Look up during the day, and you’ll see the Sun; look up at night, and you’ll (sometimes) see a full Moon.
Rather confusingly, both appear to be roughly the same size, despite most of us knowing from a very young age that the Sun is absolutely enormous, and the Moon is much smaller.
So what’s going on?
Quite often with space and astronomy questions such as this, there's a nice simple explanation, to which there's then a caveat “But it’s not actually as simple as that”.
In this case, the answer really is very simple.
The Moon’s quite small but looks bigger because it’s much closer to us, while the Sun is enormous but looks smaller because it’s a long way away.
Anyone who’s seen the episode of ‘Father Ted’ with the cows that are a long way away will know exactly what we’re talking about here!
But there’s still a bit more to it than that, because there’s also an interesting coincidence going on.
The Sun, which is roughly 400 times the size of the Moon, also just happens to be roughly 400 times further away.
That's why the Sun and the Moon, as seen from Earth, appear to be roughly the same size.
Sun
- Distance from Earth: 149 million km (93 million mi)
- Diameter: 1.4 million km (865,000 mi)
Moon
- Distance from Earth: 384,400km (283,855 miles)
- Diameter: 3,476 km (2,159 miles)
The Sun, the Moon and eclipses
That neat alignment of the Sun and the Moon's relative distances and sizes really is sheer fluke.
But it’s one that pays off nicely for us on Earth, because it’s the reason we get to see total solar eclipses, when the Moon passes between Earth and the Sun and blocks out its light.
Of course, not all solar eclipses are total – in fact, some are annular, which is to say the Moon almost covers the Sun, but doesn’t quite.
The effect of this is an annular solar eclipse, also known as a 'ring of fire' eclipse because a ring of sunlight manages to peek around the edges of the Moon.
That’s because the Moon’s orbit around Earth is far more eccentric than Earth’s orbit around the Sun.
While the Earth-Sun distance at any given moment only fluctuates by around 0.2% over the course of a year, the Earth-Moon distance can fluctuate by as much as 3.5%.
So, sometimes the Moon is a bit further away, it appears slightly smaller than the Sun and causes an annular solar eclipse when passing in front of it.
Other times, when the Moon is nearer, you’ll see a total solar eclipse.
Earth's eclipses are unique
Earth is the only planet in the Solar System where you would be able to see a total eclipse.
Venus and Mercury don’t have any moons, and while you can see solar eclipses on Mars, the Red Planet's two moons Phobos and Deimos are too small to cause a total solar eclipse.
The outer planets do have moons of sufficient size to cause a total eclipse, but they don’t have a surface that you could ever stand on to watch one.
Make the most of total eclipses while they last.
The Moon is moving around 38mm (1.5 inches) further from Earth with every passing year, so there will come a point when the Moon is too far away, and total solar eclipses will be a thing of the past.
You’ve got about 600 million years until that happens, though, so we wouldn’t worry too much!
