Why don eclipses occur every new Moon?
Eclipses only occur if the satellite of a planet is located within 0.5 degrees of the plane of the ecliptic, on a line that passes through the center of the Sun and Earth. The Moon travels along an orbit inclined by 5 degrees to the ecliptic plane, so there are only two opportunities each month — when it passes through the plane of the ecliptic — called the ascending and descending nodes. These two points connected to the barycenter of the Earth-Moon system (the point around which the objects orbit, in this case roughly Earth’s center) define a ‘line of nodes.’ Eclipses of the Sun and Moon will occur if this line of nodes coincides with the line drawn between the center of Earth and the Sun. Again, the Moon also has to be within 0.5 degrees of one or the other of the nodes so that the disk of the Sun is partially or totally covered in a solar eclipse. A similar argument explains why lunar eclipses do not happen every full Moon at the node opposite the Sun from Earth. What happens mor
