Andre-Louis Danjon (6 April 1890 – 21 April 1967) was a French astronomer born in Caen to Louis Dominique Danjon and Marie Justine Binet.
Danjon devised a method to measure "Earthshine" on the Moon using a telescope in which a prism 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 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 Lecture to the Royal Astronomical Society (in Monthly Notices of the RAS (1958), vol.118, pages 411-431).
He also developed the Danjon Limit, a proposed measure of the minimum angular separation of the Sun and Moon at which a lunar crescent is visible. However, this limit may not exist.
He was Director of the Observatory of Strasbourg from 1930 to 1945 and of the Paris Observatory from 1945 to 1963 (see biographical notice at the website of the Observatoire de Paris http://www.obspm.fr/histoire/paris/danjon.fr.shtml).
He was awarded the Prix Jules Janssen of the French Astronomical Society in 1950, and the Gold Medal of the Royal Astronomical Society in 1958.
Danjon died in 1967 in Suresnes, Hauts-de-Seine.