# 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:** 10\ **Page:** 2
**Author:** ![F.U.Shakespeare](https://avatars.discourse-cdn.com/v4/letter/f/aca169/32.png) [@F.U.Shakespeare](https://boards.straightdope.com/u/F.U.Shakespeare)\ **Post date:** [February 14, 2009, 5:47am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/21 "2009-02-14T05:47:53Z")
> [@Q.E.D](#): > > …the faster orbital velocity results in a longer orbital radius… I don’t understand this. Wouldn’t a **faster** orbital velocity (which corresponds to a shorter orbital period) result in a \*\*shorter \*\*orbital radius? Since the square of the period is proportional to the cube of the radius (Kepler’s 3rd Law)? ---
**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, 5:48am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/22 "2009-02-14T05:48:14Z")
Well, it occurs because of a cosmic coincidence. The cosmos is so big that coincidences almost never happen. When a coincidence of this magnitude happens on our own doorstep even the most hardened cynic must raise an eyebow. If the moon was just a tiny bit bigger or a little bit closer, we wouldn’t get the corona or the annular “gold ring” effect. Annulars are even better than nomal eclipses. If the moon was just a tiny bit smaller or a tiny bit further away the sun’s rays would overpower it and we’d see nothing, just a slight dimming of the sun. It’s pretty weird, I submit. ---
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [February 14, 2009, 6:10am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/23 "2009-02-14T06:10:46Z")
> [@F. U. Shakespeare](#): > > Wouldn’t a faster orbital velocity (which corresponds to a shorter orbital period) result in a shorter orbital radius? Since the square of the period is proportional to the cube of the radius (Kepler’s 3rd Law)? Yes. The moon’s rate of revolution about the earth is indeed slowing down as it gets farther away, but its orbital **angular momentum** is increasing because of the energy transferred to it by the earth’s tidal bulge. Think of it this way: the moon is hurrying to catch up with the bulge, but it’s hurrying along a path that’s leading it slightly outside its current orbit, so it’s doing bigger laps around us at the center. So from our perspective, the period of its revolution is increasing as it gets farther away, that is, it’s taking the moon longer to complete a lap around us. It [sez here:](http://en.wikipedia.org/wiki/Tidal_acceleration#Angular_momentum_and_energy) > [@](#): > > The gravitational torque between the Moon and the tidal bulge of the Earth causes the Moon to be promoted in its orbit, and the Earth to be decelerated in its rotation. As in any physical process, total energy and angular momentum are conserved. Effectively, energy and angular momentum are transferred from the rotation of the Earth to the orbital motion of the Moon (however, most of the energy lost by the Earth is converted to heat, and only about one 30th is transferred to the Moon). The Moon moves farther away from the Earth, so its potential energy (in the Earth’s gravity well) increases. It stays in orbit, and from Kepler’s 3rd law it follows that its velocity actually decreases, so the tidal action on the Moon actually causes a deceleration of its motion across the celestial sphere. Although its kinetic energy decreases, its potential energy increases by a larger amount. The tidal force has a component in the direction of the Moon’s motion, and therefore increases its energy, but the non-tidal part of the Earth’s gravity pulls (on average) slightly backwards on the Moon (which on average has a slight outward velocity), so the net result is that the Moon slows down. The Moon’s orbital angular momentum increases. ---
**Author:** ![TimeWinder](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@TimeWinder](https://boards.straightdope.com/u/TimeWinder)\ **Post date:** [February 14, 2009, 6:22am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/24 "2009-02-14T06:22:42Z")
> [@mutantmoose](#): > > The cosmos is so big that coincidences almost never happen. Uh, say what? Sure you don’t have that backwards, now? ---
**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, 6:40am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/25 "2009-02-14T06:40:22Z")
I take your point - that in an infinite universe even the most unlikely things will happen somewhere. But it’s still a strange coincidence tho, that it should happen right here, right now when there’s a civilisation below able to view it and also appreciate how unusual it is. Suppose you’re right though, infinite universe - we just got lucky. ---
**Author:** ![F.U.Shakespeare](https://avatars.discourse-cdn.com/v4/letter/f/aca169/32.png) [@F.U.Shakespeare](https://boards.straightdope.com/u/F.U.Shakespeare)\ **Post date:** [February 14, 2009, 6:51am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/26 "2009-02-14T06:51:05Z")
> [@Kimstu](#): > > Yes. The moon’s rate of revolution about the earth is indeed slowing down as it gets farther away, but its orbital **angular momentum** is increasing because of the energy transferred to it by the earth’s tidal bulge. > > Think of it this way: the moon is hurrying to catch up with the bulge, but it’s hurrying along a path that’s leading it slightly outside its current orbit, so it’s doing bigger laps around us at the center. So from our perspective, the period of its revolution is increasing as it gets farther away, that is, it’s taking the moon longer to complete a lap around us. > > It [sez here:](http://en.wikipedia.org/wiki/Tidal_acceleration#Angular_momentum_and_energy) Got it – thanks. ---
**Author:** ![TimeWinder](https://avatars.discourse-cdn.com/v4/letter/t/bcef8e/32.png) [@TimeWinder](https://boards.straightdope.com/u/TimeWinder)\ **Post date:** [February 14, 2009, 9:30am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/27 "2009-02-14T09:30:23Z")
> [@mutantmoose](#): > > I take your point - that in an infinite universe even the most unlikely things will happen somewhere. But it’s still a strange coincidence tho, that it should happen right here, right now when there’s a civilisation below able to view it and also appreciate how unusual it is. > > Suppose you’re right though, infinite universe - we just got lucky. Actually, it’s stronger than that: isn’t it a strange coincidence that: - The world’s three tallest mountains are all the same height? - That the continents all have roughtly the same land area? - That the number of species of mammals is the same as the number of species of insects? - That the Earth has the same number of moons as it does oceans? - That the distance to the sun and the distance to the moon are the same? - That the stomach of a hummingbird is exactly the right size to hold a duck? And so on and so on. The number of possible coincidences is infinite – the fact that a few of them happen to actually be true isn’t strange, unusual, or even all that interesting: it’s a guarantee from the law of large numbers. ---
**Author:** ![glee](https://avatars.discourse-cdn.com/v4/letter/g/b5a626/32.png) [@glee](https://boards.straightdope.com/u/glee)\ **Post date:** [February 14, 2009, 9:53am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/28 "2009-02-14T09:53:39Z")
How about the coincidence that there is both life on Earth AND we are here to discuss it? Coincidence … or proof of a Designer? ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [February 14, 2009, 11:02am UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/29 "2009-02-14T11:02:53Z")
A large moon also has a stabilizing effect on the rotational axis of the body it orbits, and thus a stabilizing effect on its climate, which may have been beneficial to the development of complex life forms on Earth, so it’s probably not entirely random that we ended up developing on a planet with such a well-placed and -sized moon; that such a moon at some point has just the right distance to its planet to have the same angular size in the sky that its sun does is then only a matter of the right timing, provided the moon started out as being relatively bigger (i.e. closer). ---
**Author:** ![chappachula](https://avatars.discourse-cdn.com/v4/letter/c/d2c977/32.png) [@chappachula](https://boards.straightdope.com/u/chappachula)\ **Post date:** [February 14, 2009, 7:31pm UTC](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736/30 "2009-02-14T19:31:07Z")
> [@Q.E.D](#): > > …Moon go bye-bye. Man, I love this place 🙂 'Cause if I read that at,say, Yahoo.answers, I’d think, “geez, what a stupid site” [Previous page](https://boards.straightdope.com/t/why-is-the-moon-just-the-right-size-for-a-full-eclipse/485736.md?page=1)