At this particular moment in Earth’s history – although the sun’s diameter is about 400 times larger than that of the moon – the sun is also about 400 times farther away. So the sun and moon appear nearly the same size as seen from Earth. And that’s why we on Earth can sometimes witness that most amazing of spectacles, a total eclipse of the sun – or even an annular solar eclipse like the one coming up on May 20, 2012. What are the odds?
No one knows the odds, because no one knows how many planets and moons there are in space. At this writing (early 2012), astronomers have discovered only more than 700 planets in distant solar systems so far, and we don’t know much about their moons.
Sun and moon appear same size, approximately, as seen from Earth. Image via Gabrielle Wang
EarthSky asked Myles Standish about the similarity in the size of Earth’s moon and sun, as seen from Earth. He is a mathematical astronomer and a former professor at Yale University. He worked for CalTech’s Jet Propulsion Laboratory. He has published over 300 articles, mostly in the field of solar system dynamics and celestial mechanics. He said this similarity is unique among the planets and moons that make up our local family in space, our solar system. In other words, it doesn’t occur with any other planets and moons in our immediate neighborhood of space.
But there may be millions or billions of undiscovered solar systems.
So what are the odds of the moon and sun appearing nearly the same size from Earth? No one knows.
By the way, although it’s fascinating that they are so similar, the sun and moon aren’t always the same size as seen from Earth. In fact, the moon and sun are rarely exactly the same size. The moon’s distance from Earth varies slightly over the course of a single month. So the moon’s apparent size in our sky is always changing.
For part of every month, the moon is in a far part of its orbit from Earth. At such times it isn’t big enough to cover the sun completely. If an eclipse happens then, the outer part of the sun’s surface will appear as a ring around the moon. This type of event is called an annular or ring eclipse. It’s essentially a partial eclipse. The sky doesn’t darken. You can’t look at the eclipse without special filters. Still, it’s very beautiful and fascinating to witness any eclipse – and stand in line with the sun and moon!
Annular eclipse of the sun for southwestern U.S. on May 20
Solar eclipse for North America on May 20 and Asia on May 21
What’s more, the moon is getting minutely farther from the Earth – by about 4 centimeters – every year. That means that, in past aeons, there were no annular eclipses. All eclipses were total eclipses because the moon in Earth’s sky always appeared bigger than the sun.
Now some eclipses of the sun by the moon are total, and some are annular, as seen from Earth.
But as the aeons pass, and the moon gets ever farther from us, its apparent size in our sky will continue to grow smaller. Eventually, people on Earth won’t see total eclipses. All the solar eclipses will be annular, with the moon not quite big enough to cover the sun completely.
Bottom line: The sun’s diameter is about 400 times larger than that of the moon – and the sun is also about 400 times farther from Earth. So the sun and moon appear nearly the same size as seen from Earth. And that’s why we sometimes see total or annualar eclipses of the sun – like the one coming up on May 20, 2012. What are the odds?
Very interesting and informative
The opening, “At this particular moment”, refers to a fact which is not mentioned in the rest of the story. The tidal force of the Moon on the Earth is slowly transferring rotational momentum from the Earth’s rotation to the Moon’s orbital motion. As a result, the Moon’s orbit is slowly getting larger, and in a hundred million years or so, the Moon will never look as large as the Sun, and total solar eclipses will be a thing of the past.
Courtney, yes, thank you for that additional info! Very true. The moon will eventually become too small to cover the sun completely.
It seems nothing in our universe ever stays the same …
That the two dominant objects in the sky of a planet populated by people who can observe this overlap visually is quite an amazing coincidence. The sun and moon are vastly different in size and composition and yet they line up in this very striking fashion (and occupy a similar visual ‘space’, one by day, the other by night). The chances of this being a random fluke seem remote in the extreme. Yet, what other explanation?
What it even more incredible is the fact that this “coincidence” is purely for the benefit of the human eye! It’s even more incredible that there are only 2 heavenly discs seen from earth and they both appear the same size. Life would not exist without either of them. It’s as if by design.
There are many other sun moon “coincidences”.
