STARRY SKY ABOVE SANTA MONICA

AUGUST 18 - 25
Mirek Plavec
Emeritus Professor of Astronomy, UCLA

Invisible Venus

   Venus is invisible, except... This is an interesting situation. On August 20, Venus will be at its inferior conjunction with the Sun. There is nothing much inferior about this position! In fact, it is special at least in one sense, namely that, at this time, Venus is as near to us as it can ever be, and no other planet comes that close; of course, the distance is still 43 million km. “Inferior conjunction” is the traditional astronomical term used to describe the situation when Venus is located exactly between the Earth and the Sun. Thus it is above the horizon only during the day, and therefore completely lost in the glare of the Sun and the surrounding sky. 
   However, it happens quite rarely that Venus would lie exactly on the line connecting the Earth and the Sun. In such a case, it is visible as a small black spot traversing across the bright disk of the Sun. This happens rarely because Venus is not moving about the Sun in the same plane as the Earth does. In fact, during this conjunction, Venus will be full 8 degrees (about 15 diameters of the Sun’s disk) south of the Sun. Therefore, in principle, it will never show us only its entire dark hemisphere. A very, very tiny crescent of its disk will be illuminated by the Sun. 
   Thus, for the days around August 20, a skilled observer with a good telescope could spot this crescent, always on the side facing the Sun, but he must very carefully avoid pointing the telescope at the Sun! A telescope equipped with precise setting circles is needed, as well as knowledge about the precise position of Venus. I am mentioning this here as just a mere curiosity, occurring rather rarely.

Mars to Scorpius

   Mars is the only planet visible in the evening sky. It is slowly crawling eastward towards the majestic constellation of Scorpius. You can actually see a good image of a scorpion in this constellation! Mars is slowly approaching its claws, represented by three fairly bright stars. Not far behind them, to the east, is the brightest star of the constellation, Antares. There can hardly be two celestial bodies as different in nature as Mars and Antares, yet they are related by tradition! Mars was, for ancient Romans, the worst of all gods: he was the god of war, fire, destruction, and disaster of all kinds. Poor Mars, and fire! It is rather a cold planet, being farther from the Sun than the Earth, so it gets less light and heat. And the heat it does acquire escapes easily back into space, since its very tenuous atmosphere is a very poor protective cushion. Its surface is to a large degree covered by deserts, and they give the planet its orange-to-red color. The orbit of Mars about the Sun is distinctly elliptical. All outer planets, including Mars, appear most prominent in our sky when they are at opposition to the Sun, that is, when the Earth lies exactly in between the Sun and the planet. If this opposition occurs when Mars is also as close to the Sun as it can be (at perihelion), it becomes very prominent in our night sky, second only to Venus. It does then happen that, under special atmospheric conditions, it might appear rather fiery red, at least to some people. And that’s all that associated Mars with fire and war in the minds of ancient people! Of course, if you insist that objects in the sky affect our lives or at least give us clues about the future, then you grab at the most flimsiest straw!

Antares’ Role
   How does Antares come in? It is a first magnitude star that happens to have about the same color as Mars -- its light is a bit reddish. Now the ancient Greeks venerated a god that was every bit as nasty as Mars, only had a different name: Ares. And since the star in Scorpius competed with the planet in brightness and color, it was a rival of Mars -- Anti - Ares, Antares! Here the similarity ends! Antares is one of the biggest stars we know of, a true red supergiant. 
   As the Moon travels across the sky, it occults stars that happen to lie in its path. If you watch such an occultation, you will see that the star disappears behind the Moon instantaneously. However, Antares takes a little time to disappear, since it has a tiny but not negligible apparent disk. This means that it must be a really big star. Unfortunately, it is too far away for any precise determination of distance, hence also of its size and luminosity. We estimate that it is about 400 light years away. If it were to replace our Sun, we would not be here, since its surface would lie far beyond the orbit of Mars. As a consequence, Antares radiates no less than some 10 thousand Suns! Considering its huge size, this is actually less than one could expect. Certain much smaller stars outshine this red supergiant easily. And, looking at it from another side, the setting Sun can beat both Mars - Ares and Antares as to the red color! I think you agree that I must continue writing this column, when so many questions are left unanswered? 

Planets and horoscopes

   I mentioned here last time that Venus has always been considered to be the goddess of love and beauty -- just because it shines with a bright, lovely light on spring evenings. Now we have Mars, being a devil of a god just because of its sometimes reddish color! The other associations were no more profound! Jupiter is the brightest planet after Venus, shines with a steady light, and moves among the stars at a slow, dignified pace -- can’t you see that this is a king? Powerful but essentially benevolent in all horoscopes, as a real king is expected to be? And then Mercury, encircling the Sun in just 88 days, briefly showing up in the evening and then switching to the morning sky, a fast-moving messenger of the more powerful gods, and himself a god friendly to those who are of quick mind, and, alas, also of those who are of quick hand. Who remains is Saturn, the most beautiful planet in the telescope, but outwardly perhaps shining with a sickly yellowish light. The ancients no doubt needed another nasty character, in addition to Mars, in the perpetual theater of life; and thus, poor Saturn became another bad guy.
   It so happened that, for the ancient nations, there were just seven bodies in the sky that moved among the fixed stars, and thus were believed to have some special meaning for us down here: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. And their ever-changing mutual configurations were believed to foretell the events on the Earth. Seven bodies, and seven days of the week! Just consider: Sunday for the Sun, Monday for the Moon (Mond in German), Saturday for Saturn. The other days only seemingly disagree -- we only inherited other nations’ god names for those coming from Latin. Thus Venus was Freia for the Germanic nations, and we have Friday (but viernes in Spanish and vendredi in French!); and the supreme god was Thur or Thor, and got Thursday; and no matter what his name was, it was always the same god, holding a lightning bolt always ready in his hand. In Latin, the possessive form for “Jupiter” was “Iovis”, and the Italians have giovedi, the French jeudi, the Spanish jeuves! Mars and Mercury were not robbed of their days, either; just think that Tuesday is martes, and Wednesday is miercoles! Personally, I love most the Czech version of Sunday: nedele. No god involved! Just “You don’t work.”