The perpetuation of the supermoon myth is mostly motivated by desire for publicity. But much of what we call the supermoon is just our eyes playing tricks on us.
Beware the continuing media hype about supermoons. The upcoming supermoon, technically occurring at 7:32 a.m. (EDT) this Sunday in North America, will no doubt have some in a tizzy, so here are the facts.
The term “supermoon” was first coined by astrologer Richard Nolle to describe the full or new Moon when it's less than 223,000 miles (359,000 km) from Earth. That’s about 6% closer than the average Earth-Moon distance, which is 239,000 miles.
Despite being closer than usual, supermoons are rather ordinary and, by Nolle’s definition, take place several times a year. But the media won’t hype that up — can you imagine headlines publicizing “Come out and see the Moon that's a little bigger than normal!” or “Look out for this amazing event that happens several times a year!” — yeah, me neither.
Fact vs. FictionOver the next few days, you might hear startling statistics about size and brightness increases this weekend, but don’t be fooled. This supermoon will only be about 7% larger than the average full moon. Most sources advertise a 14% size increase because they're comparing it with the smallest full moon of the year.
This weekend's supermoon will also bring the largest tides of 2013, because a closer Moon exerts a stronger gravitational pull, creating more variation between the tides. However, this variation is hardly enough to account for massive earthquakes, flooding, or volcanic eruptions. (See why here.)
The Moon Illusion
But even though the Moon isn't noticeably bigger on the night of the supermoon, at least not without some kind measuring tool, lots of folks will head out to see a huge full Moon looming over the treetops. The so-called Moon Illusion causes the Moon to appear bigger when viewed along the horizon and can fool people into believing the supermoon myth.
Nobody quite understands why the Moon illusion happens, but here are two of the theories psychologists think might explain this effect.One theory claims that our brain perceives the distance to the sky directly overhead as closer than the distance to the horizon. We think the Moon must be bigger along the horizon because we perceive the distance to the horizon as farther away. Don't buy the theory? Look at the picture to the right. The top line seems larger than the bottom one. But measure for yourself and, surprisingly, you'll find the lines are the same size. Psychologists have also suggested that an overhead Moon looks smaller because it’s surrounded by empty space, while a Moon near the horizon appears larger juxtaposed against trees and buildings. The picture on the right shows two orange circles of identical size. But the orange circle on the right appears larger than the one at the left because of its relative size to the surrounding circles.
So the supermoon might look bigger than normal if you see it in the evening when the Moon's just rising, but the real size difference isn't big enough to notice.
Then what is there to look forward to? As always, a full Moon is a great opportunity for astrophotography — it’s so bright that you only need exposures of a fraction of a second, just like daylight photos. The supermoon technically happens at 7:32 a.m. (EDT) on Sunday morning, but the Moon will shine just about as full and bright at sunset on Saturday as it will at sunset on Sunday.
And despite the fact that the supermoon isn't as extraordinary as some might have you think, any full Moon is still a spectacular sight to behold.