# Why is the moon \*just\* the right size for a full eclipse? **URL:** \ **Category:** Factual Questions\ **Created:** [February 13, 2009, 8:41pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736 "2009-02-13T20:41:27Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [February 13, 2009, 8:41pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/1 "2009-02-13T20:41:27Z")
I thought this had been asked here before but I am failing on searches and tire of the timeout trying new ones. In short, when the moon eclipses the sun in a full eclipse it is _just_ the right size to block out the disk of the sun. Is this just some bizarre cosmic chance? Do orbital mechanics always make it work that way? Something else? ---
**Author:** ![Scissorjack](https://avatars.discourse-cdn.com/v4/letter/s/c67d28/32.png) [@Scissorjack](https://boards.straightdope.com/u/Scissorjack)\ **Post date:** [February 13, 2009, 8:42pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/2 "2009-02-13T20:42:53Z")
Sheer coincidence. Amazing, isn’t it? ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [February 13, 2009, 8:45pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/3 "2009-02-13T20:45:15Z")
Actually it’s not always just the right size. When it’s farther away it’s too small, and we get an annular eclipse. If it’s closer we get a total eclipse. ---
**Author:** ![Algorithm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/algorithm/32/12204_2.png) [@Algorithm](https://boards.straightdope.com/u/Algorithm)\ **Post date:** [February 13, 2009, 8:45pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/4 "2009-02-13T20:45:32Z")
It’s not always, see [_Annular eclipse_](http://en.wikipedia.org/wiki/Solar_eclipse#Types). ---
**Author:** ![Anne\_Neville](https://avatars.discourse-cdn.com/v4/letter/a/b9e5f3/32.png) [@Anne\_Neville](https://boards.straightdope.com/u/Anne_Neville)\ **Post date:** [February 13, 2009, 8:46pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/5 "2009-02-13T20:46:13Z")
> [@Whack-a-Mole](#): > > Is this just some bizarre cosmic chance? Ding ding ding ding ding! > [@](#): > > Do orbital mechanics always make it work that way? No. The moon is moving away from the Earth. About 600 million years from now, it will have moved far enough away that there will no longer be total solar eclipses on Earth. ---
**Author:** ![tdn](https://avatars.discourse-cdn.com/v4/letter/t/94ad74/32.png) [@tdn](https://boards.straightdope.com/u/tdn)\ **Post date:** [February 13, 2009, 8:46pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/6 "2009-02-13T20:46:19Z")
> [@Whack-a-Mole](#): > > Is this just some bizarre cosmic chance? Yes. But it’s not static. It only appears that way at this particular point of time in our history. ---
**Author:** ![KlondikeGeoff](https://avatars.discourse-cdn.com/v4/letter/k/3e96dc/32.png) [@KlondikeGeoff](https://boards.straightdope.com/u/KlondikeGeoff)\ **Post date:** [February 13, 2009, 8:54pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/7 "2009-02-13T20:54:10Z")
> [@Anne\_Neville](#): > > No. The moon is moving away from the Earth. About 600 million years from now, it will have moved far enough away that there will no longer be total solar eclipses on Earth. Oh, damn, and I was looking forward to seeing an eclipse in 600002009. :mad: ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [February 13, 2009, 9:07pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/8 "2009-02-13T21:07:09Z")
> [@KlondikeGeoff](#): > > Oh, damn, and I was looking forward to seeing an eclipse in 600002009. :mad: Don’t fret. If there is anyone around the, there will be someone with a [Total Eclipse of the Heart](http://www.romantic-lyrics.com/lt14.shtml) … Shameless pre-Valentines Day plug. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [February 13, 2009, 9:25pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/9 "2009-02-13T21:25:09Z")
Thanks all…knew I could count on you! ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [February 13, 2009, 9:37pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/10 "2009-02-13T21:37:29Z")
It isn’t necessarily a meaningless coincidence. Perhaps the spectacle of eclipses that varied between total and annular was so thought provoking to those animals around to see it start to happen (as the moon drifted further away) that it helped spur evolution in the direction of species like us who would create World Wide Webs to discuss it on. Not likely, though, eh? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 13, 2009, 10:11pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/11 "2009-02-13T22:11:16Z")
