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The fiction of a supermoon

by Bruno Van de Casteele

June 23, 2013

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Donate Today, early this morning on the 23rd June, there was a "supermoon". Scared ? You shouldn't. The Full Moon was closer than it ever would be this year, and some call it a supermoon.

Impressed ? Well, you shouldn't either. The full moon was still a reasonable 356 000 kilometers away. And though it was very close, it was almost as close as last month. Furthermore, as the Moon orbits our Earth in an elliptical orbit, it does this every month, twice, when it is closest to Earth (called perigee). It's just that today, this perigee almost coincides with full moon.

But even though this makes the Moon close to the Earth when fully illuminated for us, you won't really notice. The difference between apogee and perigee however is noticeable because it makes a difference of 50 000 km, as the picture below illustrates.

Neil deGrasse Tyson explained it best in his Amazing Meeting talk in 2011, Las Vegas. The difference between a "supermoon" and a normal "close" moon is actually, as shown in his slide below, the difference between an 8 and an 8.00001 inch pizza.

In this video, we also get an explanation where the name "supermoon" comes from (you can also read it here). It was actually invented by an astrologer, Richard Nolle (I won't link to his site, if you need it you can google it). Now I think we shouldn't dismiss his term just because he's an astrologer, but his definition is really silly:
...a new or full moon which occurs with the moon at or near 
(within 90% of) its closest approach to Earth in a given
orbit (perigee). In short, Earth, moon and sun are all in
a line, with moon in its nearest approach to Earth.

90%? Why 90% and not some other number? And given that there are 24 full or new moons each year, labelling 4 to 6 each year "super"moon isn't that "special" anymore. Furthermore, by following this arbitrary "definition" I feel we give this astrologer too much credit for something which isn't even visible to any observer without instruments. Worse, it creates a platform where he or others can spout disinformation and scaremongering about earthquakes or other calamities, linked to this "alignment".

Not everyone agrees that we should ignore this term, though. Elisa Andrew, creator of the extremely popular (and excellent) Facebook page "I F*cking Love Science", posted this picture some days ago.

I think the goal is laudable. Trying to "steal" the term from the astrologers and getting people interested in looking up, can never be bad. But the picture gives the impression that it will really be an impressive sight. Which it is, but that is because it is every month an impressive sight, with or without "supermoon". In the long run, it might even be counterproductive, as casual observers might notice that indeed, there is no difference.

I think there are better ways to get people interested in astronomy. You can observe the moon through different phases, and with some binoculars on a broomstick (cheapest support you can find), a whole new world goes open to you. The Moon is interesting every day, not only during Full Moon.

Update: Eric Hall also posted an article on this blog about the "supermoon", detailing further what actually the distances were during the last twenty years. Nothing "super" about it ...

by Bruno Van de Casteele

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