# The Naked Scientists Forum

### Author Topic: Basic questions about the Moon and Light  (Read 4991 times)

#### k2director

• First timers
• Posts: 1
##### Basic questions about the Moon and Light
« on: 08/01/2004 02:49:35 »
I'm doing some fiction writing about the moon, and was curious about a couple of things. Was wondering if anyone could shed light on these questions!

1) Is the moon iluminated only by light reflected from the Earth? I tried to imagine that, but can't imagine if the moon rotates the earth every month, how it doesn't get direct sunlight at some point. Ie, come between the Earth and Sun and therefore get direct sunlight on the lunar side opposite the Earth.

2) Is one side of the moon always in darkness and is that side unchanging? In other words, if you were to plant a flag on the dark side of the moon, would that flag always be in darkness during the moon's rotation around the earth, and its own axis rotation?

3) Is it possible to be on one side of the moon that can't see the Earth, but still gets some kind of light either reflected from Earth or the sun? I'm working on a story that involves a character not being able to see our planet from his vantage point, but does that mean the character will be in pure darkness?

Any answers very much appreciated! THank you....

#### Quantumcat

• The Kitty Down Under
• Hero Member
• Posts: 894
##### Re: Basic questions about the Moon and Light
« Reply #1 on: 08/01/2004 08:50:47 »
1) Is the moon iluminated only by light reflected from the Earth? I tried to imagine that, but can't imagine if the moon rotates the earth every month, how it doesn't get direct sunlight at some point. Ie, come between the Earth and Sun and therefore get direct sunlight on the lunar side opposite the Earth.

That's why there's a time of the month when you can't see any moon!! The moon is illuminated with light from the sun. Depending on the moon's angle you get cresents and full moons and such.

2) Is one side of the moon always in darkness and is that side unchanging? In other words, if you were to plant a flag on the dark side of the moon, would that flag always be in darkness during the moon's rotation around the earth, and its own axis rotation?

hmm. I think the circle of light moves around the moon and when it's like on the side you see just a sliver of it. so, the flag would be in light sometimes.

3) Is it possible to be on one side of the moon that can't see the Earth, but still gets some kind of light either reflected from Earth or the sun? I'm working on a story that involves a character not being able to see our planet from his vantage point, but does that mean the character will be in pure darkness?

As I said the circle of light moves around the moon, just like day and night!! except when the moon is on the opposite side of the earth to the sun. hey maybe a circle doesn't move around the moon, but at least light is on different sides depending on where it is relative to the sun. (I think, someone tell me if I'm wrong please!!!)

Am I dead? Am I alive? I'm both!
« Last Edit: 08/01/2004 08:54:18 by Quantumcat »

#### tweener

• Moderator
• Neilep Level Member
• Posts: 1144
##### Re: Basic questions about the Moon and Light
« Reply #2 on: 09/01/2004 04:23:53 »
Good job Quantumcat!

But I can't leave well enough alone, so here is my stab.

1.  The main illumination of the moon is from direct sunlight.  That is what you see when you see the moon.  When the moon is full, the side facing earth is also facing the sun directly and we see the whole side of the moon in full illumination. When we see less than a full moon (let's use half as an example) the sunlight is coming from the side and while we are looking at the whole round moon, we see illumination on the side facing the sun.  As the phase of the moon changes to a cresent, we are looking at the moon with the sun almost behind the moon.  At this point, on a really dark night, you can actually see the whole round moon, with only a sliveer fully lit.  The dark part that you can still make out is illuminated by light being reflected from the earth.  If you were standing on the moon looking at the earth, you would see an almost "full" earth - the part that was not illuminated by the sun would be the same size as the part of the moon (seen from earth) that was illuminated.

2.  The surface of the moon is constantly rotating with respect to the sun and thus all parts of the moon receive sunlight and nighttime in equal amounts over the course of the 28 day rotation.  In other words, if you were at any point on the moon for 28 days, half of your time would be in sunlight and the other half would be in darkness.  The "day" on the moon is 14 [earth] days long!  Some people call the side of the moon that we can't see from earth the "dark side" but that is a serious misnomer.  There is not a dark side and a light side.  There is a side we can see from earth and a side we cannot see from earth.

3.  If you (or your character) were on the side of the moon facing away from the earth, he/she would never be able to see the earth, but would receive sunlight for 14 days and then be in darkness for 14 days.  It would be a great place for an observatory!  No light pollution, no atmosphere to muck up the observations, and you could even observe stars during the "day" (because there is no atmosphere).

As an experiment, take a ball and a flashlight into a room and turn the light off.  Set the flashlight up to point horizontally across the room at about your shoulder height (maybe before you turn the light off).  That is your "sun".  Now hold the ball (your moon) out at arms length in the beam of light and slowly rotate your body, keeping the light on the ball.  Notice how much of the ball is illuminated by the "light" and how much is in shadow.  Also notice your orientation to the ball and the light.  Think of the phases of the moon while you orbit your "moon" around your self, and you should come to understand the way the moon/earth/sun system works.

