Herts astrophysicist explains how to see the ‘blood moon’ lunar eclipse

Early risers in the UK will have a chance to see a “blood moon” eclipse this week, as the moon passes into the Earth's shadow.

The partial lunar eclipse is set to take place this Friday, 14 March - but those looking to see the celestial spectacle may need to set their alarm clocks before dawn as the eclipse begins at 03:57am UK time.

Dr William Alston, a senior lecturer at the University of Hertfordshire’s Centre of Astrophysics, said: “A lunar eclipse happens when the Earth lies between the sun and the moon. The moon, therefore, falls in the shadow of the Earth.

“A partial lunar eclipse is when only some part of the moon has entered the Earth’s shadow, while in case of a total lunar eclipse, all of the moon has moved into the Earth’s shadow.

“The shadow that the Earth casts onto the moon has two parts: the penumbra and umbra. The umbra is the part that is totally obscured and casts the darkest shadow. When the moon is fully in the Earth’s shadow, this is when we see a total lunar eclipse.”

Dr Alston said that although the event is a total lunar eclipse, in the UK, the moon “will have set below the horizon before it reaches 100% of the umbra shadow, so we will see only a partial lunar eclipse”.

“However, a very large fraction - at least 95% - will be covered before it sets, so it will be an event worth watching!”, he added.

For those looking to see the eclipse from other parts of the world, Dr Alston says the percentage of the moon that is obscured will be dependent on the observer’s location on Earth, as well as the time they are viewing it.

The penumbral eclipse in the UK will start at 03:57 am, while the partial eclipse will begin at 05:09 am. The total eclipse will start at 06:26am, but the moon will set below the horizon in London at 06:22am.

Dr Alston said: “So, UK viewers will be able to see almost all of the moon (more than 95%) in the Earth’s shadow before it sets.”

He said that during the eclipse, the moon’s surface will appear reddish, which is colloquially referred to as a ‘blood moon’.

“The red colour is due to sunlight being scattered through the Earth’s atmosphere,” Dr Alston explained. “It is the same process that causes the sky to appear red during sunsets – known as Rayliegh scattering.”

In this phenomenon, light is scattered by particles much smaller than its wavelength, causing shorter wavelengths such as blue light to scatter more than longer wavelengths such as red light.

Dr Alston said this results in red light “preferentially coming in our direction” and so the

red light passing through the Earth’s atmosphere “illuminates the moon’s surface, before being reflected back to us on Earth”.

Dr Alston continued: “Observers in the UK will still get to see most of the moon covered by the Earth’s shadow, appearing visibly red - well worth looking out for!

“This will occur at a reasonable time in the morning, making it possible for most UK observers to see as they get up and ready for work.

“As the moon is starting to set, the best place will be as high up as possible, looking west. Any buildings or trees will obscure the moon before it sets.”

For those interested in similar events, Dr Alston said eclipses usually come in pairs and a solar eclipse often occurs around two weeks either side of a lunar eclipse.

This year, on Saturday, 29 March, there will be a partial solar eclipse visible from the UK at around 9:20 am.

Dr Alston said: “Finally, one thing to note is that the March full moon is also known as the ‘Worm moon’, as it coincides with the beginning of warmer spring weather, with earthworms starting to appear and nature springing to life following a cold, dark winter.”