Earthsky FAQs
Coincidence that sun and moon seem same size?
Photo Credit: NASA
Although the sun is about 400 times larger than the moon, it is also about 400 times farther away. So the sun and moon are nearly the same size as seen from Earth.
03-01-2010 - Space
At this particular moment in Earth’s history – although the sun is about 400 times larger than the moon – it is also about 400 times farther away. So the sun and moon are nearly the same size as seen from Earth. 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 2010), astronomers have discovered only more than 400 planets in distant solar systems so far, and we don’t know much about their moons. EarthSky asked Myles Standish of NASA’s Jet Propulsion Laboratory about the similarity in the size of Earth’s moon and sun, as seen from Earth. 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!
Our thanks to:
Myles Standish
Jet Propulsion Laboratory
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
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.
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!