Starry Skies Above Santa Monica

November 7-14, 2000

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   A Third Advance Notice: Meteors! Meteors!
   For many centuries, comets were believed to be either heralds or direct carriers of all possible disasters affecting humanity: wars, fires, epidemics, deaths of kings (with a civil war often following). Something in their behavior did indeed encourage these fears: A bright comet usually appeared in the sky without warning, and had a strange shape: a bright head followed by a very long tail. Thus, if we lived in the past, and if a bright comet appeared last August, it would be held responsible for the terrorists and for anthrax!
   Today we know that the statement “A comet is a full bag of nothing” is much closer to the truth. The tail is formed by tiny dust particles and gas molecules escaping freely from the head. The density of material in it is unbelievably low And the head –- often quite big (the head of the comet of 1811 was bigger than the Sun) –- is not much denser, again very rarefied gas. The real mass of the comet is concentrated in its nucleus. This is just an agglomerate of rocks, boulders, gravel, and dust, held together loosely by ice. The nucleus typically measures only several miles across, so that the comet’s self-gravitation is very weak. No wonder that even the nucleus gradually disintegrates, especially when the comet passes near a planet. Even more powerful are occasional explosions caused by a rapid heating of the nucleus if and when the comet approaches the Sun too close, as fairly often happens.
   The gas molecules and dust simply disperse into space. The heavier pieces -– gravel, stones –- separate from the comet but essentially follow its orbit. They tend to form clouds of material either behind or in front of the comet. Even these clouds are invisible to us, except when the comet’s orbit happens to intersect the orbit of the Earth. Then, when we collide with such a swarm of stones and gravel, we observe them as meteors: they heat up by collisions with air molecules and shine briefly before they are pulverized.
   Out of the large collection of comet orbits, only a few happen to cross our own orbit. Since each comet gradually disintegrates, we do see at least a few meteors every year when we pass through such an intersection.
   It happens much less often that we run into a dense swarm of meteors, ejected by their parental comet during one of its explosions. Such an exceptional event may occur in the pre-dawn hours of the night of Saturday/Sunday, November 17/18. We hope to encounter a fairly dense swarm of meteors ejected by the comet Tempel-Tuttle.
    The reader may lift the eyebrows here and repeat after me:”…MAY occur? We HOPE? What kind of talk is it for a scientist?!” Well, realistic! We can’t see even a extremely dense cloud of meteors until we run into it! We can only rely on a prediction based on a careful examination of past events.
   And detailed studies tell us: There is a good chance for us to see plenty of meteors! How many? Probably several hundreds per hour, starting around midnight – and our chances will be getting better towards the morning, but, unfortunately, after 5 a.m. on Sunday, dawn will put an end to the show, just when it will probably be becoming really exciting. What can we do? Not many of us can travel to Hawaii or East Asia to watch the expected climax of the show – perhaps more than 10,000 meteors per hour!
   If you are interested, plan to get away from the city lights and from our endless morning fog. The best place would probably be somewhere in the desert, with a clear view of the eastern sky -– but watch the weather forecasts carefully before you go!

   Evening Planets: Two Coming, One Leaving
   The Earth is getting closer to the place where we will have the two bright planets, Saturn and Jupiter, on the opposite side of the sky than is the Sun. In more scientific terms, the time is approaching when Saturn, and then Jupiter, will be in opposition to the Sun. For Saturn, this will happen at the beginning of December. Jupiter’s opposition time will not come until after the New Year. Nevertheless, both planets are already rising in the evening. Saturn, in Taurus, rises already after 6 p.m. (the Sun sets by 4:52 p.m. on Sunday, November 11). Jupiter, located farther east, in the middle of the constellation Gemini, will rise at about 8:30 p.m., in the north-east. Saturn shines like a star of the first magnitude, and is brighter than the real star of the first magnitude, Aldebaran, which precedes Saturn. Jupiter is so much brighter that you will recognize it for sure at first sight, especially if you wait a bit after its rising time, when it gets higher in the sky.
   There is a third planet visible in the evening, Mars. The opposition time is long past for Mars -– it occurred in June. However, Mars is now moving a bit faster eastward, among the stars of Capricorn, and so it still remains visible and fairly conspicuous in the evening sky. When the sky gets sufficiently dark, look for Mars in the south-west, it has no competition there. It sets by 10:30 p.m.

   The Morning Sky:
   The Moon, Venus, and Mercury
   The Moon reaches its Last Quarter in the morning of November 8. It is then near the first-magnitude star Regulus in Leo. (By the way, the “radiant point” of the meteors I promised to you lies not far above Regulus). As the days of the week proceed, the illuminated portion of the disk of the Moon shrinks, and at the end of “our SMM week,” on the morning of Wednesday, November 14, it will be an extremely thin crescent, unlikely to be spotted, since it becomes New, and therefore completely invisible, on the following night.
   The Sun, projected into the constellation of Libra, will rise by 6:20 a.m. The first obvious signs of dawn will be perceptible about one hour before that. The two planets that currently adorn the morning sky, Venus and Mercury, will emerge in the south-east almost simultaneously – but they will rise late, after 5 a.m. Thus you must have a very good view of the eastern horizon to see them, and, on top of that, you must be an early riser, and eager to look at the sky on top of that!