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I was wondering if you could tell me about the moon and about eclipses. Why is it harmful to your vision? Ive heard that eclipses of the sun hurt your eyes. My teacher said hes seen moon eclipses that looked red. Why would that happen?
Question Date: 1999-03-11
Answer 1:

I'm not sure exactly what you want to know about the moon, but I can certainly tell you about eclipses of the moon. Let's start with the phases of the moon.

As the earth is orbiting the sun, the moon is orbiting the earth. So, you could picture a flat plane in your head, and in it lies the sun at the center, with the earth traveling around it, and the moon traveling around the earth. Sunlight is always being reflected off of the half of the moon that is facing the sun at any given time. However, based on our position relative to the moon, we may only see a half, crescent or no moon. The reason is this: for example, with a half moon...there is one whole side of the moon that is lit up by the sun, but we are only seeing half of it...so we see it as a half moon. A new moon is when the other side of the moon is being lit up, relative to our position.

An eclipse (lunar), on the other hand, happens when the earth gets between the sun and the moon and blocks the light. The earth is actually casting a shadow on the moon. (I don't know why it might be red, sorry!) A lunar (moon) eclipse is totally safe to look at.

A solar eclipse happens when the moon gets between the earth and the sun, and the moon's shadow is actually blocking out most of the sun. It is dangerous to look at a solar eclipse for the same reason it is dangerous to look directly into the sun. Because the moon doesn't completely block out the sun, there is still an area of direct rays coming out around the rim of the circle, which can be damaging to your eyes. I hope this helps you!

Answer 2:

The moon formed about 4500 million years ago (write this out on a piece of paper and have your teacher make sure you have all the zeros!!) when a body about the size of Mars collided with the earth. This giant impact created a huge vapor plume of vaporized rock that got put into orbit around the earth and eventually condensed to form the moon.

An eclipse happens when one body, say the moon, passes into the shadow of another. The redness occurs because the small amount of light that does illuminate the surface of the moon is mostly red light because red light has a longer wavelength and scatters less than blue light.


I hope this helps.

Answer 3:

Your questions are very good and ones that confuse a lot of people. There are two kind of eclipse, lunar and solar, and each one works a little differently.

Lunar eclipses happen during the full moon when the earth is between the sun and the moon. Normally, the full moon is bright because the light of the sun is shining on the full surface of the moon that is facing the earth. Sometimes, the earth comes into and exact line between the sun and the moon and blocks the light of the sun. The shadow of the earth moves across the moon until we can't see the moon anymore and this is a lunar eclipse. The moon becomes red during these eclipses because some of the sunlight shines through the earth's atmosphere before reaching the moon and is refracted so that more red light reaches the moon. This is like what happens at sunset when the sun gets close to the horizon and looks red because its light travels through so much atmosphere and is refracted. In some eclipses, even the light that goes through the earth's atmosphere doesn't reach the moon and it becomes completely dark before turning red again and the going back to the usual color of the full moon at the end of the eclipse.

Solar eclipses happen when the moon, traveling in its orbit, comes between the earth and the sun so we can't see the sun anymore. Although the sun is much much bigger than the moon, the moon is much closer so they appear about the same size in the sky. The moon can sometimes completely block the sun but this is very rare because they are close to the same size. Often you see a little bit of the sun ringing the moon or on one side of it. When an eclipse like this happens, most of the light from the sun is blocked so the sky gets darker, and everything gets a little colder for a few minutes. Still, a little of the sun shows from around the moon and it is still much too bright for your eyes. This is a very dangerous time because everything seems darker and so you want to look at the eclipse but it's just as dangerous to look at the sun, even with sunglasses.

Answer 4:

Eclipses occur when the sun, moon, and the earth are in a line so that the earth passes through the moon's shadow (solar eclipse) or the moon passes through the earth's shadow (lunar eclipse). Why do you think the moon is always full for a lunar eclipse and new for a solar eclipse?

I found a pretty good website at:


There are some links to other sites too. You should never look at the sun because the lens of the eye focuses light onto the retina. The sun is so bright that it can cause damage before you know it. During a total lunar eclipse light from the sun gets refracted through the atmosphere and hits the moon which then gets reflected back to us. Can you think of a reason why it's mostly red light that we see?

Answer 5:

You asked several questions so I'll take them in turn.

Moon and Eclipses: As you know the moon revolves about the earth in a 28 day (1 lunar month) orbit. At the same time, both the earth and the moon orbit the sun in a much larger (93,000,000 miles) and slower (365.256 days = 1 year) orbit. The solar orbit is slightly inclined to the lunar orbit-- if you drew circles to represent the orbits, you would need two sheets of paper on which to draw the circles. Also, the moon at 238,000 miles is about 1/2 an angular degree wide and the sun at 93,000,000 miles is also about 1/2 a degree.

So, now at one time each month, the moon is between the earth and the sun -- but only rarely is it exactly between so it casts a shadow on the earth. (The shadow is a solar eclipse). 1/2 month later, the earth is between the sun and the moon -- and if this happens where the two planes (paper sheets above) the earth casts a shadow on the moon -- the make a lunar eclipse.

Although you can watch a lunar eclipse without protection, it is very dangerous to look at a solar eclipse. Typical sunlight is much brighter than you might think, and staring into the sun is just like burning paper with a magnifying glass -- in this case the magnifier is your eye lens and cornea, and the "paper" that is burned is your retina-- The part of your eye that is sensitive to light.

The danger in a eclipse, is that a significant part of the light of the sun (and all of the heat) is invisible to your eye. During the eclipse, the sun appears dimmer (and it is) but it is still dangerously bright. Worse, your eye gets used to the overload -- and stops sending warning signals to you (i.e. pain) so significant damage can occur at the point of focus, and to parts of the eye which are sensed by nerves that are hit. The simplest safe way to view an eclipse is to poke a small hole with a pin in a card and look at the image of the sun on another sheet of paper held a few feet behind. (The same effect can be seen during a partial eclipse if you walk through trees -- you see hundreds of partial eclipses on the ground).

Finally, the standard argument for why a lunar eclipse looks red is that red light has a longer wavelength than yellow, green, or blue light, so is less easily scattered by the dust in the earth's atmosphere. Some of the light falling on the moon during the eclipse went very close to the earth and went a long way through the air before going back into space and hitting the moon. Blue, green, and some yellow light is scattered into other directions, but the red light goes straight. At the moon, it is the only light left -- so the moon appears red as well.

What would happen if the lunar and solar orbits were exactly in the same plane? How often would we have an eclipse?

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