Quick! Why does the moon go through phases every month?
https://griffithobservatory.org/wp-content/uploads/2021/03/moon_phases_uppergraphic-1600×800.jpg
If you answered that it’s because the Earth’s shadow falls on the moon, be sure the volume on your device is turned on before you click below.
Back you go to 8th grade science class!
The Earth’s shadow falling on the moon creates a lunar eclipse. Even though they are not terribly rare—they occur at an average rate of two a year—they do not take place every month. And they only take place when the moon is in its full phase.
Lunar phases occur because of the relative positions of Earth, moon, and sun. A diagram first:
https://edrawcloudpublicus.s3.amazonaws.com/edrawimage/work/2021-11-18/1637225286/main.png
And a video:
And when you see the moon looking like this:
https://images.immediate.co.uk/production/volatile/sites/25/2019/02/IMG_7204-35aee7f.jpg
That dimly illuminated part of the moon that you can nonetheless see is being illuminated by Earthshine. The sun is reflecting off the Earth and illuminating the moon, which reflects that light back to us. If you were standing on the moon, you would see the Earth go through phases. At the crescent phase of the moon shown above, the Earth would be a mirror image in its illumination, with most of it illuminated and a thin sliver dark.
Working out that geometry is your homework. Maybe slightly more advanced than 8th grade. We’ll call it freshman level astronomy.
Leave a Reply