The Total Lunar Eclipse of Jan 31, 2018 – Part 1

Animation of a lunar eclipse.

On the night of Wednesday January 31, 2018 a total lunar eclipse will be visible from across Australia and New Zealand*. It will be the first such eclipse visible from these locations since 2015.

The Moon will be at its full phase as it enters Earth’s shadow and this particular full Moon will also be a “Supermoon” and, for many locations, it will also be a “Blue Moon”.

It is quite safe to watch this eclipse in its entirety by eye or through binoculars or a telescope.

What is a lunar eclipse?

Eclipses of the Moon occur when the Moon moves into the shadow of the Earth. When the Moon is fully immersed in the dark part of the shadow (the umbra) we see a total eclipse of the Moon. At such times the eclipsed Moon usually takes on a dark reddish colour from the light bent or refracted onto the Moon by the Earth’s atmosphere. If the Moon is only partially immersed in the dark part of the shadow we have a partial eclipse.

An eclipse of the Moon can only happen at full Moon phase. But it does not happen every month as the path the Moon takes around the Earth is tilted by about 5° to the path the Earth takes around the Sun. Hence at full Moon the Earth’s shadow usually falls below or above the Moon.

Diagram of a lunar eclipse.

How common are lunar eclipses

On average there is an eclipse of the Moon every eight months, with a little under half of these total. The actual number of lunar eclipses in a year can range from none to a maximum of three. A total eclipse of the Moon is visible from Australia on average every 2.8 years.

When does it happen on Jan 31 2018?

Times for the eclipse vary depending on your location and whether your state follows daylight saving time.

For those in NSW the umbral eclipse, when the Moon moves into the darker part of Earth’s shadow, begins at 10:48pm (these are daylight-saving times for NSW). It is fully eclipsed from 11:51pm to 01:08am. The eclipse ends at 02:12am when the Moon finally leaves the umbra.

For other locations Table 1 gives the times.

Table from the “2018 Australasian Sky Guide” by Dr Nick Lomb
Table 1: Circumstances of the total lunar eclipse of January 31, 2018 for Australian and New Zealand cities. For Canberra please use the Sydney times. Copied from the “2018 Australasian Sky Guide” by Dr Nick Lomb. Copyright MAAS.

What will I see?

During a lunar eclipse the Moon first moves into the fainter part of Earth’s shadow, called the “penumbra”, then into the darker central part of the shadow (the umbra) then back into the penumbra before leaving the shadow completely. In practice the penumbral part is barely noticable.

Shortly after the umbral eclipse begins you will see a dark “bite” missing from the Moon’s edge. This grows until the Moon is completely inside the shadow (“totality” begins now) at which point you will notice the Moon has turned a reddish colour – I’ll explain why this happens in Part 2 of this post. The reddish colour fades to grey as totality ends and the Moon moves out of Earth’s shadow. Finally, after almost 3.5-hours the Moon exits the umbral shadow and the eclipse is all but over.

The movement of the Moon through Earth’s shadow gives a powerful impression of the real movement of the Moon in its orbit about the Earth.

Is this a Blue Moon? A supermoon? And why does the Moon turn red?

I will answer these questions in Part 2.

The Sydney Observatory Total Lunar Eclipse Event

Sydney Observatory is holding an event to observe this eclipse, with our expert Guides on hand to answer your questions and enhance your viewing experience – Find out more.

* This eclipse will also be visible from Alaska and parts of western Canada, the Pacific region, South East Asia, and most of the Asian region excluding India.

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