# Relative size/spacing of earth/moon/sun **URL:** \ **Category:** Factual Questions\ **Created:** [August 7, 2001, 4:50pm UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314 "2001-08-07T16:50:43Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 7, 2001, 4:50pm UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/1 "2001-08-07T16:50:43Z")
I’ve never been fortunate enough to be in the right place at the right time to witness a full eclipe (although we had a fairly good partial eclipse here in the south of England a couple of years back. My question: Is there some inherent reason (i.e. relative masses/gravity, orbital velocity etc) that results in the moon being just the right size and the right distance from the earth to _just_ obscure the sun at eclipse, or is it just a striking coincidence. (i.e. if it was bigger or closer, it would obscure the corona too, if it was smaller/further away, we wouldn’t have a total eclipse - if it was smaller, would it need to be closer to stay in orbit?) (also, somebody told me that during lunar eclipse (the moon passes through the earth’s shadow), the shadow of the earth is exactly the right size to _just_ cover the entire moon - is this true?) ---
**Author:** ![pldennison](https://avatars.discourse-cdn.com/v4/letter/p/9fc29f/32.png) [@pldennison](https://boards.straightdope.com/u/pldennison)\ **Post date:** [August 7, 2001, 5:12pm UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/2 "2001-08-07T17:12:27Z")
This part, I believe, is correct: > [@](#): > > Is there some inherent reason (i.e. relative masses/gravity, orbital velocity etc) that results in the moon being just the right size and the right distance from the earth to just obscure the sun at eclipse, or is it just a striking coincidence. The moon currently is moving away from the Earth by some small fraction each year. At some point in the future it will be too far away to cause a total eclipse of the sun. Similarly, at some point in the past, it may indeed have been close enough to block out the corona. There is some good information at [http://sunearth.gsfc.nasa.gov/](http://sunearth.gsfc.nasa.gov/) . I would be surprised if this was true: > [@](#): > > (also, somebody told me that during lunar eclipse (the moon passes through the earth’s shadow), the shadow of the earth is exactly the right size to just cover the entire moon - is this true?) After all, there are both partial and total lunar eclipses. ---
**Author:** ![bughunter](https://avatars.discourse-cdn.com/v4/letter/b/aeb1de/32.png) [@bughunter](https://boards.straightdope.com/u/bughunter)\ **Post date:** [August 7, 2001, 5:24pm UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/3 "2001-08-07T17:24:06Z")
First of all, astronomers agree that it’s just coincidence that the apparent sizes of the sun and moon are so similar. (Of course, science says it’s also just coincidence that we’re here to witness it at all. So if you’re faithfully inclined, no one can argue with you that it’s “God’s will.” Personally, I am comfortable with the coincidence answer.) As for the size of the Earth’s umbra (full shadow) with respect to the moon… it’s just slightly larger than the moon. If you watch a total lunar eclipse the moon spends several minutes in totality, whereas in a total solar eclipse you will seldom see the sun in totality for more than a minute. ---
**Author:** ![Podkayne](https://avatars.discourse-cdn.com/v4/letter/p/edb3f5/32.png) [@Podkayne](https://boards.straightdope.com/u/Podkayne)\ **Post date:** [August 7, 2001, 7:52pm UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/4 "2001-08-07T19:52:58Z")
The Earth’s shadow is considerably larger than the Moon. It consists of the umbra (total shadow) surrounded by the penumbra (only partial shadow) due to the fact that the Sun isn’t a point source. The umbra is roughly 2.7 times larger than the Moon. (I can show you the math, if’n ya want, cuz I jes’ done it.) It’s got nothing to do with masses or orbit. Just a conincidence. If you replaced the Moon with a can of Folger’s Crystals, the Folger’s would just keep happily trucking around in the Moon’s orbit . . . though scientific calculations show that, sparkly as Folger’s crystals are, the amount of romance on Earth would decrease by 13.8%. ---
**Author:** ![Montfort](https://avatars.discourse-cdn.com/v4/letter/m/d6d6ee/32.png) [@Montfort](https://boards.straightdope.com/u/Montfort)\ **Post date:** [August 7, 2001, 9:31pm UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/5 "2001-08-07T21:31:08Z")
> [@](#): > > \*Originally posted by Podkayne \* > \*\*If you replaced the Moon with a can of Folger’s Crystals, the Folger’s would just keep happily trucking around in the Moon’s orbit . . . though scientific calculations show that, sparkly as Folger’s crystals are, the amount of romance on Earth would decrease by 13.8%. \*\* I can just see a super-race of aliens playing a practical joke on Earthlings in a TV commercial… “They don’t know it, but we secretly replaced their satellite moon with a can of Folger’s Crystals…” Hehehe. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [August 7, 2001, 10:34pm UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/6 "2001-08-07T22:34:49Z")
Since the moon varies in its distance from the Earth, not all solar eclipses are the same. In some, the moon doesn’t completely cover the sun, but leaves a ring of light around it. These are known as annular eclipses. Others have variable periods of totality. I think the longest possible are some 7 minutes long, but more commonly totality lasts a couple minutes. For the European eclipse a couple years ago, it lasted between about 2 and 2.5 minutes, depending on where you were. ---
**Author:** ![easy\_e](https://avatars.discourse-cdn.com/v4/letter/e/eb8c5e/32.png) [@easy\_e](https://boards.straightdope.com/u/easy_e)\ **Post date:** [August 8, 2001, 6:26am UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/7 "2001-08-08T06:26:46Z")
Don’t forget that the earth is gradually slowing down and the moon is moving further away. So eventually, there will be a time when total solar eclipses will not be possible (just like there was a time when solar eclipses lasted much longer). ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 8, 2001, 7:32am UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/8 "2001-08-08T07:32:16Z")
> [@](#): > > \*Originally posted by dtilque \* > \*\*Since the moon varies in its distance from the Earth, not all solar eclipses are the same. In some, the moon doesn’t completely cover the sun, but leaves a ring of light around it. These are known as annular eclipses. \*\* I suppose the point on the earth from which you’re observing it makes a difference too; if you’re standing at the north pole. you’re a distance of (approximately) the radius of the earth further away from the moon than you would be at the equator. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [August 8, 2001, 7:33am UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/9 "2001-08-08T07:33:24Z")
Great answers people, thanks very much. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [August 8, 2001, 7:58am UTC](https://boards.straightdope.com/t/relative-size-spacing-of-earth-moon-sun/75314/10 "2001-08-08T07:58:39Z")
…and eventually, as the moon spirals outward, the term _solar eclipse_ fades from usage to be replaced by _lunar transit of the sun_. Say, how far out will the moon spiral? The asymptotic limit of orbital radius of the moon will occur when the Earth and Moon no longer rotate with respect to one another. I think angular momentum will be conserved, so how far out is that?