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A Whale of a Full Moon

A bigger, brighter full moon will herald the beginning of winter in the northern hemisphere on Dec. 22, 1999

The moon on the breast of the new-fallen snow
Gave the luster of midday to objects below....

             Twas The Night Before Christmas

Dec 19, 1999: Northern winter officially begins at 2:44 a.m. EST (0744 UT) on Dec. 22, 1999. That's the exact moment of the winter solstice, when the Sun reaches its lowest point in the sky as seen from the Northern hemisphere. The days are short and the nights are very long.

Although it's not officially winter yet, many revelers north of the equator are already enjoying snowy weather and the promise of a white Christmas. In fact, this Christmas promises to be even whiter and brighter than usual thanks to the extraordinary last full Moon of 1999.

Above: These images, based on a photograph of the full moon taken from the Lick Observatory, depict the relative apparent size of the moon when it is at apogee (right; maximum distance from Earth) and perigee (left; minimum distance from Earth). The next lunar perigee occurs on Dec. 22, 1999.

Several aspects of celestial geometry are conspiring to make the winter solstice of 1999 special. First of all, at 5:44 AM EST (1044 UT) on Dec. 22 -- just a few hours after the solstice -- the Moon makes its closest approach to Earth. The Moon circles our planet on a slightly elliptical orbit so that its not always the same distance away. At perigee, the Moon's closest approach, the Moon is 50 thousand km nearer to us than it is at apogee, its greatest distance. When the Moon is closest, as it will be on Dec. 22, it appears to be a whopping 14% larger than its minimum size at apogee.

This difference is particularly noticeable if the Moon is full at about the same time as apogee or perigee. That's exactly what will happen on Dec. 22. The Moon becomes full at 12:33 pm EST (1733 UT), less than 7 hours after its closest approach to Earth.

But that's not all.... The Earth also travels in an elliptical orbit around the Sun, with a closest approach near January 3rd. That's less than 2 weeks away. During this time of year, we're about 1.7% closer to the Sun than usual. Thus, the sunlight streaming down on the Moon will be 3% more intense on Dec. 22 than the year-round average.

Left: Our Moon's appearance changes nightly. This time-lapse sequence (Credit: António Cidadão) shows what our Moon looks like during a lunation, a complete lunar cycle. As the Moon orbits the Earth, the half illuminated by the Sun first becomes increasingly visible, then decreasingly visible. The Moon always keeps the same face toward the Earth. The Moon's apparent size changes slightly, though, and a slight wobble called a libration is discernable as it progresses along its elliptical orbit. During the cycle, sunlight reflects from the Moon at different angles, and so illuminates different features differently. A full lunation takes about 29.5 days, just under a month (moon-th). [more information]

All these factors -- the full Moon, lunar perigee, and the Earth's proximity to the sun -- will combine to make Wednesday's full Moon unusually big and bright.

How can you see it? You can't miss it! The Moon on Dec. 22 rises at 5:05 pm. local time and sets at 6:50 am. These times are approximately correct no matter where you live at mid-Northern latitudes. For the best view of this year's last full Moon, go out around midnight. That's when the dazzling orb will shine down from its highest point of the night.

Web Links

the Moon -- an overview from the SEDS Nine Planets web site

Jack Stargazer -- what's up in the sky every week from Jack Horkheimer

This week's sky at a glance -- from Sky &Telescope

The Astonishing Lunar Illumination of Dec. 22, 1999 -- from the Old Farmer's Almanac

Earth's Seasons -- A table of solstices and equinoxes from the US Naval Observatory

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For more information, please contact:
Dr. John M. Horack , Director of Science Communications
Author: Dr. Tony Phillips
Curator: Linda Porter
NASA Official: M. Frank Rose