Hello and welcome back. You and the moon both! I couldn’t resist a little Yoda-speak in the title — the Force will do that to a guy.
After delighting us with stops at Jupiter, Venus and Saturn at dawn, the crescent moon has returned to the evening sky. The bright part has also switched sides. Since the dawn crescent lies west of the sun, its sun-facing, eastern edge is illuminated. After new moon, the crescent shines to the left or east of the setting sun with its western rim etched in sunlight.
These are directions as we see them in the sky, but not for an astronaut walking on the moon. The same way the Earth has a prime meridian dividing eastern and western hemispheres, so does the moon. On our planet the 0° longitude meridian passes through Greenwich, England. Everything west of this line is considered the western hemisphere, while everything east of it belongs to the eastern hemisphere.
We use the exact same system on the moon, but instead of Greenwich, astronomers chose the fresh, bright crater Mösting A, located near the center of the moon’s face near the equator, as the basis for the lunar system of latitude and longitude. For northern hemisphere observers, when we see a half-moon in the evening sky, that’s the moon’s eastern hemisphere — equivalent to eastern Europe, Asia, Africa, Oceania and Australia. At last quarter phase, when the “left” half of the moon is illuminated, we’re seeing its western hemisphere. Of course, at full moon we see both. If you’re reading this from the southern hemisphere, these directions are reversed.
Before 1961, east and west directions in the sky were the same as east and west on the moon. So it made sense in the early 1900s to name a newly-studied lunar sea located at the eastern edge of the moon the Eastern Sea (Mare Orientale). Oops. Now that current conventions are in place, it really should be called the Western Sea or Mare Occidentale. But tradition holds.
Tonight, those of you with a wide-open western horizon can look for a baby moon just 25-28 hours old. Face west-southwest about 20 minutes past sunset and scan about 5° or two fingers held together at arm’s length above the horizon for an extremely delicate, sickle-shaped moon. Binoculars will help under less than ideal conditions. Tomorrow the moon will be nearly as thin but higher up and much easier to spot.
I’ll end with a real Yoda quote, a favorite: “Do or do not. There is no try.”