Some things to notice are:  1) The full moon always rises at sunset.  This is logical since the sun in the west and the moon is in the east.  It's like when you are almost between the ball and the flashlight - you see the full side of the ball illuminated.  Here, the side of the moon that is facing away from the earth is in darkness and the side facing the earth is fully illuminated.  Someone on the moon looking up at earth would be seeing the "night" side.

2) The new moon always sets at sunset.  Now, the moon is between you and the sun, so the side of the moon that is illuminated is facing away from you.  This is like having the ball between you and the flashlight - you can't see any of the ball that is illuminated (directly) by the flashlight.  In this case, the side of the moon facing away from the earth is now fully illuminated and the side facing earth is in darkness (except that they would be seeing a brilliant full earth, which would be quite bright!).

3) At half phase, both the earth and moon observers can see the other at half phase.  This is like when you are holding the ball in front of you and the flashlight is coming from one side.  You see the ball half illuminated because all the light is coming from the side.

This is a lot more than I intended to bore you with, but I hope it helps.

Good luck with your story!  I'd love to read it sometime.

Welcome to the forum!

----
John
« Last Edit: 09/01/2004 04:32:31 by tweener »

#### Quantumcat

• The Kitty Down Under
• Hero Member
• Posts: 894
##### Re: Basic questions about the Moon and Light
« Reply #3 on: 09/01/2004 07:11:40 »
That's heaps better than what I wrote!!! I guess the 14 day night and day seems obvious now. Didn't think of it before.

Am I dead? Am I alive? I'm both!

#### Gneiss

• Jr. Member
• Posts: 14
##### Re: Basic questions about the Moon and Light
« Reply #4 on: 10/02/2004 23:20:24 »
quote:
Originally posted by k2director

1) Is the moon iluminated only by light reflected from the Earth?

Just to complicate matters slightly…

The moon is also illuminated by the Earths light, albeit to a far lesser degree than from the sun.

If you get the chance, observe the moon when it is a thin crescent, you will notice that not only is the thin crescent (the bit illuminated by the sun) visible but the remainder of the moon can also just be made out, much fainter. This is a result of the Earth 'shining' on the moons surface and illuminating it slightly in just the same way that the moon shines on the Earth at night. This effect is known colloquially as 'Earthshine.'
« Last Edit: 10/02/2004 23:22:21 by Gneiss »

#### daveshorts

• Moderator
• Neilep Level Member
• Posts: 2583
• Physics, Experiments
##### Re: Basic questions about the Moon and Light
« Reply #5 on: 26/04/2005 14:43:37 »
I made a flash program to explain some of this stuff at:
http://www.chaosscience.org.uk/pub/public_html//article.php?story=2004062914222354
you may find it useful

#### gsmollin

• Hero Member
• Posts: 749
##### Re: Basic questions about the Moon and Light
« Reply #6 on: 26/04/2005 15:25:29 »
Of course this question would get a landslide of good responses. I can add one point: Eclipses. The fictional writing could have some good stuff with an eclipse. At the full moon, the earth is between the sun and the moon. The inclination of the moon's orbit with respect to the sun is enough to prevent an eclipse from occuring at every full moon, but occasionally the moon, earth, and sun line up and a lunar eclipse happens. The moon moves through the earth's shadow, and becomes a beautiful red color.

Your character is apparently on the far side of the moon that never faces earth. He would be in darkness at the full moon. It could be an interesting plot twist if he made an effort to see the earth at the full moon, and observed a lunar eclipse from the moon, which would of course be a solar eclipse as seen from the moon.

Later:

Since the moon turns copper-red during a lunar eclipse, it is receiving some light from the earth, even in deep and total shadow. I think this is caused by atmospheric refraction, but I can't say what the earth would look like from the moon during an eclipse. That would be important in any kind of story involving a lunar-earth-solar eclipse. Maybe somebody else can help.
« Last Edit: 27/04/2005 16:48:49 by gsmollin »

#### daveshorts

• Moderator
• Neilep Level Member
• Posts: 2583
• Physics, Experiments
##### Re: Basic questions about the Moon and Light
« Reply #7 on: 27/04/2005 20:48:13 »
The redness is caused by light that has been refracted around a corner (the sun is behind the earth remember) by the atmosphere and become red due to scattering by dust in the atmosphere. So you should see a black earth surrounded by a glowing red atmosphere - probably very eerie.
I had a go at drawing some pictures at:
http://www.chaosscience.org.uk/pub/public_html//article.php?story=20040619124507228
Just cartoons and not done properly but they may be interesting

#### The Naked Scientists Forum

##### Re: Basic questions about the Moon and Light
« Reply #7 on: 27/04/2005 20:48:13 »