Starry Skies Above Santa Monica

June 26 - July 3, 2002

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   The Sun
   - Slowly Returning to the South

   Really very, very slowly. The turn-over time was the summer solstice early on Friday, June 21, but the change will be hard to notice for some time. Thus, on Sunday, the last day of June, the Sun will rise at 5:45 (three minutes later than a week ago), and set by 8:08 p.m. – actually just one minute later (!) than a week ago. This rather surprising later sunset is due to the fact that the Sun will culminate at 12:57 p.m., while a week ago this happened at 12:55. As you can see, we can easily forget these insignificant details. You may think that astronomers should have more important problems to deal with, and they really do – all of the above is our heritage from a distant past, yet still valid.

   The Summer Moon
   The Full Moon occurred on June 24, and this coming week, the Moon will gradually lose part of its bright face, and recede into the night. I must say that it will recede rather reluctantly. The reason is that the Full Moon occurs very low, between Scorpius and Sagittarius, practically at the place where the Sun is at its lowest just before Christmas. And after that, the Moon will be moving ever higher along the ecliptic, just what the Sun does after the New Year. Since the Moon moves much faster than the Sun, the intervals between its successive rising will be quite short – for example: on June 26, it will rise by 10:07 p.m., and the next evening, it will rise only 41 minutes later, at 10:48.
   During the entire week between the two Wednesdays, the Moon will travel from Capricorn through Aquarius into Pisces, and will eventually rise after midnight. Its Last Quarter will come on July 2.

   Venus, the Lone Planet of our Evenings
   A few days ago, I was still able to spot Jupiter fairly low above the western horizon, but the domination of this celestial king is coming to its end. Jupiter is now setting less than one hour after sunset, and will be quickly lost in the glare above the western horizon.
   The only remaining evening planet is, however, the most beautiful one, Venus. It is projected between the constellations Cancer (the Crab) and Leo (the Lion), and is gradually approaching the brightest star of Leo, Regulus. The two will be fairly close together on July 10. At present, Venus sets by 10:30 p.m., and is so bright that it cannot escape your attention if you look to the west already some 40 minutes after sunset.
   Perhaps it would be fair to Mercury to mention that it is currently rising by 4:30 a.m., that is, more than one hour before the Sun, and is fairly bright, but I can hardly expect that anyone in Santa Monica to rise that early to see Mercury above the glare of all the city lights to the east of you… There will be better times to look for this fast-moving planet!

   The Stars of our Summer Evenings
   The Moon will slowly recede into the night, and if city lights permit, you may be able to see the brightest stars of our summer sky. After all, the end of the school year means the beginning of summer vacations at places much more favorable for star watching!
   It is not difficult to find and identify the brightest stars! The search can best start with Venus, which is so bright that you cannot miss it. As I mentioned above, Venus will pass close to Regulus in Leo on July 10, so the two objects will be close to each other for several nights about this date. Venus will then continue along the ecliptic to the south-east, but your eyes can move much faster than the planet. It will be fairly easy to find the next bright star along the ecliptic, Spica, in the constellation of Virgo.
   The next constellation along the ecliptic is Libra (the Scales), but it has no bright stars. You have to continue further to the southeast to find the next first-magnitude star, Antares in Scorpius, which has a reddish light.
   And then it is best to forget about the constellations of the zodiac, and simply look direct to the east. Most of you probably know the constellation of the Big Dipper, which is, on these evenings, easy to find not far from the zenith. Its “handle” points to a bright orange star Arcturus. Actually, you do not need any assistance in locating Arcturus. Just find the brightest star that is very nearly overhead – if you look at about 9:30 or so. Later on, about midnight, Arcturus will slowly slip to the west, and will be replaced at the zenith by an even brighter star, Vega in Lyra, which has a bluish color.
   So here we have a nice collection of five bright stars. Each of them represents a different type, and each of them lies at a different distance from us. Nearest to us is Vega, but even this star is pretty far by our terrestrial standards, 25 light years away. Arcturus comes next with 34 light years, then Regulus at 72, and the two stars that are most distant among these, Spica and Antares, shine at us from a distance of about 142 light years. Imagine that the rays you will be seeing now left the two stars in about 1860! Do you remember what happened in your country and among your ancestors at that time?
   Naturally, the two most distant stars among the bright ones must actually be very luminous – yet for a different reason! Antares has a cooler surface than our Sun, so each square inch of it emits less light – but this is more than compensated for by the fact that Antares is a supergiant star, with a radius about 700 times the radius of the Sun. Spica competes very well with Antares as to its actual brightness, in fact, it is more luminous – emits as much radiation as some 6,300 Suns. This is due to its much higher surface temperature, not to its much larger size, which is “only” about 8 times the size of the Sun.