There are different types of Solar Eclipses. Annular, Total and Partial.
Total Solar Eclipses are those wherein the Moon completely covers the sun. For those who have not heard the word “Annular”, it means ring-shaped. Thus Annular Solar Eclipses are those where the Sun is not fully covered, and hence you can see the sun like a ring around the Moon. Partial solar eclipses are those where the the Moon neither completely covers the sun, nor is completely “inside” the sun. Here are pictures to help you visualise.
Any planet which has a moon will have solar eclipses. But not every planet has a Total Solar Eclipse. Did you know that we are lucky to have total solar eclipses at all?
The question is not how a body as big as the sun, can be blocked by an object that is 400 times smaller, our moon? That is easy. It can be done in the same way that we can use a small coin to block the sun from our eyes by appropriately positioning the coin. If you keep it too far away from the eye, you can see that the apparent size of the coin is not big enough to cover the sun, and so you will keep bringing it closer till the Sun is fully covered.
But the catch here is that the position of the coin can be changed as you like. But what about the moon? You cannot move the moon around to your liking. At least, not yet. And this is where we are lucky, because there is an interesting coincidence here. Though the sun is 400 times bigger than the moon, the sun is also almost 400 times farther from the earth than the moon. That means, the moon is positioned almost perfectly for it to block the sun completely from our view. It is this bit of coincidence that causes total solar eclipse on Earth.
But then if the Moon and sun’s sizes are in the same ratio as their distances from the earth, why are there both Annular and Total eclipses. Should we not expect only total solar eclipses? The answer lies in the fact that the orbit of the Moon around the earth is elliptical. Hence the Moon is not always at the same distance from the earth. Sometimes it is closer and sometimes it is farther. Thus if the eclipse occurs when the Moon is farther from the earth, the apparent size of the Moon as seen from the earth reduces, and hence it wont be able to cover the sun completely. Then you get an Annular eclipse. But if the eclipse occurs when the Moon is close to the earth, it is sufficiently big enough to block out the sun and hence we get a total solar eclipse. As my wife asked, the earth’s orbit is also elliptic, which means the sun should also be at varying distance from the earth during different times of the year. But this variation does not affect the totality and annularity as much as the elliptic nature of the Moon’s orbit.
To understand when eclipses are caused, see the picture below. Of course, the sizes of neither the orbits, nor the objects are in scale. As explained before the ratio of the diameters of the Sun to the Moon is 400:1, which means if I have to draw it in proportion, you would not see the Moon :-).
But if you think about the picture a bit, a question that could come up is this. The Moon revolves around the earth every 28 days. So every 28 days there should be at least 1 day, when the sun, the Moon and the earth are aligned. Then why are we not experiencing an eclipse every month?
To understand that, we need to picturise the previous situation from a different angle. The previous picture showed the top view of orbit of the earth. See the picture below. This gives the side-on view. Here you can see that the Moon’s orbit is not on the same plane as the earth’s orbit. It is tilted relative to the earth’s orbit. Hence what you saw above will not cause an eclipse since when you see the situation side on, you realise that the three objects are actually not aligned. This scenario occurs every month but does not cause an eclipse.
So when do we actually experience an eclipse. For that, look at the picture below.
This is the time when the the sun, the earth and the Moon are actually aligned. And this is when we see solar eclipses. Similarly another 6 months from this position, the Earth will be at the opposite side, and then again we can experience eclipses. There are more details to it, but for now, this understanding will do.