If the moon looks brighter and bigger than usual this weekend, it’s not because you’ve been imbibing too much — it actually IS brighter, and though the moon cannot change size, it will also LOOK bigger from the Earth. It’s a phenomenon called a Supermoon.
Perigee And Apogee
The moon does not circle the Earth, the moon ellipticates the Earth (goes around the Earth in a path shaped like an ellipse), if I might coin a word usage. [editorial note: the word ‘ellipticate’ has been used in reference to exercising on a certain machine; this is a new usage]
Each month, the moon has a closest approach to, and a farthest departure from, the Earth; these points are called perigee and apogee. It stands to reason that when the moon is closer to the Earth, it will look bigger and brighter.
The Full Moon
The moon is full when it is on the opposite side of the Earth from the sun and the entire face is illuminated. Technically when the moon, Earth, and sun are exactly aligned, the moon is in eclipse, but any time within a day or so of alignment produces a full-face moon as near as anyone can tell with the naked eye.
The moon ellipticates the Earth in 27 days and eight hours; there is a full moon every 29 days, 12 hours and 44 minutes. ‘How’s that?’ you ask.
Well, in the time it takes for the moon to revolve once around the Earth, the whole Earth-moon system has traversed approximately one-twelfth of its path around the sun. So after going around the Earth, the moon needs another two-plus days to get re-aligned with the Earth and sun.
What Is A Supermoon?
When the full moon occurs near the time of perigee, the moon appears bigger and brighter simply because it is closer. There is a Supermoon about once a year (perigee and full moon align in an approximately 14 month cycle), and there is a longer cycle of approximately 18 years that determines Super-Supermoons. The Superest moon of this cycle was in 2011, so we are in the waning stage of the cycle. This weekend’s will be the Superest moon for quite a while.
Supermoon, August 10, 2014
Perigee brings the moon to within 221,786 miles of the Earth at about 1 p.m. EDT on August 10. This is nearly 500 miles closer than the Supermoon of July 13, and within 250 miles of the closest approach of the Super-Supermoon of March, 2011.
At its biggest and brightest, the Supermoon appears 15% bigger and 20% brighter than an average full moon.
Where And When To See The Supermoon
Since the full moon is opposite the sun, viewers in the northern hemisphere will see the moon rise before the sun sets (there are more than 12 hours of daylight). This mutes the effect until the moon gets well above the horizon and the sun’s glow fades. Decoded Science recommends viewing the moonrise on August 11, when it will occur after sunset and might be the more engrossing spectacle. There will also be an only-slightly-less-super Supermoon on Sept. 7. That will also be the Harvest Moon.
What Will Viewing Conditions Be Like Where You Are?
Summer afternoons in much of the US are often punctuated by showers and thunderstorms. In addition, sluggish weather patterns produce stagnant air and haze. The latter can make for a colorful moonrise — or simply obscure the whole show.
Currently, the forecast calls for good viewing in California, the desert southwest, most of Texas, and the entire northeast south to Virginia and west to Wisconsin. Check the local forecast where you live; even areas with thunderstorms could offer a glimpse of the Supermoon through breaks in the clouds..
If you miss this Supermoon, it is no tragedy. They are common, and though Supermoons in the next ten years or so may be slightly less Super than this one, the difference will not be noticeable to the untrained eye.
A Different View Of The Supermoon
Most people will watch the Supermoon rise, or view it in the middle of the night. For a different perspective, get up and watch it set. You probably won’t have much company: The moon sets in Boston (where I’ll be) at 4:22 on Saturday and 5:36 on Sunday. Since sunrise is around 5:45, Monday’s moonset will be after the sun is up.
Wherever you are, if the weather permits, check out this Supermoon. It will be spectacular at any hour.