Doorstep Astronomy: Where the Moon Hides
The third quarter moon, seen from Boston Massachusetts at 8:45 a.m. January 7.
Credit: Starry Night® Software

The moon will be at third quarter, sometimes called last quarter, Thursday morning, but most people won?t see it.

The moon rises approximately 50 minutes later each night through its monthly cycle around the Earth. At full moon, which was on new year?s eve, the moon rose just around sunset. By tonight, a week later, it will be rising in the southeast around midnight, after most people have gone to bed. By sunrise, it will have moved around to the south-southwest.

A couple of hours later, the sun will be well up in the sky in the southeast, and the moon will be in the southwest. The moon will be plainly visible, but most people never think to look for the moon in the daytime sky, even though it?s visible there at some point almost every day of the month.

Most people nowadays believe incorrectly that the moon is only visible at night. Many also believe that it is visible all night every night, also incorrect. Because of the Earth?s rotation, the moon, like the sun, planets, and stars, appears to rise and set every day. Because it is in orbit around the Earth, its rising and setting times change every day by approximately 50 minutes.

For the next week, as the moon moves from third quarter to new moon, it will continue to rise later in the night and spend more time in the daytime sky. On the date of new moon, Jan. 15, the moon will be very close to the sun in the sky, rising and setting together. In fact, they will be so close that the moon will pass directly in front of the sun, causing an annular solar eclipse visible over much of the Eastern Hemisphere.

A popular homework assignment is to ask students to keep track of the rising, setting, and position of the moon over a full lunar cycle of 29.5 days. Many students run into serious difficulties at this time of the month because they can no longer see the moon at night. One of the purposes of this exercise is to get them to look for the moon during daytime, not just at night.

When the moon is riding high in the southern sky around third quarter, it?s an excellent time for students to see how the phases of the moon arise. Go out on a sunny morning when the moon is in the sky and hold a ball up in the air. The sunlight falling on the ball will cause exactly the same phase to appear on the ball as on the moon in the sky, because both are illuminated by the sun from the same angle.

This article was provided to by Starry Night Education, the leader in space science curriculum solutions.