# Why are the sun and moon percieved as being almost exactly the same size? **URL:** \ **Category:** Factual Questions\ **Created:** [September 27, 2005, 11:21pm UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755 "2005-09-27T23:21:57Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![Dr.Rieux](https://avatars.discourse-cdn.com/v4/letter/d/aeb1de/32.png) [@Dr.Rieux](https://boards.straightdope.com/u/Dr.Rieux)\ **Post date:** [September 29, 2005, 7:34am UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/21 "2005-09-29T07:34:08Z")
Intelligent Design? 😃 ducking and running ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [September 29, 2005, 1:26pm UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/22 "2005-09-29T13:26:29Z")
> [@jocularjason](#): > > This person seems to think a similar experience may exist in our solar system with the moon Callisto off Jupiter --as in perfectly blocking the sun during eclipse. If I plug the numbers into **Bytegeist** ’s formula, I calculate that the Sun is 1.8 millirads wide as seen from Jupiter and Callisto is 2.8 millirads wide. So, during an eclipse, Callisto slops over the Sun with about half again as much diameter. Not _too_ bad, but not nearly as snug as the Moon-Sun fit. And certainly, there can never be a Callistan annular eclipse. If only I didn’t have to work, I’d plug all of the Solar System satellites into a spreadsheet and see if any of them do better. Some of the inner moons of Saturn look promising. Anybody got the time? > [@](#): > > I guess what I’m asking is-- how probable is it that a moon would Almost Perfectly!! blot out a sun. That’s hard to say. It depends on the range of distances that _could_ separate the Moon from the Earth. The inner limit would be established by the Roche Limit of the Earth, inside of which the Moon would be torn apart by tidal forces. The outer limit would depend on celestial mechanics–a distant enough Moon would eventually wander off due to perturbation from other bodies. I have no idea what those limits are. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [September 29, 2005, 3:30pm UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/23 "2005-09-29T15:30:59Z")
> [@jocularjason](#): > > I understand the concept of percieved size based on relative distance. I guess what I’m asking is-- how probable is it that a moon would Almost Perfectly!! blot out a sun. What’s the probability? 1 in 1. ---
**Author:** ![jocularjason](https://avatars.discourse-cdn.com/v4/letter/j/b2d939/32.png) [@jocularjason](https://boards.straightdope.com/u/jocularjason)\ **Post date:** [October 2, 2005, 3:59am UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/24 "2005-10-02T03:59:51Z")
> [@Freddy the Pig](#): > > If I plug the numbers into **Bytegeist** ’s formula, I calculate that the Sun is 1.8 millirads wide as seen from Jupiter and Callisto is 2.8 millirads wide. So, during an eclipse, Callisto slops over the Sun with about half again as much diameter. Not _too_ bad, but not nearly as snug as the Moon-Sun fit. And certainly, there can never be a Callistan annular eclipse. > > If only I didn’t have to work, I’d plug all of the Solar System satellites into a spreadsheet and see if any of them do better. Some of the inner moons of Saturn look promising. Anybody got the time? > > That’s hard to say. It depends on the range of distances that _could_ separate the Moon from the Earth. The inner limit would be established by the Roche Limit of the Earth, inside of which the Moon would be torn apart by tidal forces. The outer limit would depend on celestial mechanics–a distant enough Moon would eventually wander off due to perturbation from other bodies. I have no idea what those limits are. What’s you’re opinion about this “cozy” sun-moon relationship having an influence of the propogation of life on earth. I know it’s a wacky theory, but hear me out. What do you think eclipses looked like during the formation of the earliest multi-celled organisms. Was the moon appearing bigger or smaller at that time? Could the orientation of sun an moon and it’s highly improbable similar ratio of visible light and gravity (and who knows what else) given rise to stable conditions that may have fostered life. I guess the question I’m asking is–where was the moon when the first blips of life appear on the fossil record? ---
**Author:** ![Johanna](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johanna/32/8318_2.png) [@Johanna](https://boards.straightdope.com/u/Johanna)\ **Post date:** [October 2, 2005, 4:14am UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/25 "2005-10-02T04:14:32Z")
> [@Marley23](#): > > The Sun is 870,000 miles in diameter and roughly 93 million miles from the Earth. The Moon is 2,160 miles in diameter and about 238,857 miles away on average. That’s the basic reason they look the same size. Yes, that’s the answer. The numbers you cited tell the whole tale. The math is really simple: The sun’s real diameter is about 403 times the diameter of the moon. The sun’s average distance from the earth is about 389 times the average distance of the moon from the earth. Since the one is about 400 times bigger but also 400 times farther away than the other, the match is nearly perfect. Annular eclipses (where it would be total except that a bright ring of sun appears all the way around the edge of the moon) are explained by the times when the sun is closer and the moon is farther than average. ---
