Starry Skies Above Santa Monica

December 12-19, 2000

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   Geminid meteors coming!
   A few meteor showers come regularly every year, with not much change in hourly rates. Best known among them are the Perseids, flashing in the sky for several nights around August 13. Their only disadvantage is that the radiant (the point from which they appear to “radiate”) rises late at night, so that the best viewing conditions come after midnight. The Geminids are the only regular meteor stream that can compete with the Perseids as to the hourly number of meteors you can see. Their disadvantage is that the nights are much cooler in December. Their advantage is that they are active in the evening hours. The point from which the meteors seem to radiate is located close to the star Castor, which is the northern partner in the bright couple for which the constellation was named (the southern star is Pollux). Now this pair of stars currently rises, in the north-east, already shortly after 7 p.m., so in the evening hours, you may already expect to see quite a few meteors coming from the north-east.
   The Geminid stream is active for about three nights. The peak activity is predicted for the night of December 13/14, and you can expect to see about one meteor per minute. Although the show starts in the evening, it is nonetheless true that the peak activity will probably come when the radiant is really high in the sky, and that is later in that night. So, for most of you, the message is probably hardly more than: when you happen to see one or more meteors on Thursday evening, you have probably seen members of the Geminid meteor stream.
   The Geminids are interesting for the relatively low speed at which they penetrate our atmosphere: 35 km/s. The meteors of the August meteor shower, the Perseids, arrive at a speed of 60 km/s. This should make the Perseids brighter, but the Geminid meteors are also quite impressive, with a number of real “fireballs.” It seems that the Geminid meteors consist of a more solid material –- stones and gravel, while the Perseids may be something like “dirty snowballs.”
   The Moon is very kind to the Geminid meteor observers this year: it is New, and therefore invisible, just on December 14!

   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 southwest; it sets there after 10 p.m. It is projected into the constellation of Aquarius (the Water Bearer).
   The two giant planets, Saturn and Jupiter, now shine practically during the entire night. Saturn rises in the northwest 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. The pair is rather conspicuous in the eastern sky in the early evening hours – you should have no difficulty in recognizing it.
   Jupiter, farther to the east of this pair, now rises at about 6 p.m., and is significantly brighter -– in fact, if the Moon is absent, Jupiter is the brightest nighttime 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 southeast, 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.

   … And where is the Sun?
   The path of the Sun on the celestial sphere, the ecliptic, has traditionally been divided into 12 segments, often called the “sun signs.” And I am sure that for many readers of this column, the only contact with astronomy is just the knowledge of the sun sign under which they were born. Your sun sign is simply one constellation located along the path of the Sun. Thus each sign corresponds to a constellation through which the Sun is passing during one month. However, the Sun enters a given sign (constellation) on the 21st of each month (because our months have unequal lengths, it could sometimes be the 20th or 22nd).
   You probably know all the twelve sun signs: Aries (the Ram), Taurus (the Bull), Gemini (Celestial Twins), Cancer (the Crab), Leo (the Lion), Virgo (the Virgin), Libra (the Scales), Scorpius (the Scorpion), Sagittarius (the Archer), Capricorn (the Sea Goat), Aquarius (the Water Bearer), and Pisces (the Fishes).
   So where is the Sun these days? When astrology was “codified,” some 2,000 years ago, the early December Sun was projected into Sagittarius. Actually, the Sun is currently traversing the constellation of Ophiuchus (the Serpent Bearer). “But this constellation is not a member of the famous twelve!” you will exclaim, quite justly. Two factors confuse things here. One is a phenomenon called “precession,” which has shifted the traditional sun signs by about one, so that, at the time of the spring equinox (about March 21) the Sun does not enter Aries, but Pisces. Aries gets its turn on April 20. Correspondingly, the early December Sun should be traversing Scorpius.
   While we astronomers are not responsible for the nasty phenomenon of precession, we are responsible for the other complication. When the boundaries of the constellations were established by an international astronomical committee, an effort was made to preserve the traditional constellation patterns. And, as a consequence, the Sun actually travels only rather shortly through Scorpius, and for a longer part of the month, it is projected into Ophiuchus.