> [@](#): > > It isn’t necessarily a meaningless coincidence. Perhaps the spectacle of eclipses that varied between total and annular was so thought provoking to those animals around to see it start to happen (as the moon drifted further away) that it helped spur evolution in the direction of species like us who would create World Wide Webs to discuss it on. > > Not likely, though, eh? It’s also relevant to the relative tidal forces of the Sun and Moon, and it’s not entirely implausible that the monthly tidal cycle had something to do with evolution of life on Earth. That’d only get it into the same general vicinity, though, not necessarily a perfect match. ---
**Author:** ![DSYoungEsq](https://avatars.discourse-cdn.com/v4/letter/d/c6cbf5/32.png) [@DSYoungEsq](https://boards.straightdope.com/u/DSYoungEsq)\ **Post date:** [February 14, 2009, 2:03am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/12 "2009-02-14T02:03:05Z")
Or it could be this way because God intended it to be this way. \ ---
**Author:** ![Contrapuntal](https://avatars.discourse-cdn.com/v4/letter/c/e274bd/32.png) [@Contrapuntal](https://boards.straightdope.com/u/Contrapuntal)\ **Post date:** [February 14, 2009, 2:11am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/13 "2009-02-14T02:11:38Z")
> [@DSYoungEsq](#): > > Or it could be this way because God intended it to be this way. \ For purposes of the question at hand, please define “God.” ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [February 14, 2009, 2:54am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/14 "2009-02-14T02:54:38Z")
\>Or it could be this way because God intended it to be this way. Je n’ai pas besoin de cette hypothèse’. ---
**Author:** ![mutantmoose](https://avatars.discourse-cdn.com/v4/letter/m/a87d85/32.png) [@mutantmoose](https://boards.straightdope.com/u/mutantmoose)\ **Post date:** [February 14, 2009, 3:05am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/15 "2009-02-14T03:05:11Z")
tbh though, I’m not religious but this about the strongest argument for a god there is. All the other arguments are meh. This one is actually proper impressive - you can look up and actually see it. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 14, 2009, 3:16am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/16 "2009-02-14T03:16:21Z")
> [@DSYoungEsq](#): > > Or it could be this way because God intended it to be this way. \ Since this is GQ and not GD, no, it couldn’t. ---
**Author:** ![Rand\_Rover](https://avatars.discourse-cdn.com/v4/letter/r/db5fbb/32.png) [@Rand\_Rover](https://boards.straightdope.com/u/Rand_Rover)\ **Post date:** [February 14, 2009, 4:07am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/17 "2009-02-14T04:07:46Z")
Two questions about the moon moving farther away from the earth: 1. Why? 2. Is the rate of the moon moving away such that lunar eclipses would have looked different (i.e., more moon) to the naked eye of a caveman? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 14, 2009, 4:16am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/18 "2009-02-14T04:16:21Z")
> [@Rand\_Rover](#): > > Two questions about the moon moving farther away from the earth: > > 1. Why? The Moon’s gravity pulls up a tidal bulge on the Earth. Because this bulge is dragged around by the rotation of the Earth, it leads the Moon in its orbit somewhat. The small gravity offset caused by the bulge pulls on the Moon, speeding it up; the faster orbital velocity results in a longer orbital radius. Moon go bye-bye. > [@Rand\_Rover](#): > > 1. Is the rate of the moon moving away such that lunar eclipses would have looked different (i.e., more moon) to the naked eye of a caveman? Probably not. It’s only receding at a couple centimeters a year. ---
**Author:** ![Telperion](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telperion/32/509_2.png) [@Telperion](https://boards.straightdope.com/u/Telperion)\ **Post date:** [February 14, 2009, 5:12am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/19 "2009-02-14T05:12:37Z")
> [@Q.E.D](#): > > Probably not. It’s only receding at a couple centimeters a year. Trilobites, on the other hand, probably experienced total and lengthy eclipses quite frequently. And then forgot all about them five seconds afterwards. ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [February 14, 2009, 5:23am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/20 "2009-02-14T05:23:18Z")
> [@mutantmoose](#): > > tbh though, I’m not religious but this about the strongest argument for a god there is. All the other arguments are meh. This one is actually proper impressive - you can look up and actually see it. I’ve seen a few eclipses, and yes they are impressive. But they don’t cause me to redefine the word “argument.” [Next page](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736.md?page=2)