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
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.
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.
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.
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.
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.
Related Links
Moon
View - From BeachBrowser.com
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
TOP
