[MeganM (OP)]

John sent us this question:

We always see the same side of the moon. This infers that the moon does not spin on its axis. Why is this? Are there moons in the solar system that do spin?

Give us your best answer!

[evan_au]

We always see the same side of the moon. This infers that the moon does not spin on its axis.

Actually, the Moon rotates around the Earth.
The fact that we only see one side of the Moon means that the Moon does rotate on it's axis, once every time it rotates around the Earth.
This is equates to one rotation per 27.32166 days.
See: https://en.wikipedia.org/wiki/Moon

This is an example of "Tidal Locking", which affects all small Moons close to much larger bodies.
- It even affects Pluto and Charon, which are close enough in size that they are tidally locked to each other.
See: https://en.wikipedia.org/wiki/Tidal_locking#Moons

Exoplanets in the "Golidlocks Zone" around red dwarf stars are expected to be tidally locked to their parent star.

[Bored chemist]

John sent us this question:

We always see the same side of the moon. This infers that the moon does not spin on its axis. Why is this? Are there moons in the solar system that do spin?

Give us your best answer!

Infer does not mean imply.

[Petrochemicals]

The reason you see a cresent moon is due to the curvature of the moon and the illuminated part. It is not because the moon is partially blocked by earth. Henceforth you do actually see the dark side of the moon.

[Hayseed]

One can confuse the TWO dark sides of the moon.

There is a permanent dark side to us here on earth due to lock.

And then there is a present dark side due to sunlight.

[Bored chemist]

The "dark" side of the moon is an unfortunate term.
You can't see the far side of the moon. (because the moon is in the way).

[evan_au]

It is not because the moon is partially blocked by earth

Somewhere between 2 & 5 times a year, the Earth does come between the Sun and the Moon (at least partially).

This darkens the Moon; we call it a Lunar eclipse.

A Lunar Eclipse can be distinguished from the crescent Moon that is seen at other times, because:
- A Lunar eclipse always occurs on a Full Moon.
- A Lunar Eclipse is very reddish, and has a soft edge due to light diffracted through Earth's atmosphere
- The crescent Moon has a sharp edge, as the sunlight normally travels through a vacuum, casting sharp shadows on the Moon
See: https://en.wikipedia.org/wiki/Lunar_eclipse

[Hayseed]

Did you know that there are people who study and detect shadows within shadows.

It is used to track objects that can only be seen thru multiple reflections of multiple objects, i.e....multiple turns of line of sight.  Passively.

It's done with DSP software.   Observing a section of wall or object can detect all movement in the room.  And the shape and size and velocity of the movement.

It might get to the point where any surface can become a mirror.

It's starting to get scary.

[Petrochemicals]

It is not because the moon is partially blocked by earth

Somewhere between 2 & 5 times a year, the Earth does come between the Sun and the Moon (at least partially).

This darkens the Moon; we call it a Lunar eclipse.

A Lunar Eclipse can be distinguished from the crescent Moon that is seen at other times, because:
- A Lunar eclipse always occurs on a Full Moon.
- A Lunar Eclipse is very reddish, and has a soft edge due to light diffracted through Earth's atmosphere
- The crescent Moon has a sharp edge, as the sunlight normally travels through a vacuum, casting sharp shadows on the Moon
See: https://en.wikipedia.org/wiki/Lunar_eclipse

It happens fairly rapidly though.

[Bored chemist]

Did you know that there are people who study and detect shadows within shadows.

It is used to track objects that can only be seen thru multiple reflections of multiple objects, i.e....multiple turns of line of sight.  Passively.

It's done with DSP software.   Observing a section of wall or object can detect all movement in the room.  And the shape and size and velocity of the movement.

It might get to the point where any surface can become a mirror.

It's starting to get scary.

"Did you know that there are people who study and detect shadows within shadows."
Yes that's why we have the words  umbra and penumbra.
Since those words have been kicking around for ages it's clear that it happened before anything was "done with DSP software"

It might get to the point where any surface can become a mirror.

Yes, but small mirrors are useless.
https://en.wikipedia.org/wiki/Diffraction-limited_system

This reality is commonly ignored in TV shows where the hero zooms in on the reflection in someone's glasses or whatever.