1. When the sun is at it’s brightest in summer the moon is at it’s weakest and when the sun is at it’s lowest and weakest the moon is at it’s brightest.
2. The sun and moon rise and set at precisely the same opposite points on the horizon during the year. At equnonxes and solstices.
3. The moons face rotates at a precise rate to give only the same face towards earth (aka captive orbit.
Martyn, I agree with you about the amazing aspect of this coincidence. It’s a good one! But the other three “coincidences” you mentioned aren’t really coincidences at all.
1. High sun, low moon … you’re talking about a full moon only here. Otherwise, it’s not true. Yes, it’s true that the full moon rides lowest in the sky at the height of summer, when the sun rides highest. But that’s because – in order to show us its fully lighted hemisphere – the moon has to be opposite the sun. It’s really Earth’s tilt on its axis that makes for the high sun and low full moon in summer … no coincidence. Just how it is.
2. The sun and moon set on the same points on the horizon at the equinoxes … again, not a coincidence. That’s what an equinox is. It’s a time when the “ecliptic” – or path of the sun and moon – intersects the horizon at points due east and due west. No doubt that’s how due the idea of “east” and “west” originated.
3. Moon keeping one face toward Earth … lots of planets in our solar system have moons that do this. It’s just what happens when smaller bodies in space revolve around larger ones …
But the eclipse thing. Yes. That’s a good coincidence!
All the best,
Deborah
Yeah I’ve thought about this a bit before. How about looking at it the other way round? As cameron says, it’s earth centric – but we’re the only observers we know of and our moon just happens to (almost) perfectly cover the sun. Rather than just shrug and say it’s coincidence, maybe it’s that very fact that also led to us being here. I don’t know of any chemistry to suggest this is the case – but maybe an environment that cools or goes dark more suddenly could be the trigger for life. I’m not sure I believe it if I’m honest esp if this wasn’t the situation when life appeared anyway. I keep thinking – two people choose a random number between 1 and 10. And they both say the same number. To us it’s a 1/100 that they both said the same number. To each of them it’s just a 1/10 that the other one chose the same number as them. Maybe that’s the opposite argument though. Dunno.
Oh yes and an explanation? what about extra terrestrial design? It’s also an incredible coincidence that the moons surface has a highly light refelective dust that is ideal for reflecting the sun, without it, the moon wouldn’t be visible.
Actually, the Moon is very dark, reflecting only about five percent of the sunlight that falls on it (seven times less than the Earth, and fifteen times less than Venus). It is so dark that in images of the Earth and Moon taken by interplanetary spacecraft, the Moon’s brightness is usually increased by five or more times, to better fit our preconceptions of what it ought to look like. Admittedly, it looks very bright at night, but that is only in contrast to the dark night sky. During the day, it appears only slightly brighter than the sky around it.
Yes, it’s very dark, I got to know this during the Apollo days, and I was under 10.
It also came in my mind to cover the center of the visible side of aluminum foil, to supply natural light to our nights.
An ecological marvel or disaster?
Martyn, stop trying to slide your beliefs into this thread. It’s remarkable what we’re able to see, and how it got there is great fun speculation..people even make careers out of this speculation. So let’s enjoy!
so i think it looks the same size
The reality is, there are BILLIONS AND BILLIONS of stars and moons out there, so why would it have to be design? How do we know there aren’t billions of other stars that align in this way? The idea that this is more than a coincidence is a completely ignorant and Earth-centric idea. We’re just an atom compared to the rest of the universe.
I see this precise placement as evidence of specific design by a Creator God.
Psalm 8:3
When I consider your heavens, the work of your fingers, the moon and the stars, which you have set in place.
And the point of creating the moon and the sun in such a way that they appear the same size to the naked eye would be…?
Somewhere out there, an alien priest is telling his congregation how their three suns represent some sort of “Holy Trinity”…
I can imagine that on Luke Skywalker’s home planet, they are fond of the expression, “Shoot for the first, done by the second,” meaning that one should work on a task intending to finish by the first sunset, but since things tend to take longer than we anticipate, we ultimately complete the project just in time to see the second sunset, the “final” deadline that officially transforms day into night.
