Observations

A Supermoon on Sunday Feb 9 2020

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Supermoon behind Opera House, an Honorable Mention winnner in the Winning Sky Photos exhibition. Image by Matthew Hudson, copyright and all rights reserved.

The first supermoon of 2020 occurs in the early evening of Sunday 9 February 2020. The Moon may look a little larger than usual all night. And as it rises the familiar Moon Illusion makes it look even larger. This will be a full Moon rise worth watching!

What is a Supermoon?
The term supermoon originated in the field of astrology. Wiki claims it is “..a new or full moon which occurs with the Moon at or near (within 90% of) its closest approach to Earth [or perigee] in a given orbit.”

This definition is completely arbitrary (why not 95%?) and ambiguous (is that 90% of its distance or its orbital period?). Nevertheless, astronomers subsequently adopted the idea and have run amok with alternative, and more clear but still arbitrary, definitions.

Dr Nick Lomb in the 2020 Australasian Sky Guide uses the elegantly simple definition that “…a supermoon is when the time of full Moon is within 24 hours of perigee.” By this definition there are only two supermoons this year – on 10 March and 8 April.

But if you like supermoons you will prefer the definition in the Quasar Publishing Astronomy 2020 almanac: They define what they call the relative distance. This equals 1 when the full Moon occurs at perigee and equals 0 when full Moon occurs at apogee (the most distant point from Earth in an orbit). And any full Moon occurring at a relative distance of 0.9 or greater is a supermoon. This gets you four supermoons in 2020: 9 February, 10 March, 8 April and 7 May. Woo Hoo!

If you were wondering, the relative distance of the 9 Feb supermoon is 0.955.

What happens on February 9 2020?
So, enough of definitions. On Feb 9 2020 full Moon occurs at 6:33pm AEDT (I’ll take a Sydney perspective for this post – please make the usual adjustments for your time zone). At that time it is 362,479km from the Earth. But at that time the Moon is still below the eastern horizon. It rises at 8:08pm AEDT at which time it is about 100km closer to Earth.

And this is when you get wonderful views of a (slightly) larger, brighter (by 25%) glorious full Moon rising over the horizon just like Matthew’s image above. And the whole effect is accentuated by your brain which tosses in the Moon Illusion effect to make it look larger still!

Finally on February 11 at 07:28am AEDT the Moon reaches its perigee for this orbit when it is at a distance of only 360,461km from Earth. This is about 1.5 days after the full Moon which is why it fails to be a supermoon by Dr Lomb’s definition.

So get yourself to a beach, headland or hill on Sunday evening to enjoy the first supermoon of 2020. If you miss this one the best is yet to come because the superest-moon, the closest of the four this year, is the supermoon of April 8.

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