Solar eclipse: I think this should happen every month?

Answer

That would be the case if the moon, the earth and the sun moved in one plane. Then we would see a solar eclipse at every new moon and a lunar eclipse at every full moon. It would eventually get boring…

In reality, the orbital plane in which the Moon revolves around the Earth makes an angle of just over 5° with the plane in which the Earth revolves around the Sun, the ‘eclitpic plane’. As a result, we almost always see the moon passing below or above the sun at new moon. That can be up to five degrees below or above it, which is much more than the diameter of the moon, which is only half a degree in the sky. The Moon intersects the ecliptic plane twice per revolution, but the Earth-Moon line does not always point exactly to the Sun at that instant.

Take a look at the accompanying figure: the white plane is the ecliptic plane with twice the earth and the orbit of the moon in red. So that orbit makes an (here exaggerated) angle with the ecliptic plane and intersects it at two points, two “nodes”, and their connection is the node line. Each time the moon is shown as a black sphere in that orbit. On the left we have a solar eclipse because the moon is right there between the sun and the earth. After all, when the moon passes through the ecliptic plane, the sun happens to be in line with the knot line.

A month later, the Earth (with its moon) has moved far in its orbit. When the moon is now “new” again, we now see the moon in a small angle (blue) under the sun. So no eclipse. Here I pretend that the orientation of the node line maintains a constant direction against the stars. That is not the case, that line also rotates slowly. But that makes things even more complicated. So the main reason why we don’t see solar eclipses every month is the fact that the lunar orbit is angled with the ecliptic plane.