The Total Lunar Eclipse of Jan 31, 2018 – Part 2

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.

Part 1 described the eclipse, what to look for and when to see it. Part 2 explains why the Moon turns red during a total lunar eclipse. This eclipse also involves a Blue Moon and a Supermoon. What are these?

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

As I mentioned in Part 1 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”.

Is the full moon for this total lunar eclipse also a Blue Moon?

The modern definition holds that a “blue moon” is the second full Moon in a calendar month. But it doesn’t actually look blue and this definition is the result of a misinterpretation made by Sky & Telescope magazine in the 1940s. While many people think of the Moon as full for a few nights at a time, and really it does look full to the eye for a few days, technically the “full Moon” occurs at a very precise time – when the Moon is on the opposite side of the Earth from the Sun.

The table in Part 1 shows that the January 31, 2018 full Moon is:
– NOT a blue moon in New Zealand
– NOT a blue moon in NSW, the ACT, Victoria and Tasmania, but only because daylight-saving time pushes the full Moon into the early morning of February 1st
– IS a blue moon in South Australia, but only just, despite daylight-saving being in place
– IS a blue moon in Queensland, this time because daylight-saving is not recognised
– IS a blue moon in Western Australia and the Northern Territory, and would be even if daylight-saving were recognized, which it is not

Phew! So a red moon can also be a blue moon, but it depends where you are and who has won the battle on daylight-saving in your state!

Blue moons (by the above definition) occur about every 2.7 years. But in 2018 there is another blue Moon, this time for everyone in Australia (but not New Zealand), in March.

Is the full moon for this total lunar eclipse also a Supermoon?

The definition of a supermoon is completely arbitrary. The term was just made up by an astrologer and there are several every year. Yet even NASA promotes them and the idea has taken off!

According to Wiki a supermoon is “..a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth in a given orbit.” The Moon will be closest to Earth (in this orbit) on Tue Jan 30 (8:57pm) being at 359,152km. At the following day’s full Moon (while eclipsed) the Moon will be 360,198km away. That must make it a supermoon.

Whatever you want to call it don’t miss this opportunity to witness another grand spectacle free of charge from the universe!

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3 responses to “The Total Lunar Eclipse of Jan 31, 2018 – Part 2

  • The phrase “once in a blue moon” has always implied an element of significant rarity. Yet the modern definition has a blue moon occurring once every 2.7 years. That’s more frequently than, say, a leap year. And I would never consider a leap year to be a “once in a blue moon” event. The modern interpretation of “blue moon” is clearly faulty.

    • Roberto, Yes, I would tend to agree. In astronomical terms it is not rare. But for day-to-day life perhaps once in about 3 years is somewhat rare?

      • I still think ‘once in a blue moon’ implies significant rarity…no more frequently than once every 20 years, say.

        But it’s all academic this evening, since the cloud cover in Sydney has robbed us of any view of the eclipse.

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