio telescopes such as the Green Bank Telescope and the Lovell Telescope were modeled after Reber's design and thus, Reber and Jansky are considered the co-founders of radio astronomy.

Source: Wikipedia

Reber Radio Telescope; Wikipedia

Nuclear Fusion fueling Stars (1938)

As early as 1920, British astrophysicist Arthur Eddington first suggested that stars draw their apparent endless energy from the fusion of hydrogen into helium. In 1926, he published a paper called The Internal Constitution of the Stars on the subject.

The process was explained in detail in 1938 by German-American nuclear physicist Hans Bethe, explains how stars generate energy introducing the theory of stellar nucleosynthesis, which outlines a series of nuclear fusion reactions that turn hydrogen into helium and release enormous amounts of energy in a star's core. These reactions use the star's hydrogen very slowly, allowing it to burn for billions of years.

Sources: Wikipedia,

Fusion reactions in the Sun

The Big Bang Theory (1948)

The first one to suggest that the expansion of the universe can be traced back to an initial Big Bang was Georges Lemaître in 1931.

The theory that was defined with more detail in 1948 by Russian born theoretical physicist and cosmologist George Gamow.

During a BBC Radio broadcast in March 1949, English astronomer Fred Hoyle was the first to actually use the term by saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past."

Sources: Wikipedia, NBC News

Artist's concept of the Big Bang
Source: NBC News

Atlas of the Heavens (1948)

In 1948, the Czechoslovak Astronomical Society published a star atlas called the Atlas Coeli Skalnaté Pleso (Skalnaté Pleso Atlas of the Heavens). The charts were hand-drawn by Czech astronomer Antonín Bečvář who worked at the Skalnaté Pleso Observatory in Slovakia.

The atlas did not only contain all stars brighter than magnitude 7.75 but also all non-stellar objects like galaxies, nebulae and star clusters, that were visible in an 8-inch telescope. It was the first atlas to do so, which made it extremely popular among amateur astronomers (one website called it "the best-known and most used, drawn astronomical atlas all over the world...").

Page of Atlas Coeli
Source: Skalnaté Pleso Observatory

Bečvář included 14 new star names in his atlas. If there was any meaning for these names, Bečvář took the answer to his grave. Even now, over 50 years after his death, the origin of these 14 names still remains a mystery. You can read more about it here.

Bečvář's atlas marked the end of an era. For over 3,000 years, astronomers had created star catalogs, first written in clay tablets and on papyrus, later printed on paper. Artist had rendered elaborate drawings of stars and constellations. Now, with the beginning of the information age, high resolution cameras would replace the artists and computers would generate tables containing more data than a single person could write down in a lifetime.

Sources: Wikipedia, Skalnaté Pleso Observatory

The 200-inch Hale Telescope (1949)

On January 26, 1949, the 200-inch telescope at the Palomar Observatory near San Diego, California started its operation.

Named after US-American astronomer George Ellery Hale, who was essential in its design, the Hale Telescope was twice as large as the previous largest telescope, the 100-inch Hooker Telescope.

The Hale telescope was groundbreaking for its time and remained the world's largest telescope until 1976.

Source: Wikipedia

Hale telescope
Source: Wikipedia

The Origin of Lunar Craters (1949)

The idea that a meteor impact could significantly shape the surface of a planet or a moon was developed until the 1960s; thus until recently, the numerous craters on the Moon were thought to be of vulcanic origin.

In 1949, US-American planetary scientist Ralph B. Baldwin published The Face of the Moon, in which he described how the moon’s craters were caused by meteor impacts and not by volcanic action.

Although it still took decades until the scientific community accepted his idea, his theory revolutionized planetary astronomy.

Source: Wikipedia,

The Face of the Moon

The Cambridge Catalogue of Radio Sources (1950)

Most star catalogues list stars that are visible either with the naked eye or with the aid of a telescope. With the rapid development of radio astronomy, the need for a new kind of catalogue arose.

In 1950, three astronomers of the Cavendish Astrophysics Group (M. Ryle, F.G. Smith and B. Elsmore) at the University of Cambridge published a paper under the title A preliminary survey of the radio stars in the Northern Hemisphere.

The paper containd 50 radio sources observed by various radio astronomers.

Artist's rendering of a quasar - one of the
main sources of radio signals
Source: European Southern Observatory
It later turned out than many of these early observations containd errors. Nonetheless,it was the first paper to summarize the state of radio astronomy.

Later updates were called Cambridge Catalogue of Radio Sources and the early paper was retroactively titled the First Cambridge Catalogue.

Between 1955 and 2003, a total of eight updates on different bandwiths were issued.

Sources: Wikipedia, A preliminary survey of the radio stars in the Northern Hemisphere

With the launch of the first satellite in 1957, astronomy moved into a totally new laboratory: Space. Soon some of the world's largest telescopes would no longer be on Earth, but in orbit around it.

Chronologically, there is an overlap with our Standards and Measurements section as we moved early progress in rocketry prior to the launch of the first satellite into the Space Age section while leaving developments in astronomy and astrophysics prior to 1957 in the Standards and Measurements section.

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