Observing the Moon: Directions on the Moon (They're different since 1961)

Jack Kramer

I encountered a confusing situation one night when observing the Moon. I had used the computer program Moon Tool to get an orientation for that night, then consulted a large photographic atlas of the Moon from Lick Observatory. But things just weren't where I thought they'd be. A smaller scale atlas from Sky & Telescope agreed with the computer program. The atlases were oriented so that south was at the top as in a reflecting telescope, and the images were identical, but one source labelled lunar east to the left, while another showed it to the right. Which is correct? Some research disclosed that prior to 1961, it was common on lunar atlases to designate east to the left when north is up, as an observer in the northern hemisphere would reckon it when looking at the Moon in the sky. My trusty old Lick atlas has a copyright date of 1959. Aha! With the advent of the Space Age, it was necessary to adopt the orientation used in all mapping. Nowadays, with the atlas inverted so that south is up, to correspond to a Newtonian telescope, east is shown to the left. Guess its time to retire the pre-Space Age atlas. Oh well, after all those years of use it is falling apart now, anyway. (Like its owner!) By the way, the large Rand-McNally atlas of the Moon has north up. If this proves awkward, you might want to look for an atlas oriented toward observers, printed with south at the top.

Published in the September 1995 issue of the NightTimes