How to watch the longest lunar eclipse of the century

2016, because we’re a bit tone deaf in Britain, won’t be the last. The first visible lunar eclipse in six years isn’t until early January, when it will be the longest of its kind since 1639, lasting more than three hours.

But that will be too late to see the blood of the eclipse light up the night sky. It will be visible at the precise instant when totality occurs, when the moon creeps its little legs out of the Earth’s shadow, a moment in which it will turn a glorious red red.

There are about 100,000 people in the UK who will see it as it happens, though to look at it you need to get out in a relatively clear sky and with good weather. As far as the world of astronomy is concerned it will be the longest lunar eclipse for more than 600 years.

I love a lunar eclipse but, as always, a lunar eclipse is a great evening out, whatever your age. (One caveat: as a rule in Britain, if you go out late on a winter night to see a lunar eclipse, the next day you will be covered in snow.)

Those of us who love the eclipse from a British viewpoint can thank the European Space Agency (ESA) for cleaning up our stubble. The Sun is the only source of light and when it is eclipsed, part of the rays will be reflected back into space, and on last year’s total lunar eclipse, the Sun reflected back glowingly across the face of our planet.

If you want to observe the eclipse from Britain, you might be able to see it either on the morning of the 27 or 28 January, but the shorter the interval between the starts of totality and the end of totality, the shorter the total eclipse.

Although the moon gets back into sunlight, its light gets bent. As it enters the penumbra, a very small area of dark central shadow, the light passes through it and back out the sun. Hence you can see the central part of the shadow and the penumbra. You see the redder, edges to it when the penumbra is on the moon’s limb.

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There are still a few times over the centuries when you can see this kind of thing, through windows. The long shadow is only truly visible because the Moon is approaching the Earth and falling into the shadow. During this temporary time of view, you can see the brilliant sign of sunlight reflected back across the soft moondust clouds on the horizon. By looking up, you can also see the area of eclipse we call “sunset mania” and the fading light is a perfectly immaculate progression of totality from the penumbra to totality to the “entry” of the phases of twilight from twilight into moon phase. As the tributaries wane, the red is like the tail wagging the dog.

This will be the longest lunar eclipse of the century and only the second in four years. On 20th January 2015, the eclipse took four hours and 38 minutes. December 2018, 14th and 16th March 2020 will also be the longest lunar eclipses of the century.

When to watch

Lunar eclipse date: 27 and 28 January. Observe the beginning and end.

Penumbral lunar eclipse angle: The nearer to the centre the point of the eclipse, the more local viewing. Those of us in the western hemisphere will have the best viewing in the hours before dawn on the 28th morning, when the Moon is in its penumbral phase. If there is enough clear air, the Moon will already be low in the sky in the predawn skies.

Observers in the UK and Ireland: the moon will be a few hours from the greatest position as the total eclipse starts, and you may be in Europe (west of the Channel) in the east by the end of totality. There will be an angle of about 5 degrees between the Moon and totality in the UK. If you want to see the total lunar eclipse, observe the best points in the eastern skies.

University of Sheffield lunar eclipse expert Don Lavery

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