MadSci Network: Astronomy

Re: what creates a shadow on the moon, and explain how the sun appears to.....

Date: Fri Mar 23 20:34:53 2001
Posted By: John Metcalfe, Staff, Computing and Information Services, Texas A&M University at Galveston
Area of science: Astronomy
ID: 983998130.As

Hi, Claire,
You've asked a very good question, now let's see if I can answer it for you. First of all there are two ways that a shadow can fall on the moon. The first one is when there is a lunar eclipse. That's when the Earth passes between the sun and the moon and thus blocks the sun's light from reaching the moon. The second way requires a little bit of demonstration. Take a ball (to represent the moon) and a flashlight (to represent the light from the sun) into a darkened room. If you shine the light on the ball in a dark room, you'll notice that only half of the ball is lit up, much the same as the moon as you see it in the sky. If you move around the ball you'll notice that more or less of it appears to be lit, much like the moon as it changes phases through the month. [Although in the case of the moon's phases, it is the moon moving around the Earth that allows us to see more or less of its surface in shadow.] So you see the moon creates its own shadow on one side, just like the earth, except here on earth we call it nighttime.

[Moderator: I would say that there is a third way for a shadow to fall on the Moon's surface. The Moon is not perfectly spherical. There are mountains and craters. The mountains can also cast shadows, as can the walls of the craters. Much like the case on the Earth, shadows are longest near sunrise and sunset. An important item about shadows is that they do not require air to form. Some people are confused about the fact that the Moon does not have an atmosphere, but it does have shadows. Light does not need air to travel (it travels through outer space from the Sun to the Earth after all). Shadows occur anytime something is between the light source (Sun) and the observer (you), regardless of whether or not there is any air between the light source and observer.]

As for the second part, you may have to go to the playground to really understand. If you stand on a merry-go-round while it's turning slowly, it's kind of like if you lay on your back with the earth turning beneath you. If you then have someone stand still on the ground as you turn past them, they would represent the sun as it passes through the sky. You see, the sun doesn't actually move through the sky, the earth simply turns underneath it.

I hope these examples help you understand the world around you a little better, and don't confuse you even more. If you have anymore questions feel free to send them to us, we're always glad to help out.

John Metcalfe

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