Starry Skies Above Santa Monica

May 29 - June 5, 2002

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   A Royal Encounter
   This coming week, you will be able to see a truly beautiful meeting of the two brightest planets: Venus and Jupiter. The two will be closest together on Monday, June 3, but if our usual fog spoils that evening, don’t worry: the two planets will be forming a fairly close pair for several evenings about that date. And, as is quite appropriate, Venus will be the dominating beauty. However, do not delay your search for them: they are definitely evening objects, and set at 10:30. The Sun will be setting at 8 p.m., and some 40 minutes afterward, the western sky should be sufficiently darkened to show you at least these two “stars”. They will be shining above the western horizon (a bit to the south of the direction in which the Sun has set). This “conjunction” of the two brightest planets at such a convenient time is a rare event – thus even those of you who normally ignore my exhortations to go out and look at the sky, should make an exception this week! And I will recommend — even more strongly – another such an outing next week!

   The Brightest Star You Can See
   Forget (just for a few minutes) about the Sun and the Moon, the two brightest objects in the sky and the only two that we see as luminous disks. The rest are simply “stars”: if it were not for the glittering and twinkling appearance produced in our atmosphere, we would see them as mere points of light, shining with a quiet, steady light. True, even the brightest planets appear as mere points of light to the unaided eye. However, I have just discussed the two planets currently visible, and can now concentrate on true stars with clear conscience.
   OK, so which one among these are the brightest? Among the true stars – that distant suns – Sirius is the brightest one, but its annual glory is now over: it disappears in the south-west by 8 p.m., actually exactly when – much higher up to the north – the Sun is setting. Thus, naturally, Sirius is no longer visible.
   So which star has taken over as the ruler of the starry sky? The next brightest stars after Sirius, Canopus and Alpha Centauri, lie far down in the southern sky. You can glimpse Canopus, very low above the ocean, when the sky is clear all the way down to the southern horizon — but on January and February evenings, not now. In order to see Alpha Centauri (actually our nearest stellar neighbor in space – with its tiny companion, Proxima), you must travel to the south – at least to Hawaii. That is, in my opinion, the best choice, although from there, these stars are, at best, just above the ocean. OK, if you prefer the Atlantic side, I hope you will skip Cuba and go farther south into the Caribbean or right to South America.
   OK, so which star is the next brightest? Its name is Arcturus, and it is the brightest star in the constellation of Bootes (the Herdsman). It is easy to find Arcturus on these spring evenings (if our endless fog permits it): it is sufficient to know only one constellation, namely the Big Dipper, which is high above the north-east on these spring evenings. If you extend the handle to the south-east, you cannot miss Arcturus, and you should be able to notice its orange color.
   Arcturus is what we call “a giant star.” Its radius is 19 times the radius of the Sun, which makes its total surface 360 times larger than the surface of the Sun. Simplemindedly, one might expect Arcturus to emit 360 times more radiation than our Sun. Actually, it emits only 129 times more than the Sun. This is due to its lower surface temperature, which is about 1,100 degrees (centigrade) lower than our Sun’s.
   And the next competitor? Just a tiny bit fainter than Arcturus is Vega; the difference is only 0.1 magnitude. Vega is located farther to the north-east, and, again, it has no real competition there. Just look for it above the eastern horizon when the sky gets sufficiently dark. You will probably notice that, in contrast to Arcturus, Vega shines with a bluish light. This fact indicates its higher surface temperature, 9,600 degrees. As a consequence, Vega emits as much radiation as 48 Suns, although its radius is “only” 2.5 times larger than the radius of the Sun. Here is a bit of Physics: Double the radius, and the star will shine 4 times more; but double the surface temperature, and the radiation output will be 16 times greater.

   An Eclipse of the Sun – Coming Soon!
   I kept talking about my favorite topic, the stars, and nearly forgot that we also have the Sun and the Moon!
   The Sun rises (on Sunday) at 5:43 a.m., culminates at 12:51, and sets at 7:59 p.m. Thus we are entitled to 14 hours and 16 minutes of sunshine. This number will not improve much, since the summer solstice is quite near — it comes on June 21. The Sun will already reach 22 degrees above the celestial equator on Sunday. This means that it will be passing exactly overhead, at the local noon, at San Luis Potosi and Tampico in Mexico, and at central Cuba. The boundary between Egypt and Sudan runs many miles exactly along this parallel.
   For most of you, the Moon can probably be ignored this coming week. It was Full on May 26, and reaches its Last Quarter on June 2. Thus the Moon will gradually shrink its bright face and rise well after midnight. With respect to the stars, the Moon will swing from Sagittarius to Pisces.
   The Moon, however, should not be ignored the following week! The New Moon will pass in front of the Sun on June 10/11 and cause an annular solar eclipse, part of which we will be able to see near the time of sunset on Monday, June 10. This is, of course, a prime topic for the next week’s edition!