What is a supermoon? Is it a myth is mostly motivated by desire for publicity or is it true astronomical phenomenon?
The media like to hype the supermoon phenomenon, so arm yourself with the facts instead of the fiction: What is a supermoon?
What is a Supermoon?
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.
Supermoon: Fact vs. Fiction
When supermoons are in the news, you might hear startling statistics about size and brightness increases, but don’t be fooled. Any given supermoon may only be about several percent larger than the average full moon. Most sources advertise larger size increases because they're comparing against the smallest full Moon of the year.
A supermoon may also bring the largest tides of the year because a closer Moon exerts a stronger gravitational pull and creates more variation between the tides. However, this variation is hardly enough to account for massive earthquakes, flooding, or volcanic eruptions.
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 a 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. 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. So answers the question, "What is a supermoon?"