Tuesday, October 11, 2011

Smallest Full Moon of 2011 Occurs Tuesday

In my more than 40 years as an amateur astronomer, I've given countless numbers of people views of a variety of celestial objects through telescopes. And most people usually will tell me that the first object in the sky that attracted their attention as youngsters was the ever-changing moon.

Many may recall the attention given to our nearest neighbor in space in March when that months' full moon very nearly coincided with the lunar perigee — the point in the moon's orbit in which it is nearest to Earth. At a distance of 221,565 miles (356,575 kilometers), some media outlets even christened it the "Supermoon" of 2011 because of its unusual closeness.

But on Tuesday night (Oct. 11) we will have the astronomical opposite of a supermoon. At 10:06 p.m. EDT (0206 GMT), the moon will officially turn full, but it will be the smallest full moon of 2011.

The sky map of the smallest full moon of the year shows where it will appear on Tuesday night when it reaches its full phase.

Less than 10 hours after the October full moon peaks, at 8 a.m. EDT (1200 GMT) on Wednesday, the moon will arrive at the apogee of its orbit (the farthest point from Earth each month), which in October is a distance of 252,546 miles (406,434 km). That's only 154 miles (248 km) shy of the moon's absolute farthest point from Earth it can reach.

A puny full moon
Looking at the moon during Tuesday night, perhaps you will be struck by the noticeably small size of the moon's disk, even when it's near the horizon (which is supposed to make it appear larger as a result of the familiar "moon illusion" effect).

Last March, when the full moon was at perigee, some reported it as looking absolutely enormous as it emerged from above the eastern horizon. Not so on Tuesday, however, because the moon will very close to apogee, making this the smallest full moon of 2011.

By coincidence, October's full moon comes just days after the second annual International Observe the Moon Night on Oct. 8, in which skywatchers participated in hundreds of lunar observing events at skywatching parties around the world.

In fact, it will appear 12.3 percent smaller than the so-called supermoon of March 2011.

And because it is near its maximum distance from the Earth when full, the moon will also be traveling relatively slowly in its orbit. So both effects will combine to make its motion against the background stars especially small from night to night.

As a result, to the casual skywatcher it will seem that the moon is full for not just one night, but for three nights in a row: Monday, Tuesday and Wednesday. The defect of illumination on the moon's disk — or put another way, the dark sliver near the edge — will be very slight on Monday and Wednesday night.

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