April 6: André-Louis Danjon

André-Louis Danjon

April 6, 1890 – April 21, 1967

André Danjon was a French astronomer born in Caen.

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. Within four years of its introduction (1956), the Danjon astrolabe was being used in more than 30 major observatories. 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 was Director of the Paris Observatory from 1945 to 1963.

He won the Gold Medal of the Royal Astronomical Society in 1958. The Lunar crater Danjon is named in his honor.