1. Learn what happens during a lunar eclipse
This illustration shows the Moon's path through Earth's shadow during a total lunar eclipse. Credit: NASA | + Expand image
A lunar eclipse happens when the full moon passes into Earth’s shadow.
The Moon passes through two parts of Earth’s shadow during a lunar eclipse. You can tell which part of the shadow the Moon is passing through based on the way the Moon looks from our perspective on Earth.
You have probably noticed that some shadows on the ground are darker than others, depending on how much outside light enters the shadow. The same is true for Earth’s shadow.
The outer part of Earth's cone-shaped shadow is called the penumbra. The penumbra is less dark than the inner part of the shadow. The inner part of the shadow, known as the umbra, is much darker because Earth blocks more sunlight from entering the umbra.
As the Moon enters the penumbra, it will dim very slightly. Because the penumbra is not fully dark, you may only notice some dim shading (if anything at all).
These side-by-side images show the Moon just before the start of a total lunar eclipse (left) and just as the Moon has started to enter the penumbra (right), the outer part of Earth's shadow. You might only notice dim shading (if anything at all) until the Moon reaches the darker part of Earth's shadow, the umbra. Credit: NASA | + Enlarge image
When the Moon moves into the darker shadow, the umbra, it is very noticeable! Some say that during this part of the eclipse, the Moon looks as if it has had a bite taken out of it. That “bite” gets bigger and bigger as the Moon moves deeper into Earth's shadow.
The Moon is shown entering into the umbra during a total lunar eclipse. Credit: NASA | + Enlarge image
The total lunar eclipse officially begins once the Moon is completely inside the umbra. At this point, the Moon will look reddish-orange.
The Moon is completely inside the Earth's shadow during this stage of a total lunar eclipse. Credit: NASA | + Enlarge image
Even though a full moon occurs about every 29 days, lunar eclipses are much more rare. This is because the Moon’s orbit is tilted, so most full moons happen when the Moon is above or below Earth’s shadow. Sometimes, the Moon only moves part way into the edge of the shadow. We call this a partial lunar eclipse.
Because the Moon's orbit is tilted, it doesn't always pass directly through Earth's shadow. Credit: NASA | + Enlarge image