What causes an eclipse and why is it so rare? Today’s Video of the Day comes thanks to the NASA/Goddard Space Flight Center and features a look at what causes an eclipse. Total eclipses are rare because the timing of the new moon within the eclipse season needs to be more exact for an alignment between the observer (on Earth) and the centers of the Sun and Moon.
Lunar and solar eclipses are caused by the relative motions (or orbits) of the Earth, Moon and Sun. The Moon orbits the Earth in roughly* the same plane as the Earth-Sun orbit meaning that the Moon occasionally passes in front of the Sun in the sky causing a solar eclipse.
In fact, at new Moon — the only lunar phase when a solar eclipse can occur — the Moon usually misses the Sun altogether. Given all the variables, it’s almost surprising that we see eclipses at all. The Moon orbits Earth; both swing around the Sun. This truly remarkable coincidence is what gives us total solar eclipses. If the Moon were slightly smaller or orbited a little farther away from Earth, it would never completely cover the solar disk.
Source: NASA/Goddard Space Flight Center