Show Steps

If the Moon were directly overhead at 9 am, it would be in the waning crescent phase. If it instead is setting on the western horizon, then it must have been overhead six hours earlier, around 3 am. What phases precede the waning crescent, and which one lies overhead at 3 am?

We can revisit a single day drawn in the figure from slide #14 to check.

The Earth takes 24 hours to rotate once around in a full circle (covering a full night and day), so it takes six hours for an object in the sky to move from the eastern horizon to overhead, or from overhead down to the western horizon (the Earth rotates through 360° in 24 hours, or through 90° — a right angle — in six hours).

The Moon's eight phases cover its positions as it makes a full circle around the Earth. With eight phases for 360°, each phase covers 45° (there are two phases in each right angle). It takes the Moon a month to orbit once around the Earth, so it spends 29.5 ÷ 8 = 3.7 days in each phase.

Hide Steps

Show Solution

If the Moon lies on the western horizon at 9 am (having been overhead six hours earlier, at 3 am), then it must be in the waning gibbous phase.

Hide Solution