Starry Skies Above Santa Monica

September 25 - October 2, 2002

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   The Sun - Now Above the Southern Hemisphere
   On Sunday, September 29, the Sun will rise at 6:46, culminate at 12:43, and set at 6:40 p.m. Thus it gives us 11 hours and 54 minutes of sunshine, making the day shorter than the night.
   The Sun is projected into the constellation of Virgo, almost three degrees below the celestial equator, so it shines exactly overhead, at the local noon, above places whose terrestrial latitude is three degrees to the south of the terrestrial equator.

   Venus – About to Leave Our Evening Sky
   Venus is projected to the southeast of the Sun, thus marking the direction in which the Sun will be moving (with respect to the stars) in the forthcoming weeks. While the Sun is projected into Virgo, Venus adorns the next constellation of the zodiac, Libra (the Scales), and lies now 22 degrees below the celestial equator. And Venus appears to be receding back to the Sun. These two factors combine to shorten the time of its visibility. It now sets shortly before 8 p.m., and becomes visible at about 7:20, so you will be able to see the planet for no more than about 40 minutes, and you must search for it low above the south western horizon.

   Two Bright Planets Later at Night
   Saturn appears above the eastern horizon already shortly after 11 p.m. It is still projected into the edge of the constellation Orion – that short “peninsula” of Orion actually divides the two adjacent constellations of the zodiac, Taurus (the Bull) and Gemini (the Celestial Twins). Saturn shines as an ordinary star of the first magnitude, and in its vicinity there are several stars that compete with it as to brightness. Below Saturn are two such stars in Orion, Betelgeuze (fairly close to Saturn), and Rigel, farther to the south.
   Jupiter is still a morning object, rising as it does shortly before 2:30 a.m. Jupiter is projected into the constellation of Cancer, which has no bright stars. The nearest bright stars are the two stars in Gemini, Castor and Pollux, preceding Jupiter in the sky, and then Regulus in Leo, which follows Jupiter. None of them can compete with Jupiter in brightness. The only competitor could be Sirius in Canis Major (Big Dog), which appears in the sky at about the same time as Jupiter, but lies much farther to the south, so the two objects would be difficult to confuse.

   The Moon - Fading and Receding into the Night
   The Full Moon occurred in the morning of September 21, Saturday. That date was very close to the date of the autumnal equinox (which actually occurred two days later). At the time of the equinox, the Sun crosses the celestial equator, and as the Full Moon is located opposite to the Sun in the sky, it should also be located on the celestial equator. It is not the fault of us, the astronomers, that a strict condition like this one is actually seldom accurately met.
   The Moon does not move in the same orbital plane as the Earth orbits the Sun; its orbital plane is inclined by more than five degrees to the plane of the ecliptic One consequence of this is that eclipses are, unfortunately, fairly rare phenomena. At the time of its September Full Moon phase, the Moon was located five degrees below the ecliptic, too far off for an eclipse to occur. Incidentally, since this Full Moon was not located exactly opposite to the Sun, its face was not 100 percent illuminated as seen by us; however, the deviation is practically negligible.
   After September 21, the Moon started to lose part of its illuminated disk, and to recede into the night, but rather slowly. Why “rather slowly”? On the following days, the Moon was moving not only to the east but also to the north, imitating the behavior of the Sun in the spring months. You know that in the spring, the Sun rises earlier and earlier every successive day, until it reaches the summer solstice on or about June 21, three months after the spring equinox.
   Now the Moon does the same, but at a much faster pace. What the Sun accomplishes in three months – moving from the equator to its northernmost position (23.45 degrees north), the Moon accomplishes between September 22 and 29, or approximately a week.
   Naturally, the eastward motion of the Moon makes its rising times come later and later. The usual time interval between two successive risings is about 50 minutes. However, when the Moon moves also northward, this can reduce the interval significantly. Thus on September 25, it will rise at 9:10 p.m., and on the next evening at 9:45, making the interval only 35 minutes.
   The Moon will be a late evening object till the night of September 29/30, when it will rise 5 minutes after midnight. The Last Quarter comes on September 29. After that, it will be less than 50 percent illuminated, so in the first nights of October, it will be a shrinking crescent visible above the east in the morning hours. The New Moon comes on October 6.