Starry Skies Above Santa Monica

July 18-25, 2000

Mirek Plavec
Emeritus Professor of Astronomy, 
UCLA

   Evenings with the Moon and Mars
   The Sun is slowly receding from the Northern Hemisphere back to the equator, and days are slowly getting shorter again. However, the warmest days and nights typically come some 4 to 6 weeks after the solstice, so the forthcoming evenings are favorable for star watching. The Moon cooperates, too, and does not interfere. It is New on Friday, July 20. On the following evenings, it will slowly emerge from the vicinity of the Sun and shine as a thin crescent after sunset.
   On Sunday, July 22, the Sun will set at 8:01 p.m. When the sky gets a little bit darker, it might be possible to spot a very thin crescent of the Moon (only 8 per cent of its disk will be illuminated!) above the western horizon; it sets shortly before 10 p.m. On the following evenings, the crescent will gradually grow in thickness, recede from the Sun, and set later. The First Quarter comes on Friday morning, July 27. Between Sunday and Thursday, the Moon will travel from Leo to Virgo. The very thin crescent will shine near Regulus in Leo, and the First Quarter Moon will be projected to the north-east of another first magnitude star, Spica in Virgo. Still later, on the evening of July 29, the Moon will shine close to a third first magnitude star, Antares in Scorpius, and the two objects will form a nice trio with Mars.
   However, you don’t need the Moon to find Mars. Mars is still the brightest star-like object in our evening sky. When the brightest stars become visible, towards the end of the evening dusk, Mars is already quite high above the south-eastern horizon. It culminates above the south before 10 p.m., and the revolving sky carries it past the meridian towards the south-west, where Mars will set after 2 a.m.

   The Morning Planets 
   Soon after Mars disappears below the south-western horizon, the brilliant Venus appears in the north-east by 3 a.m. Another bright planet, Saturn, actually rises about half-an-hour before Venus. Saturn is much fainter than Venus, but still shines as a first-magnitude star. About an hour after Venus, the next brightest planet, Jupiter, rises in the north-east. And then Mercury comes up about one hour after Jupiter, that is, before 5 a.m.. By that time, the eastern sky will already become bright, heralding the arrival of the Sun, which will rise shortly before 6 a.m.

   The Evening Stars
   Easy to find are the six brightest stars of the evening sky. No star map is needed to find them. The dominating Mars above the southern horizon is without competition as to its brightness. And once you identify Mars, you will easily find Antares, just a few degrees to the west of Mars.
   On the evenings of July 25 and 26, the Moon will pass above the first-magnitude star Spica in Virgo. When the Moon is not available for this identification, then you can use the Big Dipper, a constellation that almost everyone knows. You will find the Big Dipper high above the north-western horizon on any of these July evenings. If you extend the direction of the least two stars of its handle, you will find a bright, orange-colored Arcturus in the constellation of Bootes (the Shepherd). And then extend this arc once more to the south, and you will find Spica.
   The remaining three first-magnitude stars of our summer sky, Vega, Altair, and Deneb, form a big triangle in the eastern sky. Easiest to identify is the brightest among them, Vega, located not far to the east from the zenith. Deneb shines to the north-east of Vega, at the top of a large stellar cross, professionally known as Cygnus (the Swan). Altair, to the south of the above two, belongs to another “birdie” constellation, Aquila (the Eagle).
   Each of these six stars actually represent a different kind of stars, and they appear almost equally bright to us since their two fundamental parameters are suitably combined: their distance from us, and their luminosity (light output). And again, luminosity depends on two other fundamental parameters: the star’s size and surface temperature. 
   In their apparent brightness (i.e. as they appear to us), these six stars would rank, starting with the brightest one, as follows: Arcturus, Vega, Altair, Antares, Spica, Deneb. If the six stars were placed at the same distance from us, their order of brightness would be quite different: Deneb (ranked 6 in apparent brightness), Antares (4), Spica (5), Arcturus (1), Vega (2), Altair (3).
   Clearly, Deneb must be the most distant star among these six. Its distance is near 1500 light years, with a fairly large margin of uncertainty. Nearest star is Altair, 16 light years, and then Arcturus and Vega come with a distance of about 25 light years.
   Deneb is a supergiant star, fairly hot on the surface, and rather big: its radius is 100 times larger than the radius of our Sun. About 64,000 Suns would be needed to match the luminosity of Deneb. Antares is a huge red supergiant, with a radius more than 400 solar radii; but its cool surface temperature lowers its light output to “only” 30,000 Suns. Compared to these two supergiants, Spica has a much more modest radius, about 4 solar radii; but it is fairly hot on the surface, and therefore still manages to produce a luminosity equal to 6,000 Suns. Altair is the most modest producer, it radiates only as much as 9 Suns.