The full moon is called a 'supermoon' when it happens during one of its closest approaches to the Earth. The term was defined by astrologer Richard Nolle about 30 years ago, as "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." This can happen up to 6 times per year. This year, it happened on May 26th, then on June 22nd (the 'superest' supermoon of the year), and tonight is the third. The next looks like it might be on December 17th.
This all happens because the moon's orbit around the Earth isn't a circle; it's an ellipse. Its closest approach to the Earth (perigee) is around 354,000 kms, and its farthest distance (apogee) is around 410,000 km. However, its actual closest and farthest distances, as well as how far away it is when it's a full moon, varies from month to month. A supermoon is when the full moon happens when the moon is somewhere between (roughly) 354,000 kms and 360,000 kms away from us.
This animation from YouTube shows it nicely:
Astronomers observe a different event for the full moon coinciding with its closest approach to the Earth. This is the 'perigee full moon', and it's a bit more uncommon. The perigee full moon happens when the moon is full when it's actually at its closest approach to the Earth, every 13-14 months. The 'supermoon' back on June 22nd was also this year's perigee full moon, and this month perigee and the full moon miss each other by less than a day.
As Mr. Nolle's definition states, the supermoon can also happen with the new moon. This isn't quite as spectacular an event, of course, since the new moon is when the far side is lit up by the sun and the near side is dark, but you can be guaranteed that if there was a solar eclipse during a super new moon, the entire disk of the sun would vanish behind the moon.
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Now, even though this is a supermoon, the difference in size between this and any other full moon is actually pretty hard to see, especially just by eyeing it. The supermoon is definitely brighter, though, and that's certainly more noticeable.
There are times when the moon actually does look noticeably bigger, though. However, as Phil Plait, the Bad Astronomer (and one of my personal heroes), pointed out a few years ago, that's actually an optical illusion, and a really strong one at that. It all has to do with perspective, and how easily our brain can be tricked on matters of scale.
(Photo courtesy: REUTERS/Yannis Behrakis)
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