Coordinate An Evening With The Moon You Will

The morning crescent lies to the right or west of the sun-Earth line, so we see its eastern edge in sunlight. After new moon, the moon swings to the left (east) of the sun, so we see its western edge illuminated. Bob King

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.

Latitude and longitude on the moon are anchored by the small, nearside crater Mösting A (hidden under the white 0° circle) near the center. When you look at a full moon as shown here, lunar west is to left and lunar east to the right, exactly the opposite of directions in the sky. John Reid / Wikipedia

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.

Mare Orientale, the Eastern Sea, are the dark patches inside the oval at the moon’s western edge. Tycho crater is at lower right. Gregory H. Rivera, CC BY-SA 3.0

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.

The day-old moon returns at sunset tonight, but it will be challenging. Or you can wait till tomorrow, when it will much easier to find. On Feb. 10 (Sunday), it will pair up with the planet Mars. Stellarium

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.”


2 Responses

    1. astrobob

      Really cool, Carol. Thanks. You can see how overexposed it is compared to the Hayabusa images because the Earth is not totally black.

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