December 3 supermoon is the only one of the year

Occurrence marks an ideal skywatching opportunity.
By Laurel Kornfeld | Dec 05, 2017
The full Moon of Sunday, December 3-4 will be the only supermoon of 2017.

Supermoons occur when the Moon, which has an elliptical orbit, is at perigee, the closest point in its orbit to the Earth, at either the new or full phase.

"The Moon's orbit is not a perfect circle. It's more like an oval, so at times it's closer to the Earth than it is at other times. The difference in distance between these close and far points can be as much as 30,000 miles," explained Michelle Nichols, director of public observing at the Adler Planetarium in Chicago.

"For this year's supermoon, the perigee distance will be approximately 222,135 miles in the early morning hours of December 4 (in the US)," she said.

The Moon will appear about 16 percent brighter and seven percent larger on December 3-4. Because of a phenomenon known as the Moon illusion, it will look especially big when it is low on the eastern horizon at sunset and low on the western horizon at sunrise.

While the Moon appears bigger when low on the horizon, this is an optical illusion or trick played by a person's eyes.

However, for those who want to photograph the supermoon, doing so when it is low on the horizon is ideal, as it can be imaged against a background landmark.

Pictures can be taken with DSLR cameras and even with iPhones. NASA photographer Bill Ingalls advises those using DSLRs to capture the Moon against a background person or location and those with iPhones to lock the focus by placing their finger on the image of the Moon on the screen and adjust the exposure by sliding their finger either upward or downward.

Although this will be the only supermoon of 2017, it is the first in a series of three. The next two will occur on January 2 and January 31.



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