Tonight’s moon comes up in the northeastern sky in Taurus the Bull. Since it’s five days past full phase, its light has diminished enough so that sky watchers in rural areas can again see the Milky Way slicing across the southern sky. We’re in for a treat because the moon will be very close to the Pleiades or Seven Sisters star cluster. It rises relatively early around 8:45 p.m., but the best time to see this fine pairing is at 10 p.m. or later, when they’ve cleared trees and buildings. While not a great naked eye event, due to the lunar glare, the view in binoculars will be outstanding. Both will fit easily in the same field of view and stand in striking contrast to one another.
This past Saturday night my older daughter had to determine the moon’s phase for a homework assignment. She hesitated but then got the correct answer: waning gibbous.
The moon fills out from the time we first see it as a thin crescent in evening twilight up until full moon, when the side facing us is 100% illuminated by the sun. The line separating the daylight from the nighttime portion of the moon is called the terminator. Night after night, the terminator moves to the left or east exposing more of the moon. Picture the terminator as the line of advancing sunrise. Features that were previously in darkness rotate into the sunshine as the angle between the moon, Earth and sun widens with each passing night. Unlike Earth, where the nights last a matter of hours, a typical lunar night lasts almost two weeks. That’s because it takes the moon almost a month (27.3 days) to rotate once on its axis. A night on the moon is a very chilly experience. With no atmosphere to speak of and plenty of darkness to go around, temperatures plummet to an average of 250 below Fahrenheit.
When the moon is full, it’s directly opposite the sun and all lit up the same way someone might shine a big, bright light directly into your face. The terminator disappears briefly at full but soon reappears along the moon’s western edge the day after.
Have you noticed the moon looks a little out of round the past couple nights? The western edge is missing because the sun is now setting there after the long lunar day. Slice by slice, the terminator, which is now the advancing line of lunar sunset, gobbles up the moon, converting it from full to waning gibbous and then to a half. The half or 3rd quarter moon refers to the moon having completed 3/4 of its orbit around the Earth since new moon.
Several nights later, the moon’s phase will have narrowed to a thin crescent, and by October 7 – new moon – we’ll see no moon at all. It will lie almost directly between us and the sun in the daylight sky. The side facing the sun will be fully illuminated, but the side facing Earth will be in complete shadow. A day later the cycle begins anew with a fresh and thin crescent low in the west at dusk.