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Lunar eclipse

The heliocentric model contains one remnant from the geocentric model. Can you find it?

Nicolaus Copernicus, On the Revolutions of the Heavenly Spheres, 1543  The geocentric model correctly assumed that the moon orbits the earth. We now know this is the reason for the phases of the moon. Why is this? First let's take a look at a sequence of photos taken of the moon over 29 days:

A sequence of photos taking over the course of 29 days. Image Credit: Tom RuenThe following is a detailed animation that covers an entire year of the earth/moon cycle. In the center there is a large image of the moon as seen from earth each evening. Take this time to convince yourself why the moon has phases:  

Notice that the moon passes directly through the shadow of the earth on days when there is a full moon. Only when the the sun, moon and earth are closely aligned like this is it possible to observe a lunar eclipse. Several cultures have myths related to lunar eclipses. The Egyptians saw the eclipse as a pig swallowing the moon other cultures view the eclipse as the moon being swallowed by other animals, such as a jaguar in Mayan tradition, or a three legged toad in China.

Total lunar eclipse. Image Credit: Jiyang Che

However, the lunar orbit is not perfectly aligned with the earths orbit. It’s shifted by around 5 degrees. This means the moon does not always pass exactly through the middle of the Earth’s shadow, causing the eclipse time to be much shorter. In some cases the eclipse doesn’t occur at all as the Moon sweeps outside the shadow cone.

 

Moon passing through the umbral and then penumbral shadow only - Image: Peter Collingridge

Variations on an eclipse

Astronomers classify eclipses into three types depending on the position of the Moon around the Earth’s two shadows: umbral and penumbral:

Penumbral and umbral shadows. Image credit: Saregd

Total lunar eclipse

This occurs when the entire moon passes through the earth’s umbral shadow. This video follows the moon during a total eclipse:

Partial eclipse

This occurs when the moon passes through the earth’s penumbral shadow but only a portion of the much darker umbral shadow.

Image Credit: Alfredo Garcia, Jr

Penumbral eclipse

In some cases the moon only passes through the penumbral shadow of the earth, these are much harder to observe as there is little change in illumination. Look closely as this is one of the most subtle phenomena in naked eye astronomy:

Look very closely, do you see the change? Image credit: John Walker'

Challenge questions

Could the length of an eclipse tell us something about the size of the Moon relative to the Earth?

We have 12 full moons per year, how come we always have far fewer lunar eclipses?

Next, we have an interactive simulation which will allow you to explore these lunar eclipses in more detail.