Starry Skies Above Santa Monica
August 25- 31

Mirek Plavec

Emeritus Professor of Astronomy, UCLA

Red Planet, Red Star
   This was our main topic last week: planet Mars (Ares for the ancient Greeks) and its competitor in color and brightness, Antares in Scorpius. Looking at both, you may agree that they are reddish, but you may also disagree; at this low light level, color is very subjective. Moreover, my experience is that your color impressions may vary from night to night, somehow related to the weather and the state of the atmosphere. In any case, I am sure you will agree that the setting Sun can be much, much more distinctly red, when you watch it ìsinking into the waves of the Pacific. The reddish hue of Mars is due to its deserts; Antares appears reddish because, at its rather low atmospheric temperature, it radiates mostly red photons. Thus the question remains:

Why Is the Setting Sun Red? 
   I explain it (in my opinion) very carefully to my UCLA students in the large classes on elementary astronomy. And then, I ask this question on the test, and offer five possible explanations. In spite of my careful exposition of the problem before, there are always quite a few students who opt for one of the three wrong answers. I will present them to you first. a) Everything cools off towards the evening, so why not the Sun? Well, many a thing in nature does tend to cool off towards the evening, because it gets less light and heat from the Sun; but, of course, the distant Sun radiates quite impartially at noon or any time later. b) As the sun sinks towards the water level, its rays have to pass through the vapor and fog above the ocean. There is, of course, something to this argument, and it may contribute to the reddish appearance of the Sun. However, quite often the atmosphere above the ocean is quite transparent (or you are watching the setting sun in a desert) and yet the sun gets its reddish color just above the horizon. c) When setting, the Sun is 40 times father away from us than at noon. There is always someone who accepts this explanation, which makes me red, rather than the Sun; but I must admit that the number 40 does come from my lecture, so I succeeded at a 5% level. 

The Correct Answer
   To my great regret, no student has as yet accepted my ingenious and original suggestion, namely, d) When the Sun sets here, it rises above the Red China. Obviously, my students lack my experience that, in a Communist country, everything must be red, so why not the Sun? And now, under e), we come to the correct explanation, which, as usual, requires a lengthy talk. We live at the bottom of a fairly thick atmosphere. And the molecules of air, be it molecules of nitrogen or oxygen, play a trick with the light passing through: they scatter the radiation. We can visualize it as a simple game between a molecule of air and an elementary quantum of radiation, which we call a photon. No harm is done if you imagine that the photon is also a kind of a particle. Now, in the process of scattering, the photon bounces off an air molecule, like a ball. It is not destroyed, and it is not changed either: it is simply sent into a different direction, essentially at random. There is plenty of space in between the air molecules for many photons to squeeze through without being scattered. It is true that some air molecules exist as high up as 1,000 km above the sea level. However, the density of air decreases very rapidly with altitude, as anyone knows who ever tried mountaineering. If squeezed to a layer of the same density as it has at sea level, our atmosphere would be only 8 km thick. In other words, between you and a mountain 8 km away is the same amount of air as above your head. 

Photons & Molecules
   When the Sun shines above your head, its rays have to pass through that many air molecules; some photons are scattered away, but most of them arrive to you as if nothing happened; the Sun is just a little bit fainter than it would be without the atmosphere. However, when near sunset, its rays have to pass a much longer path through the atmosphere, especially through its denser parts. The photons encounter approximately forty times more molecules than at noon, and forty times more of them are scattered away from their direct path to you. The Sun appears much fainter than at noon.
   And! The air molecules are not impartial in their act of scattering! They scatter violet and blue photons much more voraciously; then come the green and yellow photons; and the orange and red photons suffer least. All colors come to you at noon in sufficient numbers and the sun is white or somewhat yellowish. However, towards the evening, more and more of the violet and blue photons are filtered away, and the sun becomes more and more orange or even reddish.

Skyful of Photons
   I have said that scattering does not destroy the photons, only redirects them in all directions. So where are they? All over the sky, during the day! After several more bounces off air molecules, a fraction of them come to you from all directions in the sky; and since the atmosphere predominantly robs the direct solar radiation of the blue photons, the cloudless daytime sky is blue! If my test question were: Why is the daytime sky blue? I would certainly offer this optional answer: The air molecules are blue in color, but you recognize it only when there are very many of them. No! Or: the air reflects the blue color of the ocean -- but it is just the other way around. 

The Duke’s Surprise
   After this tedious story, you need something lighter, if not too pleasing: How to cast a horoscope: Centuries ago, astronomy and astrology were often mixed. The famous Italian astronomer, Galileo Galilei, agreed just once to cast a horoscope for his sovereign, the Duke of Tuscany. Galilei knew only too well that “no stars don’t tell nobody nothing” (decipher this as you like!) , so he “cast the horoscope” promptly: “Your majesty will have a long, healthy, and pleasant life, and will sire many children.” This prophesy pleased the Duke very much, but unfortunately did not prevent a serious disease, and the duke died within six weeks, leaving no issue, as our lawyers like to call children. Naturally, Galilei could not know what would happen to his boss. And, even if, by some miracle, he could know, he would be a fool to say it, for the Duke could have said: “My dear Galilei, I may die in six weeks, but you will for sure die this very afternoon.”