So I’ve been on some lovely Astronomy sites lately, and every post is plagued by “Flearthers” or “Flat-Earthers.” They come in to say every picture of space is fake, the Earth is flat, and they drive the signal-to-noise ratio into unusability.
One result is that when people ask honest questions, like “Isn’t the Moon always full in space?” they get a lot of push back because people think they’re Flearthers. That has to get sorted out before their question gets answered.
I created the following graphic that demonstrates the relationship of a (semi-) spherical object, light, and Point Of View with regard to apparent phases. It’s light on my kitchen counter falling on an egg.
The day-night terminator passes over the moon’s polar region at a right angle to the direction of light from the sun.
Apparent phase of the moon depends on our POV in relation to the moon. Since the Earth rotates at 27.~ times the moon’s orbital period, and the moon is in tidal lock with the Earth, we will see every “phase” of the moon, but only one side of it.
I didn’t rotate the egg as I took each side-photo, so it is not in tidal lock with my phone. But otherwise the principle is the same.
- Related: “Flat-Out Wrong” the Earth and rotation speed