André-Louis Danjon (; 6 April 1890 – 21 April 1967) was a French

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either o ...

born in

Caen Caen (, ; nrf, Kaem) is a commune in northwestern France. It is the prefecture of the department of Calvados. The city proper has 105,512 inhabitants (), while its functional urban area has 470,000,earthshine Earthlight is the diffuse reflection of sunlight reflected from Earth's surface and clouds. Earthshine (an example of planetshine), also known as the Moon's ashen glow, is the dim illumination of the otherwise unilluminated portion of the Moon ...

" on the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...

using a

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...

in which a

prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...

split the Moon's image into two identical side-by-side images. By adjusting a diaphragm to dim one of the images until the sunlit portion had the same apparent brightness as the earthlit portion on the unadjusted image, he could quantify the diaphragm adjustment, and thus had a real measurement for the brightness of earthshine. He recorded the measurements using his method (now known as the Danjon scale, on which zero equates to a barely visible Moon) from 1925 until the 1950s. Among his notable contributions to astronomy was the design of the impersonal (prismatic)

astrolabe An astrolabe ( grc, ἀστρολάβος ; ar, ٱلأَسْطُرلاب ; persian, ستاره‌یاب ) is an ancient astronomical instrument that was a handheld model of the universe. Its various functions also make it an elaborate incli ...

based on an earlier prismatic astrolabe developed by François Auguste Claude which is now known as the Danjon astrolabe, which led to an improvement in the accuracy of fundamental optical astrometry. An account of this instrument, and of the results of some early years of its operation, are given in Danjon's 1958

George Darwin Sir George Howard Darwin, (9 July 1845 – 7 December 1912) was an English barrister and astronomer, the second son and fifth child of Charles Darwin and Emma Darwin. Biography George H. Darwin was born at Down House, Kent, the fifth chi ...

Lecture to the

Royal Astronomical Society (Whatever shines should be observed) , predecessor = , successor = , formation = , founder = , extinction = , merger = , merged = , type = NG ...

. He also developed the "Danjon limit", a proposed measure of the minimum

angular separation Angular distance \theta (also known as angular separation, apparent distance, or apparent separation) is the angle between the two sightlines, or between two point objects as viewed from an observer. Angular distance appears in mathematics (in pa ...

between the Sun and the Moon at which a lunar

crescent A crescent shape (, ) is a symbol or emblem used to represent the lunar phase in the first quarter (the "sickle moon"), or by extension a symbol representing the Moon itself. In Hinduism, Lord Shiva is often shown wearing a crescent moon on his ...

is visible. However, this limit may not exist. He was Director of the

Observatory of Strasbourg The Observatory of Strasbourg is an astronomical observatory in Strasbourg, France. Following the Franco-Prussian War of 1870–71, the city of Strasbourg became part of the German Empire. The University of Strasbourg was refounded in 1872 ...

from 1930 to 1945 and of the

Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its hist ...

from 1945 to 1963. Danjon was the President of the

Société astronomique de France The Société astronomique de France (SAF; ), the French astronomical society, is a non-profit association in the public interest organized under French law ( Association loi de 1901). Founded by astronomer Camille Flammarion in 1887, its purpos ...

(SAF), the French astronomical society, during two periods: 1947–49 and 1962–64. He was awarded the

Prix Jules Janssen The Prix Jules Janssen is the highest award of the Société astronomique de France (SAF), the French astronomical society. This annual prize is given to a professional French astronomer or to an astronomer of another nationality in recognition ...

of the Société astronomique de France in 1950, and the

Gold Medal of the Royal Astronomical Society The Gold Medal of the Royal Astronomical Society is the highest award given by the Royal Astronomical Society (RAS). The RAS Council have "complete freedom as to the grounds on which it is awarded" and it can be awarded for any reason. Past awa ...

in 1958. Danjon died in 1967 in

Suresnes Suresnes () is a Communes of France, commune in the western suburbs of Paris, Île-de-France. Located in Hauts-de-Seine, from the centre of Paris, it had a population of 49,145 as of 2016. The nearest communes are Nanterre, Puteaux, Rueil-Malmais ...

Hauts-de-Seine Hauts-de-Seine (; ) is a département in the Île-de-France region, Northern France. It covers Paris's western inner suburbs. It is bordered by Paris, Seine-Saint-Denis and Val-de-Marne to the east, Val-d'Oise to the north, Yvelines to the we ...

.Bibliothèque nationale de France: Danjon André
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References

20th-century French astronomers Members of the French Academy of Sciences Scientists from Caen Recipients of the Gold Medal of the Royal Astronomical Society Academic staff of the University of Strasbourg 1890 births 1967 deaths Presidents of the International Astronomical Union {{France-astronomer-stub