And I bet there’s all sorts of weird/interesting/spiritual/funny/profound/silly expressions and ideas revolving around the relatively brief window of time in which one sun has set, while the other has yet to do so.
You get the idea. I hope.
P.S. – The next time you open a brand new container of Country Crock spread, it’s a safe bet that you won’t see the face of Jesus or the silhouette of Mary, but instead, the uncanny, buttery image of… well, something that would likely make you blush! :-X :-)
In fact the sun is NOT 400 times bigger than the moon, you are TOTALLY LOST! how can u be writing articles? if the sun is 1,300,0000 times earth’s size how in the world r u saying 400 times bigger than the moon?
noone,
The diameter of the sun is about 400 times greater than the moon’s diameter. Although the VOLUME of the sun is about 1,300,000 times the volume of Earth, the sun’s DIAMETER is some 109 times the Earth’s diameter.
Bruce
The answer to Boo’s ?, is that physical reality is a stage on which the Dance of Life is being proformed and the Sun represents the Father and the Moon represents the Mother. Number 1 the Sun, number 2 the Moon. This Truth is verified by the fact Sunday is the first day and Monday is the second day. This is true only in the USA and Canada, the others are not yet up to speed and begin their week with Monday and end with Sunday. The 7 days are based on the Sun, Moon and the 5 ancient planets known for thousands of years. The modern planets are not included since the time frame for this Dance and Teaching is very long standing. The Sun and Moon are mates and their child is the Key to all Human Evolution. Be advised that the marrige of the Sun and Moon is so very special and blessed, that those that see the truth of it will enter into greater knowledge of the way things are and will be. Love & Peace, Home
Obviously it was God who put it there this way. It takes much more faith to believe it is just a coincidence.
Dear Pete, If you knew what you juat said, you would be justly ashamed. Besides abusing the word FAITH, (Which simply put, means Spiritual Courage) You Have Obviously Misunderstood Truth preceeds Fact and Therefore, SINCE GOD DID ARRAINGE IT TO BE SO FOR OUR BENEFIT, WE CAN NOT IN GOOD FAITH DENY THAT THERE IS NO COINCIDENCE HERE AT ALL BUT ONE OF THE SPIRITUAL WONDERS OF OUR TIME, SINCE AT LATER TIME, THE PAIR WILL NO LONGER BE SIZED APPROPRIATELY, SINCE THAT LESSON WILL NO LONGER BE NECESSARY. Pete, your statement shows just how much Humanity needs to learn the True Story Of Just How Reality Is Made Manifest! Metaphysics proceeds Physics.
In earlier times, Astrology was Truely the Logic of the Astro, or the understanding of the innerness, which then manifests as the physicality or material nature. Facts and Truth must at long last be as ONE. That lame comment you made is the reason there is a legal seperation of Church and State. Some day when fools are wiser the Church and State will be One. Truth to us all, Home
Home Free,
You are ranting. Grow up.
Man,
you can’t even go on a thread out of scientific interest without running into bible thumpers these days can you?
[...] Miksi kuu ja aurinko näyttävät maasta katsottuna melkein samankokoisilta? [...]
How about the fact that the Earth’s diameter, along with the Moon’s distance from the Earth and the Moon’s diameter (and their distance from the Sun) cause the Earth’s shadow to exactly cover the Moon during a total lunar eclipse?
Can this be counted as another highly improbable coincidence?
Hello
With all my respect, I believe there is no thing called “coincidences” in
Our universe. Other wise it will be a miss…….
Regards
Ha_we
We have only had a 500,000 year period where this worked out – that’s almost less than the amount of time that sentient hominids have been on this planet. And we only have another 250k years left of this “coincidence”.. I certainly wonder about it.
Me too, Rick. I wonder about this, and lots of things.
Deborah
It really doesn’t further useful scientific exploration to permit “Creationist” proselytizing SPAM and Trolling to flood message boards with impertinent, endless circular arguments. Ther are plenty of religious, conspiracy theory, and paranormal hoax sites where people can type their tirades ad nauseum. Goldilocks belongs in fairy tales.