The precision of a solar eclipse

I was wondering, is there a physical law that explains why the moon exactly covers the sun in total solar eclipses? Besides that both the moon and the sun are relatively spherical, I was wondering if the moon’s relative distance from Earth and the sun is not random. If the moon or the Earth were more massive or less massive, or if the moon was smaller and denser, would total eclipses not take place? For short, how did total eclipses come to be so precise?

Sheer luck apparently; I asked this question a while back; I’ll be back in a mo with the link.

Relative size/spacing of earth/moon/sun

The previous link was incomplete. This one should work.

Thanks; not sure what happened there.

i don’t think a solar eclipse is ever really ‘total’- that is, the shadow of the moon covers the entire brightened half of the earth. most eclipses as of late of which i’ve heard have only been visible in asia or south america. maybe australia.

but my point is is that the shadow of the moon eclipses a part of the earth, and during that full eclipse other parts of the earth have partial eclipses, but the shadow never covers the entire earth.


No, although the moon exactly covers the entire disc of the sun (from our viewpoint); that’s what isaac is wondering about.

No the size of the moon is purely fortuitous. Sometimes it is apparently just larger than the sun and sometimes just smaller (both because of the eccentiricity of the moon’s orbit around the earth and of the earth’s around the sun). But when the earth was young, the moon was only a tenth as far away. Then solar eclipses would likely have convered the earth and lasted hours, but would not have been nearly as spectacular; more like an unusual night. On the other hand, tides vary as the cube of the distance, so instead of an 80 foot tide at the Bay of Fundy, there could have been a 16 mile high tide! Ok, there probably wasn’t but instead of an ordinary 5 foot tide, you would have had a mile high tide! That would have been quite a sight.

exactly. and no matter how small or big the moon may theoretically be, the moon will entirely cover the disc of the sun to someone standing in the moon’s shadow.


Is there still a confusion about the OP? The apparent (i.e. angular) size of the moon and sun, as seen from the surface of the earth, is almost exactly the same. Since the orbits aren’t circular, sometimes the sun is slightly larger, sometimes smaller. When the moon is slightly larger, it can cast a tiny shadow on the earth - anywhere else on the earth the sun will only be partially obscured. If the moon were smaller, nobody will see a total eclipse - it will always be an annular eclipse, where the moon only covers the middle portion of the sun. If the moon were much larger, a larger shadow will be cast on the earth. More people will see a solar eclipse, but it won’t look as pretty because the moon will cover not just the disk of the sun, but also the surrounding corona.

I think it’s quite a coincidence. But apparently nothing more than that.

The coincidence is that the point of the Moon’s shadow is just about exactly at the surface of the Earth. Were the Moon smaller, but in the same position, one would need to go into space to see a total eclipse.

Total eclipse, by the way, just means that there’s a point on the Earth where the Sun is completely covered, not that it’s covered for the whole Earth. The latter couldn’t happen unless the Moon were larger than the Earth.

[nitpick]Or if the sun was smaller than the moon and relatively close[/nitpick]

How long before someone says that it’s proof that God exists.

It’s a fantastic natural phenomenon, we had a 94% eclipse here about a year ago and the whole city stopped to watch. We had a great view from the roof of our office.

I’d love to see a total eclipse.

It’s proof that G… oops, sorry.

(actually, I was about to say how long before some cynic says “How long before someone says that it’s proof that God exists”, really, I was)

[nitpick[sup]2[/sup]]Or if the Earth, Moon, and Sun were all exactly the same size.[/nitpick[sup]2[/sup]] :slight_smile:

ok ok, physics/astronomy guy to the rescue…

The apparant area of the disk of the sun and the moon are not exactly the same, they vary quite a bunch due to the elliptical orbit of the moon and, to a much lesser extent, the elliptical orbit of the earth around the sun.

It is much more common for an annular eclipse to occur, where the moon’s disk isn’t large enough to cover the entire surface of the sun, even though the moon’s position is centered on the sun’s. What you get is a “ring-around-the-moon” situation and eclipse totality never happens. Note that an annular eclipse is different than a partial eclipse, a partial eclipse is when the viewer is in the secondary shadow cast by the moon (i.e. the moon’s disk isn’t centered on the sun’s).

IIRC, annular eclipses happen with about 4 times the frequency of total eclipses.

Here’s a page with some very good examples of partial, annular, and total eclipses so you can compare:

That should put all of those wacky divine creationist serendipity theories to a proper burial…

doh! i completely misunderstood…

gulap- snarf… swallow

eating crow. tasty stuff.


[nitpick[sup]2[/sup]]Or if the Earth, Moon, and Sun were all exactly the same size.[/nitpick[sup]2[/sup]] :slight_smile: **
Hmmm, they would need to be able to pass through each other in that case and I can’t be sure that an eclipse would result from that scenario.

I’m not sure what you mean here…