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 ...

born in Caen to Louis Dominique Danjon and Marie Justine Binet. Danjon devised a method to measure "

earthshine Earthlight is the diffuse reflection of sunlight reflected from Earth's surface and cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended ...

" 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 of ...

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 observ ...

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 The Danjon scale is a five-point scale useful for measuring the appearance and luminosity of the Moon during a total lunar eclipse. It was proposed by André-Louis Danjon in 1921, when postulating that the brightness of a lunar eclipse was rela ...

, 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 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 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 187 ...

from 1930 to 1945 and of the Paris Observatory from 1945 to 1963. Danjon was the President of the Société astronomique de France (SAF), the French astronomical society, during two periods: 1947–49 and 1962–64. He was awarded the Prix Jules Janssen 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 awar ...

in 1958. Danjon died in 1967 in

Suresnes Suresnes () is a 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-Malmaison, Saint-Cloud a ...

, Hauts-de-Seine.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