DANJON SCOPE DIAGRAM: GREGG DINDERMAN / S&T; MOON IMAGE: SEAN WALKER / S&T; CLOUDS OVER EARTH FROM ISS: NASA Objective lens Prism 2 Fully open Diaphragm Prism 1 Partly closed Focal plane SIMBLE BUT PRECISE André-Louis Danjon's earthshine telescope used a pair of prisms to provide two identical side-by-side images of the Moon, as shown in the illustration at right. A cat's-eye diaphragm could be adjusted to dim one of the images until the Moon's sunlit portion had the same apparent brightness as the earthlit portion of the unadjusted image. varies not only daily due to changes in cloud cover, but also seasonally (faintest in August and brightest in October) and even from year to year. Observing only from sites in France, Danjon and Dubois derived values for Earth's albedo that were erroneously high because the comparatively bright Eurasian landmass to their east reflected more sunlight onto the Moon than the oceans did. To eliminate this source of error, during the 1950s and 1960s Harvard College Observatory astronomer Fred Whipple and Gustav Bakos of the University of Waterloo in Canada independently established global networks of Danjon earthTOWERING CUMULONIMBUS The stark contrast in brightness between a dramatic cloudscape and the underlying terrain was captured by the crew aboard the International Space Station in 2016. Such clouds have an albedo of 0.7 to 0.9, while open bodies of deep water have an albedo of only 0.06. shine telescopes at sites widely separated in longitude. Bakos soon discovered a second source of error: the brilliant specular reflection of the Sun off the surface of Earth's oceans when overlying clouds are absent. This sun glint can brighten earthshine considerably. The third and largest source of error in the pioneering French work proved to be the extremely nonlinear variation in the reflectivity of the Moon as its phase changes. At full Moon, the brightness of every point on the entire lunar surface shines a whopping 40% brighter than just one day earlier or later. This is caused by a phenomenon known as coherent sk yand tele scope .o r g * MARCH 2023 25