If you’ve ever witnessed your first lunar eclipse unknowingly, you might have been a bit shocked to see the Moon turn deep red, so in this short post we’ll go through why the Moon reddens during this spectacular event.

The Position of The Sun, Moon, and Earth

During a solar eclipse the Moon passes in front of the Sun and blocks all or most light. in a lunar eclipse we pass in front of the Sun and block light from travelling to the Moon.

This may sound very similar to a New Moon, but the positioning and shadows are completely different. Here’s two diagrams to hopefully explain the difference. Beware: neither are to scale!

When Earth or The Moon casts a shadow, it creates and umbra and penumbra. The umbra is the darkest part (the black and deep red parts of the diagrams), but in the penumbra some normal light will dissipate in that area as well.

In a lunar eclipse we need to focus on the umbra, because although no light falls directly onto that area, there is one sneaky exception.

Where did the red light come from?

Earth’s atmosphere does all the work here. When sunlight (made of lots of colours of light) passes through the atmosphere, the shorter wavelengths of light such as blue and ultraviolet scatter extremely well and make our beautiful blue sky.

Longer wavelengths of light, most notably red, do not scatter that well. They actually refract as they pass through Earth’s atmosphere, meaning the red light bends! Refraction arises simply because the light (which was travelling through a vacuum) is now travelling through a different medium (air).

The red light will refract again once it re-meets the vacuum of space. In this case it bends out into the umbra! Please excuse the awful diagram.

Now in comes The Moon, with nothing but red light to fall onto it. And thus begins a beautiful lunar eclipse!