Astro Net

(3) a. The Brightest Moon of the Century

Since moonshine is really reflected sunlight, the full Moon is on the average slightly brighter in mid-winter, when Earth is closest to the Sun. In addition, the distance of the full Moon also varies, due to the ellipticity ("eccentricity") of the Moon's orbit. On 22 December, 1999, the two effects combined to produce the brightest full Moon of the century. On the preceding day, NASA released the following "Educational Update":

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    This year will be the first full moon to occur on the winter solstice, Dec. 22nd, commonly called the first day of winter. Since a full moon on the winter solstice occurs in conjunction with a lunar perigee (point in the moon's orbit that is closest to Earth), the moon will appear about 14% larger than it does at apogee (the point in it's elliptical orbit that is farthest from the Earth). Since the Earth is also several million miles closer to the sun at this time of the year than in the summer, sunlight striking the moon is about 7% stronger making it brighter. Also, this will be the closest perigee of the Moon of the year since the moon's orbit is constantly deforming. If the weather is clear and there is a snow cover where you live, it is believed that even car headlights will be superfluous.

    On December 21st. 1866 the Lakota Sioux took advantage of this combination of occurrences and staged a devastating retaliatory ambush on soldiers in the Wyoming Territory.

    In laymen's terms it will be a super bright full moon, much more than the usual AND it hasn't happened this way for 133 years! Our ancestors 133 years ago saw this. Our descendants 100 or so years from now will see this again.

    Remember this will happen December 22, 1999.

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(Superfluous or not, we hope drivers that night kept their headlights shining!)

Additional Exploring

"Astronomy Picture of the Day" devoted two sites to the above phenomenon:

On 12.22.1999 it showed the difference in the apparent size of the Moon,
      between its closest and most distant positions.
On 1.13.2000 it showed a striking picture of the "brightest moon."

Next Stop: #4 The angle of the Sun's Rays


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