**Author:** ![lieu](https://avatars.discourse-cdn.com/v4/letter/l/c2a13f/32.png) [@lieu](https://boards.straightdope.com/u/lieu)\ **Post date:** [October 2, 2005, 4:55am UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/26 "2005-10-02T04:55:02Z")
Math aside, it’s really as simple as **Anne Neville** suggested. After Orpheus collided with the initial Earth and the impacted material coalesced, it was about 14 times it’s present size. I’m not positive about the timing but have read proposals that dinosaurs hunted under much brighter conditions at night than we now witness due to the much greater reflective surface area present on the moon at that time. It’s since compacted and additionally is moving away from us at the rate of about and inch and a half a year. Eventually it’s gonna be so small as to make your question invalid. Timing my friend, coincidence based on timing. The sun and moon won’t be changing appreciable sizes but in time your observed perspective will no longer be similar and, thus, valid. ---
**Author:** ![snailboy](https://avatars.discourse-cdn.com/v4/letter/s/97f17d/32.png) [@snailboy](https://boards.straightdope.com/u/snailboy)\ **Post date:** [October 2, 2005, 5:36am UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/27 "2005-10-02T05:36:21Z")
Here’s a list of apparent angles in milliradians of the sun and moons from certain planets. I got the values from [NinePlanets](http://www.nineplanets.org/nineplanets.html). **Earth:** Sun - 9.29 Moon - 9.05 _(Curiously, that implies that most eclipses should be annular, not total.)_ **Mars:** Sun - 6.10 Phobos - 2.44 Deimos - 0.522 **Jupiter:** Sun - 1.79 Io - 8.60 Europa - 4.68 Ganymede - 4.92 Callisto - 2.55 **Saturn:** Sun - 0.972 Epimethius - 0.755 Janus - 1.18 _(pretty close)_ Mimas - 2.11 Enceladus - 2.18 Tethys - 3.59 Dione - 2.97 Rhea - 2.90 Titan - 4.21 Iapetus - 0.410 **Uranus:** Sun - 0.484 Cordelia - 0.520 _(very close)_ Ophelia - 0.593 _(pretty close)_ Bianca - 0.746 Cressida - 1.06 Desdemona - 0.921 Juliet - 1.31 Portia - 1.67 Rosalind - 0.771 Belinda - 0.907 Puck - 1.79 Miranda - 3.63 Ariel - 6.06 Umbriel - 4.40 Titania - 3.62 Oberon - 2.61 **Neptune:** Sun - 0.309 Naiad - 1.21 Thalassa - 1.60 Despina - 2.79 Galatea - 2.55 Larissa - 2.59 Proteus - 3.54 Triton - 7.61 Nereid - 0.0617 **Pluto:** Sun - 0.235 Charon - 59.6 So while our moon is the closest in apparent size to the sun, Cordelia is quite close itself, with a couple of others comparable as well. Thus I conclude that it is pretty rare for the moon to be that close in size, but it isn’t extremely rare. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [October 2, 2005, 5:58am UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/28 "2005-10-02T05:58:21Z")
> [@Bytegeist](#): > > So depending on where the Moon is in its orbit, and where the Earth is in its orbit, the Moon can appear either slightly smaller or slightly larger than the Sun. Hence we sometimes have annular eclipses, and sometimes total. And on rare occasions, a hybrid eclipse where part is annular and part total. There was one such last year in the South Pacific where the first and last parts were annular while the middle was total. At two points during that eclipse, the apparent sizes were exactly the same. For those not comfortable with milli-radians, Sky & Telescope has these figures in its latest issue: Moon 29.94 to 33.66 minutes of arc (there are 60 minutes of arc in one degree) Sun 31.46 to 32.53 minutes of arc ---
**Author:** ![jocularjason](https://avatars.discourse-cdn.com/v4/letter/j/b2d939/32.png) [@jocularjason](https://boards.straightdope.com/u/jocularjason)\ **Post date:** [October 2, 2005, 6:12am UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/29 "2005-10-02T06:12:38Z")
Those are exactly the numbers I was looking for. thanks for finding that. I wish our planet was more special than that, but alas. Still I wish the Hubble telescope could tell us if other solar systems had similar data. If we’re able to go there ourselves someday–I hope they address this topic. > [@](#): > > So while our moon is the closest in apparent size to the sun, Cordelia is quite close itself, with a couple of others comparable as well. Thus I conclude that it is pretty rare for the moon to be that close in size, but it isn’t extremely rare. ---
