Categorized | Featured, Sci-Tech

A Bigger, Brighter Full Moon

By Andrew Cooper

Lunar Perigee and Apogee

The visual appearance of the moon at perigee and apogee.

This month the full Moon will be just a little bigger and brighter than usual. This Friday’s full Moon, the “Long Night Moon” as it occurs just before the winter solstice, may seem unusual if you take a moment to step outside and view it.

The Moon’s orbit is not exactly circular, close, but not quite. Like all orbits, the Moon’s path around the Earth is properly described by an ellipse. This means that at some point the Moon is a little further away, or closer to us, as it orbits our world. The difference is small, about 31,000 miles (50,000km) for an object that is 238,800 miles (384,400 km) away on average. When talking about orbits, the furthest point is called apogee, while the nearest point is called perigee.

This Friday lunar perigee occurs at 21:38UT (09:38am HST) and full Moon occurs at 16:37UT (06:37am HST December 12th). At perigee the moon will be at a distance of 221,560 miles (356,567km).

This month both perigee and full Moon will occur at almost the same time, only about four hours apart. This is just a happenstance of timing, and does not occur regularly. Because the moon is a little closer, this full Moon will be 14% bigger than it would appear at perigee, the furthest point in it’s monthly orbit.

This also means that this full Moon will be quite bright, about a third brighter than a typical full Moon. Since we are approaching the start of winter here in the northern hemisphere this Moon will also rise higher in the sky. As a full Moon occurs when the Moon is opposite the Sun, a low winter Sun results in high winter full Moons. A bigger, brighter full Moon rising high into the night sky.

If you have a chance, view the Moon as it sets Friday Morning or rises Friday evening. It should rise above the horizon shortly after the Sun sets. Sunset is about 5:45pm here on the Big Island with moonrise about 6:05pm on Friday, December 12th. Does it seem a little larger than usual? The first full Moon of January 2009, occurring on the 10th, will also be quite close, the biggest and brightest full Moon of 2009. If you miss this full Moon you get a second chance next month, though not quite as big and bright as the Long Night Moon.

Some useful links…

Fourmilab Apogee and Perigee calculator www.fourmilab.ch/earthview/pac…
US Naval Observatory Data Services (Moon phase, seasons, and more) aa.usno.navy.mil/data/

Andrew Cooper is an engineer, an amateur astronomer, a telescope maker and more. You can find his website at www.darkerview.com

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