Starry Skies Above Santa Monica

January 23-30, 2002

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   The Sun -
   Coming Back Very Reluctantly
   Last time, we compared the two consecutive Sundays, and found that during the week between them, the sunrise improved only by 2 minutes, and the sunset by 7 minutes. This time, things are still not much better. On Sunday, January 27, the Sun will rise at 6:54, a gain of only 3 minutes per week, and will set at 5:18, which is 6 minutes later. Thus we gain pitiful 9 minutes more daylight over the week. We should not complain, of course – this is about the time of the deepest winter for us here in Southern California, and imagine how many millions of people would happily accept this terrible winter we are having here!

   The Moon Rules
   Not much star watching can be done this coming week, since the Moon will dominate most of the nights. It will be exactly Full in the afternoon of Monday, January 28, but for a casual observer, it will appear “almost Full” essentially all week. On Wednesday evening, January 23, its disk will be 73% illuminated, and it will shine in Taurus, somewhat to the west of a nice fairly close pair of bright objects — planet Saturn and the first-magnitude star Aldebaran. Actually, the Moon will pass so close to Saturn that it will occult it, but the event will be visible only from Japan and its neughbors. We will have a similar bad luck with Jupiter: the Moon will pass close to it between Friday and Saturday, and will occult it for observers in Scandinavia. The Full Moon will be projected into Cancer, where there are no bright stars anyway, so we will not lose much. And then the Moon will continue into Leo, and pass above its first-magnitude star Regulus.

   Still Three Bright Evening Planets
   Mars crossed the celestial equator and moves through the constellation of Pisces (the Fishes).Mars has faded significantly since the time when it ruled our summer sky with its reddish glare, and now shines only as a star of exactly magnitude 1.0. However, it travels through a region of the sky where there is no competing bright star, and therefore Mars remains fairly prominent, but it sets fairly early, by 10 p.m. We must keep in mind that Mars is orbiting the Sun at a speed (of 24.1 km/s) that is not so much lower than our speed (29.8 km/s) – so that we are currently running away from it, leaving it behind, but Mars behaves like that nasty driver whom you have overtaken on the freeway but the lights of his car are still unpleasantly reflected by your rear view mirror. Fortunately, Mars is much less unpleasant!
   The two big planets, Saturn and Jupiter, shine high now above the eastern horizon in the early hours of the evening. The starry sky we see now in the evening is fairly rich in bright stars: at about 7 p.m., you can see 10 stars that qualify as true stars of the first magnitude. Saturn, by its brightness, fits among them, but Jupiter outshines even the brightest star, Sirius. Saturn culminates above the south already at 8 p.m., and Jupiter comes to the meridian two hours later. Even Saturn is sufficiently bright to be identified easily (it stands above, and close to, the reddish Aldebaran), but if you need additional help in identification, the Moon will be ready to assist on the evenings of January 23 and 24. On the following two evenings, the Moon will be fairly close to Jupiter.
   Venus is now formally an evening star. It passed by the Sun eastward on January 14, but since it moves far behind the Sun, on the opposite side of its orbit as seen by us, Venus makes very slow progress in receding from the glare of the Sun. Currently, it sets only 11 minutes after the Sun.

   A Little Bit More About Canopus
   Yes, this second brightest star in the sky does deserve a few words! As I told you last time, it is so far south in the sky that it actually passes overhead at Punta Arenas, at the southernmost tip of South America – yet we can spot Canopus from here, very low above the ocean in the south, currently between about 9 and 11 p.m.
   A star appears bright to us if it produces enough light (to speak as a professor of astronomy, I should say “if it has a sufficiently high luminosity”), and if it is not too far away from us in the universe. The apparently brightest star in the sky, Sirius, wins mainly because it is relatively very close to us (“only” 9 light years). Canopus appears just a bit fainter than Sirius, but it is shining at us from a much greater distance, namely 116 light years. Therefore, Canopus must actually produce much more radiation than Sirius does.
   There is actually one more factor unfavorable to Canopus: its surface temperature (about 7,400 degrees) is lower than that of Sirius (9,200 degrees); therefore one square inch of the surface of Canopus emits less light than the same surface unit on Sirius. Thus, if Canopus still wins, it is due to its significantly larger size. And Canopus does win: it emits as much radiation as almost 1,900 Suns, while Sirius manages to shine only as about 22 Suns. The radius of Sirius is about twice the radius of the Sun, but Canopus is what astronomers call “a luminous giant”: its radius is about 26 times the radius of the Sun. If Canopus replaced Sirius, it would appear to us about as bright as the First Quarter Moon – but we certainly would not like Canopus replacing our Sun! We are quite happy with what we have!