Starry Skies Above Santa Monica

October 9-16, 2002

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   The Sun - Moving Southward in Virgo
   On Sunday, October 13, the Sun will rise at 6:57, culminate at 12:39, and set at 6:21 p.m. Thus it gives us 11 hours and 24 minutes of sunshine, making the day shorter by 15 minutes, compared to the preceding week.
   The Sun is moving through the constellation of Virgo, approaching Libra; it is currently located eight degrees below the celestial equator. On the Earth, this corresponds to Trujillo in Peru and to Recife in Brazil.

   Visible Planets: All Five in Principle, Two in Reality
  Yes, with some effort and luck, you might be able to see all five naked-eye planets in one night. But is it worth the effort? Only two are fairly easy to see, and even the two require some determination, since they shine in the morning hours.
   As mentioned above, the Sun sets at 6:21. Venus sets 40 minutes later, at 7:01 p.m. It is a very bright planet, but the western sky remains very bright for quite a time after sunset, making it difficult to see even such a bright object. The so-called “nautical twilight” ends at 7:15 p.m., and by that time the brightest stellar objects should become visible. So there is a slight chance that you may still spot Venus — very, very low above the southwestern horizon just a few minutes before it sets.
   But if you have a telescope, or at least good binoculars, try to see Venus! It is currently fairly close to the Earth. You know, of course, what we, the astronomers, mean by “fairly close”: 49 million km. Yet it will show you a fairly large, but quite thin crescent, resembling the Moon when it appears first above the west after the New Moon phase.
   When Venus disappears, the sky will gradually become darker, stars begin to shine, but no naked-eye planets are visible until several hours later.
   The first to show up will be Saturn, emerging above the northeast after 10 p.m. This planet shines as an ordinary star of the first magnitude, but a look at it through a telescope is very rewarding: you will see Saturn’s unique rings, surrounding the planet along its equator.
   Larger in the telescope is the disk of Jupiter: it is a larger planet than Saturn, and nearer to us. However, at present, you would have to wait for Jupiter till close to 2 a.m., since it is projected into the constellation of Cancer. Thus it will be better to wait a few more months for a comfortable evening seeing of this planet and its bright moons.
   Just for completeness, I must mention that Mercury shows up above the east by 5:30 in the morning, and is fairly bright – but for us, near the ocean, it rises above the huge city to the east of us, which certainly does not contribute to its visibility, even though the times of the dreadful smog appear to have passed.
   And while Mercury may be hard to see even if you are willing to search for it that early, then Mars is really hopeless. It rises shortly before Mercury, but shines currently only as a star of the second magnitude (approximately like the stars in the Big Dipper), and this, combined with its position, makes me say what my little grandson likes to say (a bit too often): “Forget it!”

   The Moon: Dominating Our Evening Sky
   Even if you manage to escape the city lights, this coming week is not very suitable for observing the distant universe: the Moon is quickly becoming too bright.
   Tonight, October 9, the Moon is still a very thin crescent, its disk being only 18 percent illuminated. It will shine close to the first-magnitude reddish star Antares in Scorpius. The crescent will set by 9 p.m.
   On the next several evenings, the illuminated portion of the Moon’s disk will gradually grow, and at the same time the Moon will shift to the east. However, it will remain located unusually far to the south from the celestial equator: more than 25 degrees on October 11 and 12, farther to the south than the Sun ever comes. The reason is that the orbital plane of the Moon is tilted by more than five degrees to the plane of the Earth’s orbit about the Sun. That latter plane is projected onto the celestial sphere into the great circle of the ecliptic, and the projected circle representing the Moon’s orbit goes five degrees above and then again below the ecliptic.
   This has several interesting consequences. An unpleasant one is that eclipses are rare. So let’s talk about one — merely interesting — consequence. The Sun, when the summer solstice comes, shines overhead (at the local noon) for people living at the southern tip of Baja California, or those living in Havana, Cuba. The Full Moon, when located at the northernmost point of its orbit, shines exactly overhead over Orlando, Florida.
   After October 13, the growing Moon will begin to move slowly northward during its overall eastward motion. This motion will carry it in front of the stars of Sagittarius, Capricorn, and eventually into Aquarius, where, on October 16, the Moon will shine as an 84 percent illuminated disk. It will then reach its Full Moon phase next week, on Saturday, October 21, and will then shine for the entire night – but definitely not overhead for us!