Why doesn't a lunar eclipse occur every time the moon revolves around Earth in its monthly orbit?

Answer 1

Great question, and an important one. Maybe tough to describe in text, but I'll try it:

Picture the earth orbiting the sun. Picture the orbit like a gigantic dinner plate ...

the sun is at the center, and the earth is sliding around the edge of the plate.

Now picture the moon orbiting the earth. Picture the moon's orbit like a little

saucer ... the earth is at the center, and the moon is sliding around the edge of

the saucer.

The key is that the saucer is not in the same plane as the plate; it's tilted about

5-1/2 degrees to it. So: Most of the time when the moon is directly in front of

the earth or behind it, it's usually far enough above or below the line from sun

to earth (that line is on the plate) to avoid an eclipse. When the moon is in front,

its shadow usually misses falling on the earth; when the moon is behind, it usually

misses passing through the earth's shadow.

If I'm still coming through clear, here's a punch-line to wrap it all up: In order for

an eclipse to occur when the moon is directly in front or behind the earth, those

points in the moon's orbit have to be pretty near the points where the rim of the

saucer intersects the surface of the big plate ... called the 'nodes' of the moon's

orbit. Just another way of saying that the moon has to be practically on the surface

of the plate, or else the shadows don't line up.

Does that do anything for you ? Leave a message on my message board if this helps.

It's a real challenge to try and explain it without making pencil sketches.

Answer 2

If the moon orbited the earth in exactly the same plane as the earth orbits the sun, we would see a lunar eclipse at every full moon. The orbital plane of the moon around the earth is actually a little bit tilted from the orbital plane of the earth around the sun, so it only occasionally is in the right position for a lunar eclipse. By the same token, solar eclipses would be much more common if the two orbital planes were the same.

Answer 3

Merely because Moon's orbit around our planet has variable angle with the ecliptic plane.