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Eclipses and the path of light: Geometric optics

How do we get a solar eclipse?

Details about this, and the Earth-moon system in general, are here: Kaiserscience Earth-moon system.

But here are the basics:

solar eclipse

Solar Eclipse via Shutterstock


How do we get a lunar eclipse?

First we need to know about the three types of shadows.

umbra (Latin “shadow”) is the innermost, darkest part of a shadow.
Where the light source is completely blocked.

penumbra (Latin paene “nearly”) is where only a portion of the light is obscured.
An observer in the penumbra experiences a partial eclipse.

antumbra (Latin ante, “before”) is where the occluding body appears entirely contained within the disc of the light source.
An observer here sees an annular eclipse, in which a bright ring is visible around the eclipsing body.


Here we see rays of light from the Sun, hitting the Earth. This happens 24-7.

Behind the Earth the three types of shadows always exist, 24-7.

If the moon passes through one of these regions, then we get one of these types of eclipses.


What are the conditions for a lunar eclipse?


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