Observations

The Total Lunar Eclipse of July 28 2018

Animation of a lunar eclipse.
The total lunar eclipse of 28 Aug 2007. Photos and animation by Geoff Wyatt

Before sunrise on Saturday July 28, 2018 a total lunar eclipse will be visible from across Australia and New Zealand*. It will be the second such eclipse visible from these locations this year. It will be the longest eclipse visible this century (counting from 2001 to 2100) at 102 minutes 57 seconds.

The Moon reddened by light passing through Earth’s atmosphere will be accompanied by Mars, the “red planet”, in the western sky. Mars is presently at opposition and is at its brightest and closest for 15 years. However, a planet-wide dust storm which blew up in late May has turned Mars a yellow colour. From most of Australia and New Zealand you will see a reddened (possibly very dark) Moon beside a bright, yellow Mars both descending towards the western horizon. This will be visible from any location (city, country, coastal or inland) where you have a clear view of the sky down to the western horizon.

From some locations the Moon and Mars will set before the eclipse ends.

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 because 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.

This is the second total lunar eclipse visible from Australia and New Zealand in 2018, following a 3-year drought of such eclipses. The next total lunar eclipse visible from Australia and New Zealand will be in May 2021, however there is a partial lunar eclipse on the morning of July 17 2019.

When does the eclipse happen on July 28 2018?

Times for the eclipse vary depending on your location.

For those in NSW the eclipse will be first visible when the Moon begins to move into the darkest part of Earth’s shadow (the umbra) at 04:24am. The Moon is fully eclipsed from 05:30am and the Moon sets, still in eclipse, at 06:55am. Meanwhile, at 06:24am civil twilight begins and the sky brightens until the sun rises at 06:51am.

In general, look to the west from about 04:30am (Sydney and eastern states), 04:00am for Adelaide & Darwin and 02:30am for Perth. Enjoy the eclipse until the Moon sets, the Sun rises or the eclipse ends!

Further timings and times for other locations are provided in Table 1.

Table of timings for total lunar eclipse of July 28 2018
Table 1: Circumstances of the total lunar eclipse of July 28, 2018 for Australian and New Zealand cities. For Canberra please use the Sydney times. * are times of moonset. 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 – explained below. The reddish colour fades to grey as totality ends and the Moon moves out of Earth’s shadow. Finally, 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.

The reddening of the Moon for this eclipse will be less noticeable once morning twilight begins. And the Moon moves almost precisely through the centre of Earth’s shadow so the Moon may appear a very dark red indeed.

The Moon will be beside Mars. The yellow “Red Planet” accompanied by the reddened Moon will descend together towards the western horizon. From some locations (Brisbane, Sydney, Canberra) both Mars and the Moon set before the eclipse ends. Only from Perth is the full eclipse visible.

Why does the Moon turn red during a total lunar eclipse?

When the Moon is fully immersed in the dark part of Earth’s shadow (the umbra) it takes on a dark reddish colour due to light being bent or refracted onto the Moon by the Earth’s atmosphere. But there is a little more to it than that.

The Sun, Earth & Moon are perfectly aligned during a total eclipse. This is called a syzygy. Light from the Sun passes through Earth’s atmosphere and is deflected (or refracted) into the shadow and onto the Moon as if the atmosphere was acting like a prism. You might notice that the blue light is bent more by the prism. Why doesn’t the Moon look blue? The red, orange and maybe a little of the yellow light passes right through Earth’s atmosphere and is refracted towards the Moon but the green, blue, indigo and violet light (yes, that’s all the colours of the rainbow!) are scattered in all directions into the atmosphere. If you look up you will see this blue light – its the blue of the sky.

There is one more effect that determines the colour of the Moon when it it totally eclipsed. As well as the refraction and the scattering there is dust in Earth’s atmosphere that blocks some of the light from reaching the Moon. This just darkens the red colour.

The refraction, scattering and blocking effects vary slightly from eclipse to eclipse so the Moon’s exact colour also varies. It can be brown, reddish brown, coppery or even a blood red. Hence the term “blood Moon”.

Why is this the longest total lunar eclipse this century?

This will be the longest total lunar eclipse this century (from 2001 to the year 2100) at 102 minutes and 57 seconds because the Moon passes almost precisely through the centre of Earth’s shadow (so it takes longer to traverse the shadow) and the Moon is also at its furthest from Earth (apogee) where it travels at its slowest.

How often does a total lunar eclipse occur so close to a favourable opposition of Mars

This total lunar eclipse occurs within 24 hours of a favourable (or perihelic) opposition of Mars, and on the same calendar day in some parts of the world. How rare is that? I made a careful check of some planetary opposition tables (by Jean Meeus) and lunar eclipse tables (Eclipsewise). And such a pairing has not happened since the year 0 (well, year 1 to be precise!) and won’t occur again in the next thousand years. As best as I can determine this is a once in three-thousand year event! In fact, it occurs even less often than that but the tables I referred to only cover the years 1 to 3000. The last time a plain old bi-annual Mars opposition occurred within 24 hours of a total lunar eclipse was in the year 792.

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.

* In some parts of the world this eclipse occurs on July 27, 2018. This eclipse will also be visible, at least in part, from Asia, Africa, Europe and South America. Only north America misses out this time.

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