Before we start looking in detail at the Moon, I would just like to clarify the issues of up, down, east, and west! Standard astronomical telescopes, such as refractors and reflectors, always show the Moon upside down. Schmidt-Cassegrains are even more complicated as they often come supplied with a 90 degree prism that makes the eyepiece more comfortably placed, but creates a mirror image. However, historically, the Moon has always been sketched with south at the top, as seen through a reflector or refractor from the northern hemisphere. In this chapter, as in all the planetary chapters, south is at the top, i.e., the Moon is upside down compared to how it looks to a naked-eye, northern hemisphere observer. East and west can be confusing, too. Prior to the space probe era, features on the Langrenus/Petavius/Mare Crisium side were on the west and features on the Aristarchus/Gassendi side were on the east, as was the extreme eastern limb feature called the Mare Orientale, or "Eastern Sea." However, the International Astronomical Union (IAU) reversed this ruling in the space probe era and so Langrenus, Petavius, and the Mare Crisium are now in the east and Aristarchus and Gassendi are in the west. Mare Orientale is now on the western limb: yes, you've got it . . . the "Eastern Sea" is on the western limb of the Moon!
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