Lunar eclipse

I’m not really a morning person. After being woken by the alarm every morning during the week, I usually like a bit of a lie in on a Saturday morning. But not yesterday morning, yesterday I was up even earlier than I am during the week to talk about Friday evening’s pernumbral lunar eclipse on BBC Breakfast. So I just thought I’d write a short post to explain what happens during a lunar eclipse.

When we see the Moon in the sky it is reflecting light from the Sun; it doesn’t produce its own light. A lunar eclipse occur when the Moon passes through the shadow of the Earth, where the Sun’s light can’t reach. If you look closely at a shadow – any shadow, even your own shadow when walking down the street on a sunny day – you can see that it does not have a sharp edge. The central part is the darkest, but the edge is a bit lighter and possibly looks fuzzy. The central part of the shadow is called the umbra, the fuzzy edge is called the penumbra.

A lunar eclipse occurs when the Moon passes through the Earth’s shadow. The central, darkest part of a shadow is called the umbra; the ligher edge is the penumbra. Image: NASA

A total lunar eclipse occur when the whole of the Moon passes through the umbra, the darkest part of the Earth’s shadow. In that instance, the light from the sun cannot directly reach and illuminate the Moon. Some light does bounce off the Earth’s atmosphere and reach the Moon, but the different colours or wavelengths that make up white light get separated: it is only the red light that reaches the Moon, the blue light gets lost en route. This is why the Moon looks red during a total eclipse, and is sometimes refered to as a Blood Moon. If Earth didn’t have an atmosphere no light at all would reach the Moon during a total eclipse and it would appear completely dark (not that we’d be here to see it if there was no atmosphere 😉 )

If the Sun, Earth and Moon were all perfectly aligned in the same plane, then we would see a lunar eclipse every month. But they aren’t perfectly aligned, the Moon’s orbit around the Earth is sometimes a little above, and sometimes a little below, the Sun-Earth plane.

Somtimes just part of the Moon passes through the umbra, and then we see a partial lunar eclipse. The Earth’s shadow will appear to take a bite out of the Moon. A partial eclipse is also seen in the run up to, and immediately after, a total eclipse, as the Moon moves through the Earth’s shadow.

Friday’s eclipse was a penumbral lunar eclipse, when the the Moon passes through the penumbra, the lighter, fuzzy edge of the Earth’s shadow. A penumbral eclipse is more subtle, it is not as dramatic as a total or partial eclipse, and can be quite hard to see – the Moon would just have looked a little darker than normal. I have to admit that I looked at it on my way home form work on Friday night and couldn’t see any difference!

There will be another 3 penumbral eclipses visible in the UK in 2020: 5th June, 5th July and 30th November. And if you live in a different part of the world you can search for eclipses visible in your region on

So how do you explain this to the nation in just a minute on Breakfast TV at 8:30 on a Saturday morning? Well I have my trusty football-Earth and tennis ball-Moon, and like to ask the presenters to pretend to be the Sun! A simple demonstration makes it much easier to explain than just words – and hopefully easier for viewers to understand too.

I guess Bonnie Tyler didn’t think Pernumbral Eclipse of the Heart had quite the same ring to it! 😉

About Sarah Crowther

I'm a Post Doc in the Isotope Geochemistry and Cosmochemistry group. I study xenon isotope ratios using the RELAX mass spectrometer, to try to learn more about the origins and evolution of our solar system. I look at a wide range of samples from solar wind returned by NASA's Genesis mission to zircons (some of the oldest known terrestrial rocks), from meteorites to presolar grains.
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