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Longest Lunar Eclipse of the Century

What is a lunar eclipse, why is this one special and what have they taught us?

On Friday 27th July 2018 much of the globe will see a ‘blood moon’ as the Moon is totally eclipsed by the Earth. 

In the UK the Moon will rise in the southeast at 9.10 pm with maximum eclipse occurring at 9.22 pm until 10.13 pm, the Moon will continue to be partially eclipsed until 11.19 pm. The eclipse and blood red coloured moon will be visible across the globe with only North America and Greenland missing out entirely.

What is a lunar eclipse?

A lunar eclipse occurs when the Earth passes between the Sun and the Moon, casting the Earth’s shadow across the Moon. Although not particularly rare lunar eclipses are less common than solar eclipses with a maximum of 3 occurring in any given location per year, sometimes no lunar eclipses are seen at all.

The red colour of the Moon during a lunar eclipse is due to the scattering of sunlight through the Earth’s atmosphere, as red light has a longer wavelength it is scattered less than the shorter blue and violet wavelengths meaning more red light reaches the Moon causing it to appear red in colour. If the Earth had no atmosphere there would be no scattering of light and the Moon would appear completely dark like during a new moon.

What is so special about this lunar eclipse?

Although lunar eclipses are not particularly rare what makes this one special is the length of time that it will last. The Moon will be totally eclipsed by the Earth for 1 hour and 43 minutes which is the longest eclipse so far in this century and is only 4 minutes less than a lunar eclipse can theoretically last. 

The reason this eclipse will last so long is because the moon will pass through the centre of the Earth’s shadow, the first central lunar eclipse since 2011. Central lunar eclipses occur when part of the moon passes through the centre of the Earth’s shadow and are the darkest type of eclipse; these eclipses are relatively rare, between 2001 and 2050 only 10 will occur.

What have lunar eclipses taught us?

One of the earliest suggestions that the Earth was round came from Aristotle  who wrote in his book ‘On the heavens’ about how the shadow of the Earth that can be seen to fall on the Moon during an eclipse is always circular in shape.

Ancient greek astronomers used lunar eclipses to compare the daytime and nighttime skies, measuring positions of stars relative to the middle of the moon during the middle of the eclipse. It is believed that this lead to the discovery of precession; the wobble of Earth as it spins on its axis. By looking at historic lunar eclipses Hipparchus of Nicea realised that constellations shifted in the sky over a 26,000 year cycle. This knowledge became a vital building block that Newton later used to explain Earth’s oblate shape and planetary precession.

Ancient records of lunar eclipses are still being used today, particularly in determining the rate at which the rotation of the Earth is changing.


Emma Collier

Emma studies Physics with Astronomy at the University of Southampton.

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