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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.
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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 discernible
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). [|
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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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