2017 Super Solar Eclipse, Pt. 2 - Why Don't Eclipses Happen Every Month?

As we said in our first installment in this series, solar eclipses are about light and shadows. As the moon passes between the Sun and the Earth, the moon blocks light from the Sun creating a shadow that falls onto the Earth. Seems simple enough, doesn't it? But how can our little moon block all the light from our gargantuan sun? And why doesn't this happen every month?

First, the size. The sun is 400 times farther away from us than the moon is. The sun is also 400 times bigger than the moon. This situation creates a 1 to 1 ratio of size over distance. Viewed from Earth, the two objects now appear to be the same size. So when the moon passes directly between the sun and a point on Earth, the moon appears to completely cover the sun and the sun is said to be eclipsed by the moon. If the moon were smaller or even farther away, there would be no total solar eclipses. In fact, the moon is slowly orbiting away from Earth. At some point in the earth’s future, total solar eclipses won’t happen anymore.

(Try this at home: Use a soccer ball for the sun and a tennis ball for the moon. Now hold the tennis ball directly in front of the soccer ball and adjust the tennis ball’s distance from the soccer ball so that the tennis ball seems to just match the soccer ball’s size. This is exactly what’s happening between the sun and the moon as seen by an observer on Earth!)

Second, why not every month? If the orbits of the Earth and the moon were on the same plane, there would be solar and lunar eclipses every month. But the moon’s orbit has a slight tilt - about 5 degrees from horizontal. This means that only two points of the moon’s orbit line up directly with the Earth’s orbit every month. If the moon’s orbit weren’t tilted, we’d have a solar eclipse every month! The tilt of the orbit, the rotation of the moon’s orbit and the Earth-Moon system’s revolution around the sun, limit the number of solar eclipses to just two or three per year. Add in a few other orbital variations and solar eclipses in specific locations become very rare. The last total solar eclipse in Philadelphia: 29 Jul 1478. The next? 01 May 2079!

Next time, Einstein and the 1919 Solar Eclipse!