First-Quarter Moon

• ‹ Previous program
• 309 of 1968
• Next program ›

Many calendars have little round symbols scattered about a week apart. Some are light, some are dark, and some are half and half. One of those "half-and-half" symbols is on the calendar today. It represents the first-quarter Moon. It means that the Moon stands at a right angle to the Sun, so it rises around noon and sets around midnight.

The name "first quarter" is a little bit misleading. At first and last quarter, sunlight illuminates half of the lunar hemisphere that faces Earth. It looks as though someone took a big knife and sliced the Moon right down the middle. Because of that, a lot of people call it a "half Moon." And in terms of the Moon's appearance, they're absolutely right.

But the terms "first quarter" and "last quarter" refer not to the Moon's appearance, but to its position in its orbit around Earth.

The beginning point is new Moon, when the Moon passes between Earth and Sun. About a week later, the Moon is one quarter of the way through its cycle, so it's called the first-quarter Moon -- the Moon that's on display tonight. A week later comes full Moon, and last-quarter Moon completes the cycle about a week after that. The entire cycle actually takes 29 and a half days.

Tonight's Moon is part of a beautiful lineup that includes the two brightest planets in the night sky. Jupiter is well to the right of the Moon at nightfall, with brighter Venus about the same distance to the right of Jupiter.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.