From time to time I check out the sun and moon data at the U.S. Naval Observatory.?

Question

I have noticed that sometimes the moon is 100% lit for more than one day. Why is this ?

Answer 1

Hi JC!

This happens every month and it's perfectly normal. Usually, the moon appears full for more than 48 hours around the official moment of the full moon.

Here's why:

Phases of the moon are governed by the position of the terminator, the line of sunrise and sunset as it moves across the lunar surface. When it's around the time of first quarter or last quarter, you can easily see how the terminator has moved from night to night, noticeably fattening or thinning the moon.

At the time of full moon, the terminator lies along the edge of the moon instead of being face-on. Along the edge like that, it is harder for us to notice motion from night to night. The terminator is moving just as fast as at the quarter phases, but we don't notice the changes for a few days.

The same thing happens at new moon, which is why the moon seems to disappear for about 48 hours when it is passing near the sun.

Look at a globe at home and you'll see what I mean. Places at the edge of the globe are hard to see and seem to meld together, while the ones you're looking directly at are clear. The full moon is like that.

Answer 2

Although it takes only 27.322 days on average for the moon to complete one orbit around the earth (the sidereal month), as a result of the earth's orbital motion around the sun it requires about two additional days for the earth, moon, and sun to acquire the same relative geometry. So on average the number of days between two similar phases (e.g. between one full moon and the next full moon) is about 29.531 days. This period is referred to as a lunation, lunar month, or synodic month. The actual number of days in a lunation can vary from about 29.272 to 29.833 because the velocities of the moon and of the earth are not constant in their elliptic orbits, and because of gravitational interactions with other bodies in the solar system. [1][2]