Q1.
When you checked the Moon at the same time on successive days (for example, at sunrise), did you see it in a different part of the sky?
Answer
Yes, and the movement is completely predictable, because the phase of the Moon and its distance from the Sun in the sky are the same piece of information.
| Days after full Moon | Phase you see | Angle between Moon and Sun in the sky | Where it is at sunrise |
|---|---|---|---|
| 0 | Full circle | About 180° | Setting on the western horizon |
| About 7 | Half circle | About 90° | Overhead |
| About 11 | Crescent | About 45° | In the eastern sky, well above the Sun |
| About 15 | Not visible | Nearly 0° | Rising with the Sun — lost in daylight |
Why it happens: Sunlight lights one half of the Moon. How much of that lit half we can see depends only on the angle from which we look at it — and that angle is exactly the Moon–Sun separation in our sky. So as the Moon slides towards the Sun day by day, the bright part must shrink at the same time. One observation, two effects.
Tip: Turn this round and it becomes a useful skill. If tonight's Moon is a fat gibbous and waxing, it is roughly 135° from the Sun, so look in the east a few hours before sunset. A waning crescent is only about 30° from the Sun — look low in the east just before sunrise.