How it occurs?
So let us find out how this phenomenon occurs. To understand this fully you need to comprehend the manner in which the Earth and Moon move relative to the Sun. The picture in figure 3 depicts the manner of revolution of the Earth around the Sun, and the Moon around the earth, the arrows giving the direction of motion. Just keep in mind that though the orbits shown are circular actually they are elliptical in nature. Now after seeing this picture you can imagine that if this were true, a solar eclipse should occur say every 24 hours when the Moon comes in between the Sun and the Earth, and a lunar eclipse should occur with same frequency, but we know that this does not happen. So is this picture wrong?
Well not really, but the only problem is that this picture shows (or we assume on seeing it) that this motion is coplanar or in the same plane but it is NOT so. The plane of the motion of the moon is different from the plane of motion of the Earth (remember the Sun is stationary as far as the solar system is concerned). The picture in figure 4 should make this clearer. You can see that it shows four positions of the Earth around the Sun and the relative position of the Moon when it is "New" and "Full". It clearly shows the angles and positions relative to each other and the positions in which eclipse should occur and when it cannot occur despite all three bodies in seemingly straight line.
So far as the general information about an eclipse goes, but how does an annular solar eclipse occur specifically? If you have not read my previous article about a penumbral lunar eclipse I suggest that you go through it by clicking here so that you can refresh you memory about the terms umbra and penumbra, as I will not be explaining them in detail again in this article.
An annular solar eclipse occurs when the Earth lies in the umbra region of the moon's shadow BUT the tip of the umbra does NOT touch the surface of the Earth. Because if the tip touches the surface of the Earth, then we will have a total solar eclipse and not an annular eclipse. You can think of a simple analogy that you are standing in front of a circular light bulb having a small circular disc of cardboard in your hand. Now think of the bulb as the Sun, the cardboard piece as the Moon and your eye as the Earth. If you place the disc at such a distance from the eyes that it fully covers the bulb, then it is equivalent to a total eclipse, while if you move it in such a manner that you just see a ring of the light bulb and the disc covers all but the outer periphery of the bulb, then it can be likened to an annular solar eclispe.