Earlier in our blog we talked about how eclipses occur during “New Moon” phases. So, why is it that a solar eclipse does not happen every month during a New Moon?

There would be one every month if the Moon’s orbital path were aligned with the Earth’s orbit around the Sun. In reality, the Moon’s orbital path is actually slightly tilted at a small angle of five degrees, when compared to the Earth’s orbit around the Sun. This is enough to make syzygy a rarer event. If the Moon’s orbital path were not tilted at all, syzygy would happen every month, and total solar eclipses would happen much more frequently. The tilt causes the moon to be above or below the Earth during its “New” phase, making its umbra miss the Earth completely. This is expressed in the image to the right, with the small squares being the plane of the moon’s orbit, and the large square is the plane of the Earth’s orbit around the Sun. When the moon’s orbital path intersects (or is at the same angle) as the Earth’s orbit around the Sun, it’s called a node. Eclipses only occur when the two nodes (pictured below as N and N’) line up in syzygy, point towards the Sun, AND have the Moon at either node. Solar Eclipses happen when the Moon is at N’, and Lunar Eclipses happen when the Moon is at N.