Starry Skies Above Santa Monica
March 28 - April 4, 2001

Mirek Plavec
Emeritus Professor of Astronomy, 
UCLA

   Spring Sky: Bright Moon, Fewer Planets, Fewer Stars
   As spring progresses, the winter constellations, so rich in bright stars, gradually disappear in the west, and the spring constellations are beginning to dominate our evening sky. And they become visible later and later, as the Sun continues to move northward along the ecliptic, and we get more and more daylight. And, on Sunday, April 1, we will also begin Daylight Saving Time, so from now on, the times I give here will be expressed in the Pacific Daylight Saving Time. 
   On Sunday, April 1, the Sun will rise by 6:40 (a gain of 10 minutes per week), and set by 7:14 p.m., a gain of 5 minutes per week, so our day will last 12 hours 34 minutes. On that day, the Sun will be already almost 5 degrees above the celestial equator, and will pass, at local noon, through the zenith of Santa Fe de Bogota, Colombia, and of Cayenne, Guyana.
   The Moon was “New” in the afternoon of Saturday, March 24. After that, the Moon has been climbing fast to the north-east, and within about 3 days, it became visible after sunset, as a thin crescent. On Wednesday, March 28, the Moon will already set as late as 11 p.m., and you will be able to see it that evening just below the planet Saturn. It will still be a very thin crescent: only 17% of its disk will be illuminated. A bit thicker crescent will then shine east of Jupiter and above Aldebaran on the next evening, March 29. The Moon will reach its First Quarter in the wee hours of April 1, and you will be able to see it below — but close to — the Gemini Twins, Castor and Pollux. By Wednesday, April 4, the Moon will appear to be almost Full, and will shine above Regulus in Leo (the Lion).
   Jupiter and Saturn are still easily visible in the evening sky. You can spot Jupiter already by about 7:45 p.m.; search for this bright planet about half-way between the south-western horizon and the zenith. Saturn, located to the west of Jupiter, will show up somewhat later, since it is much fainter than Jupiter, but still competes in brightness with Aldebaran. Both planets are moving eastward with respect to the stars of Taurus (the Bull): Saturn is now to the south of the star cluster Pleiades, and Jupiter is above the less compact cluster of the Hyades. Aldebaran is the brightest star in that group, but actually does not belong to it. The entire assembly of planets and stars in Taurus disappears in the west not long after 10 p.m. 
   After midnight, Mars emerges deep in the southeast, and culminates above the south about one hour before sunrise. Mars is projected almost on the ecliptic (the apparent path of the Sun among the stars), but close to its southernmost spot, between the constellations of Scorpius and Sagittarius. Curiously, that segment of the ecliptic actually passes through the southern outskirts of the constellation Ophiuchus, which is not part of the traditional “zodiac.”

   The Disappearing/Reappearing Act of Venus
   In the preceding issue, we said “Good-bye” to Venus, which had for many weeks adorned the evening sky after sunset. On March 30, Venus passes through its “inferior conjunction” with the Sun (will be located between the Earth and the Sun), and switches from west to the east of the Sun. Thus it will change from the Evening Star to the Morning Star. To use the traditional names, Venus switches from “Hesperus” to “Lucifer.” The last name actually means “bringing light,” but by some twist of meaning, this name became equivalent to “the devil.”
   Venus certainly does not deserve being associated with the devil! In the forthcoming months, it will shine beautifully above the eastern horizon for those who must leave their homes very early and drive eastward before the sunrise. This week, Venus remains too close to the Sun. At the end of the week, on Thursday, April 4, the Sun will rise by 6:36, and Venus by 5:45 – a bit too close for comfort. However, Venus will very soon become prominent as the morning star: it is just bypassing us in our orbit about the Sun, and will get ahead quite fast. 
   The orbiting of the planets about the Sun is really not a fair race! Those that move at a higher orbital speed also have a shorter orbit. Thus Venus moves ahead at a speed of 35 km/s, while the mean speed of our Earth is only 29.8 km/s. We are not moving exactly at a constant speed. When we are closest to the Sun, in early January, we move a bit faster (at 30.3 km/s), while at the beginning of July (when we are farthest from the Sun), we slow down to 29.3 km/s. The variation, as you can see, is quite small, and it is even smaller for Venus, which moves in an orbit that is even closer to a perfect circle than is our orbital ellipse. Venus completes one orbit about the Sun in only 225 days.
   The two large but distant planets, Jupiter and Saturn, move much more slowly: Jupiter at 13 km/s, Saturn at 9.7 km/s; and their orbital periods are close to 12 and 29 years, respectively. This is why they appear “almost stuck” at their places in the sky, with respect to the stars.