IN BRIEF
- A rare blue micromoon will be visible this week.
- The moon will be "tens of thousands of times brighter than the brightest star".
The phrase "once in a blue moon" is about to feel a little more literal, with a rare lunar event set to light up the sky this week.
Australians will be able to spot a rare blue micromoon on Sunday night — and you can see it quite easily.
"It is incredibly bright. It's like natural light pollution," Laura Driessen, a postdoctoral researcher at the Sydney Institute for Astronomy, told SBS News.
"It's tens of thousands of times brighter than the brightest star in the night sky."
The event is considered uncommon because it combines two separate astronomical phenomena at the same time: the blue moon and the micromoon, something skywatchers do not get to see very often.
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Here is everything you need to know about the rare blue micromoon:
What is a blue moon?
The first thing you need to know is that the moon will not be blue, and it has nothing to do with the phrase "once in a blue moon".
"There are two different types of blue moon: a seasonal blue moon and a calendar blue moon. This one is a calendar blue moon," Driessen said.
"It's not blue. It's just a name. And then the definitions that we have of a blue moon are actually really recent, less than 100 years old.
"The first written record of someone describing a seasonal blue moon was from 1937, and the calendar blue moon ... is even more recent than that."
A calendar blue moon is the second full moon in a month — there was another full moon earlier in May. A seasonal blue moon is the third full moon in an astronomical season with four full moons.
According to Driessen, "every couple of years we get an extra full moon in a year".
What is a micromoon?
What makes this event even rarer is that the blue moon will also be what astronomers call a "micromoon".
"At one point in its orbit, the moon is closer to the Earth, and that closest point is called the perigee. And another point in its orbit, it's a little bit further away from Earth, and that's called the apogee," Driessen said.
"A micromoon is when the full moon happens close to that apogee point. So the moon is just a touch farther away than it is in other parts of its orbit, so it looks a little bit smaller."
However, don't expect to see a tiny moon in the sky on Sunday night, you might not even be able to tell the difference.
It's estimated to be 6 per cent smaller than the average full moon.
"Human eyes won't be able to tell the difference. You can only tell the difference if you have a photo of an average moon next to a photo of a micromoon," Driessen said.
"It will look like a beautiful full moon to us."
When is the best time to see the blue micromoon?
Australians will be able to see the blue micromoon in the evening sky on Sunday 31 May, and it will be visible for the whole evening.
"A full moon happens when the moon is on the opposite side of the Earth to the sun. So that means that the moon is gonna be big and bright, and it's gonna be up all night," Driessen said.
"If you're a person who stays up late or someone who gets up early, so long as you're outside and it's dark outside and the sky is clear, you'll be able to see it, and you really won't miss it."
But if you want to still choose the best time to watch, you might mark 6.45 pm on your calendar, as that is when the moon will reach peak fullness.
Where to watch the blue micromoon?
Good news, you don't need to change your location to see the rare blue micromoon.
"As long as you have a view of the night sky and it's not cloudy. Anywhere in Australia, you'll be able to see it," Driessen said.
To truly enjoy this astronomical event, find a darker location away from light pollution that can interfere with your view.
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