Starry Skies Above Santa Monica

December 5-12, 2000

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   Earliest Sunset
   Yes, it is coming this week, namely on Friday, December 7, at 4:44 p.m. Actually, the time of sunset, within the precision of 1 minute, remains the same between December 1 and December 12; only after that, sunset begins to come later, but the change is very slow at first: on December 31, sunset comes at 4:54, that is, 10 minutes later.
   In spite of this slowly increasing duration of the afternoon, the length of the day continues to decrease until the time of the winter solstice, which comes on December 21. This means that the time of sunrise must still be coming later at the present. And indeed: on December 7, the Sun rises at 6:45; on the day of the solstice it rises at 6:55, and on the last day of the year, the Sun rises at 6:58. The shortest day coincides with the solstice day.
   Small changes in the time of sunrise and sunset do not play any important role in our lives -– although, when I drive from UCLA to Pacific Palisades on Sunset Boulevard just close to 5 p.m., the accurate time of sunset plays a significant role, since the setting Sun shines, on certain days, directly into the driver’s eyes, making driving a bit hazardous.
   For people living well to the north of us, the solstice means a lot more. We live at latitude 34 degrees, and our shortest day lasts 8 hours 53 minutes. Prince George and Edmonton, in Canada, lie near latitude 54 degrees, where the shortest day lasts only 7 hours 22 minutes. As you go farther north, the length of the day decreases very rapidly. Thus people living in Helsinki, Finland, or Saint Petersburg in Russia (perhaps still better known as Leningrad), at latitude 60 degrees, can enjoy the winter solstice sun for less than 6 hours. People living in Iceland, at latitude 65 degrees, have a day of only 3.5 hours. And people living north of the “Tropic of Cancer,” i.e. north of latitude 66.5 degrees, will not see the Christmas Sun at all.
   What does the term “solstice” mean? From the name one could guess that the Sun somehow stops in its motion and stands still. This would be, of course, a naïve interpretation, since the Earth continues to move smoothly in its orbit, and this motion is reflected in the apparent motion of the Sun with respect to the stars. Thus the Sun, as seen by us, continues to move along its apparent orbit among the stars (the ecliptic), and steadily proceeds eastward at a rate of nearly 1 degree per our complete day (24 hours). This number is easy to understand: the circle has 360 degrees and the year has just a bit more, 365.2442 days. This is not a chance similarity!
   However, the ecliptic is tilted to the celestial equator (the projection of the terrestrial equator onto the celestial sphere) by somewhat more than 23 degrees. Thus, as the Sun moves eastward along the ecliptic, it also moves somewhat to the north or to the south, depending on the time of the year. On September 22, the Sun crossed the celestial equator, moving southward. We had the Autumnal Equinox, day and night were of the same length. Since that date, the Sun kept moving southward, and will reach the southernmost point of the ecliptic on December 21. Then it will begin to move northward. The moment when the Sun reverses its motion from southward to northward is called “solstice,” a not very illuminating term – and actually a bit confusing. The Czech term is “slunovrat,” indicating that the Sun begins to “return,” i.e. comes back north.

   Where the Planets Are
   Mercury and Venus, the two planets closest to the Sun in space, happen to be currently projected close to the Sun in the sky, too, and are therefore invisible.
   The first planet beyond the Earth, Mars, is also getting closer to invisibility, yet can still be seen in the evening above the south-west; it sets there after 10 p.m.
   The two giant planets, Saturn and Jupiter, now shine practically during the entire night. Saturn rises in the north-west already before sunset. It forms a nice pair with Aldebaran, the first-magnitude star in Taurus (the Bull). Saturn is brighter and yellowish, Aldebaran tends to have a reddish color. Jupiter, farther to the east of this pair, now rises at about 6:30 p.m., and is significantly brighter -– in fact, if the Moon is absent, Jupiter is the brightest night-time object. Next to Jupiter in brightness is Sirius, a real star in the constellation of Big Dog (Canis major). While Jupiter rises in the northeast, Sirius rises in the south-east, and shows up at about 8:30 p.m. By that time, the entire beautiful constellation of Orion, located between Jupiter and Sirius, is already above the eastern horizon.

   The Moon Also Deserves a Few Lines…
   The Moon starts our week in its decreasing phase (it was Full on November 30). On December 5, it is located in the constellation of Leo, and rises before 10 p.m. On each consecutive night, it rises almost one hour later, and passes through Virgo into Libra. Only an extremely thin crescent remains at the end of our “SMM week.” Then the Moon becomes invisible; the “New Moon” comes on December 14. We should be thankful to the Moon, since the meteor shower of the Geminids will have its peak activity on the night of December 13/14. More about the Geminids next time!