Starry Skies Above Santa Monica

August 15-22, 2002

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   Venus – Farthest from the Sun
   I mean: only as it appears to us. Venus goes around the Sun in a very nearly circular orbit, so its actual distance from the Sun virtually never changes, and is always close to 108 million kilometers, compared to our average distance of 149.6 million kilometers. Since Venus orbits inside our orbit, it can never be seen opposite to the Sun in the sky, as the outer planets (starting with Mars) are. Venus cannot even reach 90 degrees deviation from the Sun, in which case it would be on the meridian when the Sun is setting. The greatest deviation from the Sun that Venus can reach is just about one half of that deviation, which for the outer planets is called the “quadrature.”
   Thus Venus deviates either westward from the Sun, and precedes the Sun in the morning, or eastward, and then it sets after the Sun and is visible in the evening, after sunset. This is the current situation. Venus reaches its “greatest elongation east” on August 21. The term “east” is no mistake. We see Venus in the west, but it is located to the east of the Sun.
   From the point of view of an observer, not all of the “greatest elongations east” are equal. The favorable case comes when the Sun is located in the southernmost part of the ecliptic. Then Venus deviates not only to the east but actually to the northeast, since it is then located higher up on the ecliptic.
   This is not the current case. The Sun is still rather close to the northernmost point of the ecliptic, so Venus is located on the descending branch, at a point that the Sun will reach in October. Therefore, for us, on the northern hemisphere, Venus sets relatively early after the sunset, in spite of its “greatest deviation” along the ecliptic. Consider the real numbers: the Sun sets by 7:38 p.m. (this number is accurate for Saturday), and Venus follows at 9:33, less than two hours after sunset.
   Nevertheless, Venus is still brilliant and prominent above the western horizon, if you look at or after about 8:10 p.m. It is currently projected into the constellation of Virgo.

   Two Morning Planets
   The other bright planets (however, no match for Venus), Jupiter and Saturn, are now located among the morning stars. Saturn shines in Taurus and rises shortly before 2 a.m., and the much brighter Jupiter comes up after 4:30 a.m. Jupiter is now projected into the constellation of Cancer, where there are no bright stars at all.
   The only competition to Jupiter can be the (real) star Sirius, which comes up much farther to the south, after 4 a.m. For Saturn, which shines as a star of magnitude zero, there is more competition among the real distant stars. Not far from Saturn are three prominent stars. Rising before Saturn is Aldebaran, the “red eye of the Bull,” Taurus. To the south of Saturn is an even redder star, Beteigeuze in Orion. And farther to the east is Procyon, the “preceding dog star” – preceding Sirius, that is. You will be able to see all these objects much more comfortably, in the evening sky, when winter returns to us.
   Two other planets can be seen with the naked eye, if the conditions are favorable: Mercury and, especially, Mars. Currently the conditions are not favorable. Actually, Mars was in conjunction with the Sun on August 10. It was right behind the Sun, on the opposite side than we are, and therefore completely lost in the daily glare of the Sun.
About Sizes and Scales…
   Your neighbor’s house may be pretty big, freeway 10 is pretty wide, and Santa Barbara is rather far away… This is our daily scale of things.
   An astronomer works on a different scale. The diameter of the Sun is 1.4 million kilometers, and already this is difficult to comprehend. So let’s reduce the Sun to the size of a basketball! On that scale, the Earth will orbit the basketball at a distance of 32 meters (or 106 feet), Jupiter will be more than five times farther out, and the mean radius of the orbit of the most distant planet, Pluto, will be 1.27 kilometers or 0.8 miles. This may be, very approximately, the size of the UCLA campus, say.
   OK, where, on that scale, will be the nearest star, Proxima Centauri? Almost exactly in Tokyo, not in our local Little Tokyo, but in Tokyo, Japan! And this is the nearest star!
   The center of our Milky Way Galaxy will then be on the Moon…
   For these very large distances, let us change the scale. Our Milky Way Galaxy floats through space like a huge ship – so let’s make it an impressive ocean liner, some 1,000 feet long. Then the nearest such galaxy, M31 in Andromeda, will be another such liner at a distance of 4.2 miles.
   Emptiness, emptiness everywhere. However, stars tend to live in pairs, triples, or even larger groups. Thus the nearest star, Proxima Centauri, is a member of a triple system, generally known as Alpha Centauri. Actually, Proxima is an outsider in this system. The brightest star of the triple system happens to be quite similar to our Sun, and is closely associated with a somewhat cooler star. The two orbit their common center of gravity in a period of 80 years. Proxima lies pretty far from this pair, about 2 degrees (roughly, four diameters of the Moon) away, and shares the motion of the closer pair through space – in this way, it was identified as a member of the system by Innes in 1915.