Starry Skies Above Santa Monica

October 3-10, 2000

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   One Night with Planets and Stars
   This time, we will follow the astronomical events of the Saturday/Sunday night, October 6/7. The Sun is already more than 5 degrees below the celestial equator, in the constellation of Virgo, and sets by 6:30 p.m. The astronomical night, when you should see stars without any interference from Sun’s rays in our upper atmosphere, starts before 8 p.m. However, very soon the Moon will rise -- on Saturday night shortly after 9 p.m. -- and since it is, on the weekend, still almost Full, it will dominate the sky and allow only the brightest stars to remain visible.
   It has been even worse on the preceding nights. The Moon was Full in the morning of Tuesday, October 2. When the Moon is full, it -- naturally -- stands opposite to the Sun in the sky. This means that the Moon is projected into the constellation of Pisces (the Fishes), a few degrees above the celestial equator. On the following nights, the Moon travels through the constellations of Aries and Taurus. This is a path that leads in the northeastern direction, and therefore on the following nights, from Wednesday through Saturday, the Moon remains “an early riser” in the sense that it rises early in the evening, is still nearly full, and therefore the visibility of stars is limited.
   Nevertheless, if the sky is clear, you should have no problem in finding the planet Mars on any evening. You can find it in the south (it has culminated, directly above the southern point of the horizon, already at 7 p.m.). No star shining above the horizon at that time is brighter than Mars; moreover, you can identify the planet by its yellow-reddish light. As the evening progresses, Mars descends to the south-west, and disappears before midnight.
   For the next visible planet, you should look to the east, where Saturn emerges already before 10 p.m. Saturn, shining as a star of the first magnitude, has been slowly receding westward with respect to the stars, which in this case means that it is moving back to the (equally bright) orange-reddish star Aldebaran, the brightest star in Taurus (the Bull). Saturn is still located somewhat to the northeast of Aldebaran.
   The Moon will pass quite close above Saturn, but this will happen at the time of our noon on October 7. This means that on the night of October 6/7, you will see the Moon preceding Saturn (the Moon will be to the west of it), and when the following night comes, the Moon will be already behind Saturn, moving towards Jupiter.
   Jupiter now rises just shortly before midnight, and the Moon will pass above the planet on October 9, again during our day, so when you look on the night of October 9/10, you will find the Moon already to the east of Jupiter. Both objects are projected into the constellation of Gemini. You should have no problem in identifying Jupiter: it is the brightest object in the sky after midnight.
   A successful competition comes only when Venus rises above the eastern horizon, but this happens very late (or shall I say very early in the morning?), at 5 a.m. Venus is moving from Leo to Virgo, and coming ever closer to the Sun. Clear signs of dawn show at about 5:45, and before 7 a.m., the Sun will appear above the horizon.

   Two Planets Leaving our Night Sky
   Naturally, no planet is actually leaving and none is coming into our solar system. I only mean their visibility in our sky. The two planets that “are leaving” are Mars and Venus. They have adorned our starry sky for quite a few months, but the time of their glory is coming to an end. Mars was especially easy to see, being an evening planet. We are -- at least most of us -- essentially evening people. Unless we must get up early, we prefer to stay up longer, and spend evenings with our friends or in front of the TV.
   Perhaps when seeing your friends off, many of you may look at the starry sky, if clouds, fog, and city lights permit. Then you may still spot Mars in the south-west, as described above. The best time to see Mars was in June, when Mars was in opposition to the Sun. That was the time when the Sun, the Earth, and Mars lay on a straight line, in this order, and therefore Mars shone all the night — it was rising at sunset and setting at sunrise. Since June, Mars has been moving eastward, but our Earth has been moving faster, leaving Mars more and more behind. For star watchers this means that Mars appeared to set earlier and earlier, and remains visible only in the evening.
   Venus, orbiting closer to the Sun than the Earth, has been behaving differently. It travels in its shorter orbit faster than the Earth, and is now getting farther and farther from us on that part of its orbit than carries it, from our point of view, behind the Sun. As our Earth rotates on its axis once per day, it makes it possible for us to see Venus rise before the sunrise. However, the apparent separation from the Sun has been steadily decreasing.
   In July, Venus rose 3 hours before the Sun. At the moment, the interval of visibility has shrunk to 2 hours, and at the end of November, Venus will be hardly visible at all. It will rise only 50 minutes before sunrise, when the sky background above the east is already quite bright.