**Author:** ![RM\_Mentock](https://avatars.discourse-cdn.com/v4/letter/r/e274bd/32.png) [@RM\_Mentock](https://boards.straightdope.com/u/RM_Mentock)\ **Post date:** [October 2, 2005, 2:31pm UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/30 "2005-10-02T14:31:36Z")
> [@Anne Neville](#): > > Yes, it is rare. In fact, it hasn’t been true during much of Earth’s history, either, and it will eventually cease to be true as the moon moves further from the Earth. Eventually, h\*ck, less than 24 hours from now. 🙂 Madrid, Spain around 9 o’clock Greenwich tomorrow, the sun will be visible completely surrounding the moon ---
**Author:** ![Freddy\_the\_Pig](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@Freddy\_the\_Pig](https://boards.straightdope.com/u/Freddy_the_Pig)\ **Post date:** [October 3, 2005, 6:48pm UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/31 "2005-10-03T18:48:26Z")
> [@snailboy](#): > > (Curiously, that implies that most eclipses should be annular, not total.) As indeed they are. About 60% of all total-or-annular eclipses are annular. If that seems surprising, it’s only because annular eclipses don’t get much publicity because they’re unexciting. > [@jocularjarson](#): > > I guess what I’m asking is-- how probable is it that a moon would Almost Perfectly!! blot out a sun. I thought of a simpler way to approach this question. Let’s take it as a given that the Moon is at its current distance, and ask what is the likelihood that it would be sized correctly to fit so snugly over the Sun. Let’s further define a “snug” fit as one close enough so that eclipses can be sometimes total and sometimes annular. The lower bound on the Moon’s size is obviously something a little greater than zero. For an upper bound, let’s use the size of the Earth itself–if it were bigger, after all, the Moon would be the primary and the Earth the satellite. The Earth is four times the diameter of the Moon, so our range of possible moons would run from 0 to about 120 minutes of arc. As we have seen, the Sun ranges in apparent size from 31.46 to 32.53 minutes of arc. Because of its elliptical orbit, the Moon will vary from 0.9447 to 1.0585 times its average apparent size. So the minimum possible _average_ size to create a total eclipse, under the most favorable conditions, is 31.46 / 1.0585 = 29.72 minutes of arc. The maximum possible average size to create an annular eclipse under the _least_ favorable conditions is 32.53 / 0.9447 = 34.43 minutes of arc. In other words, out of a range of 120 minutes worth of possible values, a span of 4.71 minutes, or 4% of the total, produces a fit snug enough to generate a mix of total and annular eclipses. It makes the coincidence seem a _little_ less amazing. ---
**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:** [October 3, 2005, 8:41pm UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/32 "2005-10-03T20:41:01Z")
For eclipses on other planets, remember that you don’t necessarily need to be standing directly under the moon and Sun. They could both be on the horizon, in which case you’d have about a planetary radius more distance from the moon (but no significant change in the distance to the Sun, of course). This would tend to bring Callisto and Cordelia closer to a perfect fit (though I don’t think it would be enough for Callisto, and I’m not sure about Cordelia). Meanwhile, if the Moon-Sun coincidence has any influence on life on Earth, it’d be due to the tidal effects. The tidal force due to a celestial body is proportional to the body’s density times the cube of its angular size. Currently, the Moon’s angular size is about the same as that of the Sun, and the Moon’s density is about twice as great, so the Moon has about twice the tidal effect on the Earth as does the Sun. I suppose it’s conceivable that there’s some reason it might be advantageous to have comparable tidal effects from two different celestial bodies. ---
**Author:** ![clairobscur](https://avatars.discourse-cdn.com/v4/letter/c/839c29/32.png) [@clairobscur](https://boards.straightdope.com/u/clairobscur)\ **Post date:** [October 3, 2005, 9:53pm UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/33 "2005-10-03T21:53:21Z")
> [@Chronos](#): > > Meanwhile, if the Moon-Sun coincidence has any influence on life on Earth, it’d be due to the tidal effects. . Hasn’t there been some study concluding that on the long term , the Moon has a stabilizing influence on the Earth orbit, hence preventing it from being too erratic over the course of millions of years and making life possible? ---
**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:** [October 3, 2005, 10:27pm UTC](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755/34 "2005-10-03T22:27:27Z")
> [@clairobscur](#): > > Hasn’t there been some study concluding that on the long term , the Moon has a stabilizing influence on the Earth orbit, hence preventing it from being too erratic over the course of millions of years and making life possible? I can’t find a cite, but IIRC the theory is that the Moon keeps the obliquity of the ecliptic from shifting chaotically (keeps the Earth tilted at about 23.5 degrees to the plane of its orbit). [Previous page](https://boards.straightdope.com/t/why-are-the-sun-and-moon-percieved-as-being-almost-exactly-the-same-size/323755.md?page=1)