19 August 2021

Once in a 'blue moon', you say? That'd be this Sunday (and it's not as rare as you thought)

| James Coleman
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Full monon

The best time to view a full moon is early evening, just after sunset. Photo: Michelle Kroll.

It’s actually happening. On Sunday night, a blue moon will take to the skies over Australia.

The rare celestial phenomenon will be visible from sunset on 22 August, meaning all those things we thought highly unlikely are probably about to happen.

Dr Brad Tucker has worked as an astrophysicist at the ANU Mount Stromlo Observatory for 12 years and says that one definition of a ‘blue moon’ is “the second full moon in a month”.

“But this rarely happens because the lunar cycle is 29 days long and a calendar month is 30 to 31 days long.”

Hence the expression, ‘once in a blue moon’, describes something extremely unlikely or rare.

Brad says that this Sunday’s blue moon is the other definition.

“It will be the third full moon of four this season. Again, there is usually only one moon a month and therefore only three a season, but if the timing’s right, you can get four in. This will be the third full moon of four between the June equinox and the September equinox.”

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However, even a blue moon is rarely blue.

“It has nothing to do with how it looks; it’s just the rarity,” Brad says.

“If the moon looks blue or any other colour, it’s because of the earth. We saw that when we had the bushfires in 2019 when the sky was coloured differently because the dust and smoke particles were refracting the light.”

He compares it to Mars, where the level of carbon dioxide makes the sun appear blue.

“The same way we get pink and orange sunrises and sunsets, Mars gets blue.”

About five times a century, the earth is treated to what’s known as a ‘double blue moon’. This is because February, which has 28 days in a common year and 29 days in a leap year, can never have a monthly blue moon. In fact, some years, it has no full moon at all or a ‘Black Moon’, and January and March end up having a blue moon each.

The last double blue moon was in 2018 but won’t be seen again until 2037.

Sunday’s blue moon will also come alongside Jupiter and Saturn from Friday, 20 August.

“Right now, in the night sky, we have these two planets near each other. If you look towards the west after the sunset, Saturn’s on the top and Jupiter’s the brighter star closer to the horizon,” Brad says.

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On Sunday, the moon will be right alongside Jupiter.

“It’s a nice pairing that we’ll get treated to this weekend. Even space is getting in on the interior decorating ‘rule of threes’,” he says.

“It’s visible everywhere, but a full moon is always best viewed in the early evening as a full moon always rises at sunset, and when you see it rising in the east, that’s when you get the best views of it looking big and beautiful.”

Brad says we won’t see another blue moon for another year or two and sees it as an illustration of how the moon and planets have very concrete orbital patterns, whereas our calendar marches to the beat of a different drum.

“When people have used the lunar calendar instead of our Gregorian-style calendar, you really get differences in time.”

Later this year, in September, he says we’ll also have the Geminid meteor shower.

“That’s a good annual shower that’s visible right across the Southern Hemisphere, and particularly good in Australia. This year should hopefully be decent, but meteor showers are always hard to predict.”

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