August will end with a 'blue' supermoon. Here's the best night to see it

This coming week we'll experience a lunar rarity.

We'll witness a supermoon and a blue moon at the same time.

Let's break down what that actually means and what to expect.

Technically, the full moon will occur on Thursday, August 31.

To get even more technical, the Perth Observatory says the exact time when the moon will be its fullest will be 9:35am AWST (11:35am AEST, 10:35am ACST) — and at that point, the sun will be up.

So, the observatory's Matt Woods says Wednesday night will be the best night to see the supermoon in Australia.

Look up at dusk, just as the moon is rising.

"It's always best to see low on the horizon," Mr Woods says.

"There's that optical illusion where it looks bigger than it does."

But while Wednesday is the big night, Mr Woods says Thursday night will be worth a look too.

"It will still look pretty impressive," Mr Woods says.

It's when there's a full moon at the same time the Moon is closest to Earth.

The Moon's orbit around Earth isn't a perfect circle.

So there are times in the Moon's orbit when it's closer to Earth — that's called the perigee.

When this happens, the Moon is 363,396km from Earth.

So because it's closer to us on Earth, the Moon looks bigger.

"Supermoon" isn't really a technical term, but it's generally applied when the Moon is 90 per cent or closer to its perigee.

The opposite of this is the apogee, when the Moon is 405,504km from Earth

When a full moon coincides with the apogee, it's called a micromoon.

Mr Woods says this week's supermoon will look seven per cent bigger than a normal full moon.

It takes special camera tricks to make the Moon look gigantic.

Generally, a supermoon appears 14 per cent bigger and 30 per cent brighter than a micromoon.

NASA's Jet Propulsion Laboratory Education Office says it's not that noticeable.

"A 14 per cent increase in the apparent size of something that can be covered with a fingernail on an outstretched arm won't seem significantly bigger," NASA says.

It's less about the size and more about brightness — which will be more noticeable.

"It should look brighter," Mr Woods says.

We sure did — it was most visible on the nights of August 1 and August 2.

That's what makes this supermoon even more … super.

It won't just be a blue moon or a supermoon, it'll be both at the same time.

No, but you're not silly if your mind went there.

Occasionally, the moon can look like it's a different colour.

The moon appears to have a reddish brown tinge during a lunar eclipse, which also has the nickname 'blood moon'.

And, according to a NASA post from David R Williams, it has looked blue-ish before.

"The term 'blue moon' is believed to have originated in 1883 after the eruption of Krakatoa," he wrote.

"The volcano put so much dust in the atmosphere that the Moon actually looked blue in colour.

"This was so unusual that the term 'once in a blue moon' was coined."

Since the late 1940s, 'blue moon' has been used as a phrase that describes when two full moons fall within the same calendar month.

But only the second full moon is described as a blue moon.

That's all because of an article in an astronomy magazine.

"This colourful term is actually a calendrical goof that worked its way into the pages of Sky & Telescope back in March 1946, and it spread to the world from there," Kelly Beatty wrote in an article published on the magazine's website in 2009.

Sky & Telescope explained that its writer James Hugh Pruett "made an incorrect assumption about how the term had been used in the Maine Farmers' Almanac".

The Old Farmer's Almanac is an American annual magazine that contains things like weather pattern forecasts, moon phases and gardening tips and has been in print since 1792.

It takes roughly 29.5 days for the Moon to go through a whole cycle.

And most months go for 30 or 31 days, so it's not that common for a full moon to happen within the same calender month.

Because February will only ever have 28 or 29 days, a blue moon would never fall in February.

They happen once every two-and-a-half years, NASA says.

But about four times a century we get two blue moons in the one year — the last time that happened was in 2018.

And they always happen one after the other — so we'll get three or four in a row.

The first supermoon of the year was on July 3.

Friday, September 29.

That'll be the last supermoon of the year.