Starship Asterisk*

Can anyone explain how it is that if the sun and the moon are both visable in the sky, the moon does not have to be full? If they are both visable, what causes the shadow on the moon? Another planet?
Thanks

I did my best:

It has to do with the relative position of the Earth, Sun, and Moon. Here's a nice option for visualization if you think about a 3,4 right triangle (base is 3 units in length, height is 4 units in length. For this analogy to work you will have to think about looking down on all objects from above. Place the Earth, our reference point, at the right angle of the triangle. Place the Moon at the end of the 3 unit long side and place the Sun at the end of the 4 unit long side. Now what this means physically is that one side of the earth is illuminated and one side of the moon in illuminated, but the side of the moon that is illuminated (taking the reference point for the moon to be at the exact center of the side that is illuminated) is almost 90 degrees away from the center of the side of the moon you would view from Earth. This means that the moon will appear almost half full. Now let's pretend that we are on the earth at a location where it is 6 in the afternoon (assume that it's summertime and that the sun doesn't set until say 9 pm, and the center of the portion the Earth that is illuminated is at 12 noon). From there assuming that you have an unobscured view of both the Sun and the Moon you would see that the sun is in the western portion of the sky and that a half full moon is in the eastern portion of the sky.

The more interesting situation would be to see a full moon during the day since that means that if the Sun and the Earth formed a line then the Moon would be falling around the relative location of the line if you extended past Earth. I'd like to see how some one can explain that because I don't feel like thinking that hard at the moment.

Hope that helped.

Here is another good way to visualize it. At night, enter a dark room or go outside with a friend (outside at night is better). Have your friend shine a flashlight on you. The flashlight is the Sun and you are the Earth (or more specifically, your vantage point from Earth). Hold a tennis ball out at arms length in your shadow created by the beam of light. This is the eclipsed moon. Now slowly rotate yourself and as the tennis ball emerges from your shadow, it is a full round sphere. As you slowly rotate, the represented lunar globe will slowly wane to a half sphere, then to no sphere at all. There is no shadow being cast on the tennis ball except the shadow of the ball itself as your vantage point changes.
The same thing happens with the moon.

drep wrote:If they are both visable, what causes the shadow on the moon? Another planet?
Thanks

Hi Drep,

The shadow is caused by the moon itself. The moon is visible because the sun is shining light on it's surface. The sun can only light half of the moon at any given moment and that is the half that is directly facing the sun at that moment. The only time we see a full moon is when the sunlit half is directly facing Earth. A "new moon" is when the sunlit half is facing directly away from the earth so all we see is the unlit side. If the sunlit half is facing 90 degrees from our vantage point on earth then we see half of the moon lit and the other half dark. The phases of the moon are just a result of us seeing the sunlit half of the moon from different angles.

When a body (such as the Earth) casts a shadow onto another body (such as the Moon) it is known as an eclipse. This happens, for example, when the Earth is directly between the Moon and the Sun and would not occur when both are visible in the sky at the same time. It is also much more rare than the routine progression of the phases of the moon.


Don

Great question