EXPLAINER

A Super Blood Moon is headed our way this Wednesday

A Blood Moon is the nickname for a Total Lunar Eclipse due to the Moon's red appearance. Picture: Shutterstock
A Blood Moon is the nickname for a Total Lunar Eclipse due to the Moon's red appearance. Picture: Shutterstock

On Wednesday, all of Australia will be treated to a truly spectacular treat: a Super Blood Moon.

Technically called a Total Lunar Eclipse, a Blood Moon is the nickname due to the Moon's red appearance.

Just like us during the day, the Earth also casts a shadow out into space. A big, Earth-sized shadow that extends for hundreds of thousands of kilometres.

It is amazing to think that even the Earth casts a shadow - something we often don't think about. As the Moon goes around the Earth, at the rate of about every 29 days, sometimes the Moon goes into this shadow and we get a lunar eclipse.

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When you look at the Moon during the Total Lunar Eclipse, you are seeing the Sunrise and Sunset of the Earth lighting up the Moon.

In order for the shadow to be perfectly aligned with the Moon's position, the Moon must be about the exact opposite side of the Earth compared to the Sun. This is also how a Full Moon happens, when the Sun fully lights up the side of the Moon that we see. This is why we always get a Total Lunar Eclipse during Full Moon.

If the Moon goes around the Earth about once a month, as we see with the Full Moon, why don't we get a Lunar Eclipse every month?

It turns out, the Moon wobbles a bit as it goes around the Earth, by about 5 degrees. Sometimes it is a bit higher compared to the Earth as it goes around, and sometimes lower.

In order for the Moon to perfectly go into the shadow, it needs to be aligned with the Earth. Sometimes it just skims the shadow and we get a Partial Lunar Eclipse.

While the Earth casts a shadow into space, a little bit of light skims through the Earth's atmosphere and into space.

Just as Sunrise and Sunset are an orange or reddish colour, so is this light that skims through the Earth's atmosphere and out into space.

As the Moon goes into the shadow, it also goes into this coloured light, so while the Moon darkens, it also turns red.

When you look at the Moon on Wednesday during the Total Lunar Eclipse, you are seeing the Sunrise and Sunset of the Earth lighting up the Moon.

Wednesday's Moon will also be a Supermoon, or a perigee-syzygy Moon. The orbit of the Moon not only wobbles, but isn't perfect a circle - it varies by about 50,000 kilometres. Sometimes the Moon is closer to the Earth, and therefore a bit bigger and brighter.

This is the case with the Full Moon next Wednesday when it also happens to be a Total Lunar Eclipse, so a Super Blood Moon is born.

The action begins at 7.44pm on the East coast, where the Moon will slowly darken and turn red for about one and a half hours.

Then totality will happen, when the Moon is fully in the shadow and red, occurring between 9.11 and 9.25pm.

It will then slowly drift out of the shadow, ending just before 11pm.

  • Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU
This story A Super Blood Moon is headed our way this Wednesday first appeared on The Canberra Times.