Starry Skies Over Santa Monica: Marking Time Celestially

Mirek Plavec
Emeritus Professor of Astronomy, UCLA

The Sun will almost complete its run through the constellation of Gemini (Celestial Twins) in the coming week. Wait a minute, will say those of you who were born between June 21 and July 22, our sun sign is Cancer, so the Sun should be in Cancer Indeed, it was in Cancer at the time when Ptolemy codified the rules of astrology, and that was about the year 150 A.D. Since that time, a rather unpleasant dynamic process called precession has shifted the monthly positions of the Sun by almost exactly one constellation of the zodiac. Thus, if you worry whether you are a Cancer or a Gemini, remember that (1) everyone else has a similar problem, and (2) it does not matter anyway.
   Of course, it is the Earth that orbits the Sun, and not vice versa. However, to us it appears that the Sun is going around the celestial sphere, and completes one full circle, or 360 degrees, in 365.25 days. The two numbers are similar; is it a mere chance? Not at all! For ancient scientists, the apparent orbit of the Sun was the most important circle, so they decided to divide the circle into 360 degrees. It would be foolish to introduce a division into 365.25 degrees! The number 360 is easily divided into equal parts, say 90, 60, 30, 20, etc.; but what would you do with 365.25?
   Astronomers often use time units instead of degrees, and then 24 hours equal 360 degrees, 1 hour is equivalent to 15 degrees, and one degree corresponds to 4 minutes of time. The Sun moves eastward among the stars by nearly 1 degree per day. The Earth completes one rotation on its axis, turning from west to east, in 23 hours and 56 minutes. The remaining 4 minutes are needed to catch up with the Sun and make it culminate at noon every day, because it is the Sun that is important for our time reckoning. You cannot see the Sun's motion among the stars, but you can fairly easily see its consequences. Pick up any star that happens to be projected above a terrestrial object (a tower, chimney, tree) and note the time when that happens. On every of the following nights, the star will be at the same place 4 minutes earlier than at the preceding night. And, similarly, every star will set 4 minutes earlier every consecutive night. This ever earlier setting time is fairly easy to notice if you try, on every clear evening, to see the planet Venus. Venus itself is not fully affected, since it is moving eastward among the stars. However, this motion has slowed down significantly, and Venus is still quite close to the first-magnitude star Regulus in the constellation Leo (the Lion), currently just a bit to the south-east of it. It is Regulus that suffers from this setting rule, and as it sinks to the horizon, it is more and more difficult to see. Even Venus is above the western horizon for an ever shorter time, but is still easy to pick up because of its high brightness.

Crescent Moon

A thin crescent Moon will form a beautiful trio with Venus and Regulus on Thursday evening, July 15. If the sky is clear, be sure to look at about 8:30 - 9 p.m.! By that time, the Moon will be 3 days old. On the evenings July 14 through about July 18, the Moon itself is worth looking at! When the sky gets a little bit darker, you will be able to see, in addition to the brilliant crescent, the rest of the Moon's disk dimly illuminated. The dim light is often called the ashen light of the Moon, but much more correct is the name earthshine; for it is us who are illuminating the lunar surface! Just imagine watching the configuration from some distant point in space: When the Moon is new, it stands between the Sun and the Earth. An astronaut standing on the Moon will, at that time, see the Earth as full, i.e. a fully illuminated disk. And what a sight it is! The disk of the Earth has a diameter about 3.7 times larger than the diameter of the Full Moon; the surface area of the Earth's disk is 13 times larger, and the Earth reflects solar light much better (almost 6 times) than does the lunar surface, which is actually very dark. Now a few days after the new moon, when we see a thin lunar crescent, a hypothetical inhabitant of the Moon would see the Earth just a bit past the full phase (with a thin dark crescent not illuminated). The light of the Earth is still relatively bright, and illuminates the lunar countryside perceptibly. A small fraction of that light bounces off back to the Earth and this is what we see as the dimly illuminated part of the Moon's disk. As the Moon's phase increases towards the first quarter, the Earth is no longer Full for the lunatics, and, moreover, the sun-illuminated portion of the Moon's disk becomes very bright. Your chance to see your own light on the Moon disappears but will come again next month!

Seeing Mars

As the Moon moves rapidly among the stars, it will shine just above Mars on Tuesday evening, July 20. If you have not been able to locate Mars, here is a splendid opportunity! However, finding Mars is quite easy on any other evening. Mars is still fairly bright (of course, no match for Venus), and the orange (sometimes almost red) hue will identify it easily. Mars is now high in the south when the sky gets dark, and is slowly receding eastward from the first-magnitude star Spica in Virgo, which is white (or slightly bluish) and is still distinctly fainter than Mars.
   By the time the first-quarter Moon meets Mars, on July 20, it may be good to remember that already full 20 years have elapsed since that memorable night when we watched the first man to land on the Moon!

Radio Astronomy

As the Moon grows in brightness, the starry sky behind it becomes less and less conspicuous. Thus, instead of pointing to you an interesting object you could see, let’s talk about quite an inconspicuous star (of magnitude 13, i.e. difficult to see even in a very good amateur telescope) that made history in 1963. That star is in the constellation Virgo. It entered astronomy rather inconspicuously. When World War II ended, many British scientists and engineers who had participated in the development and use of radar, were looking for new jobs. Some of them decided to develop a new field of astronomy—radio astronomy. During the war, radar detected radiation coming from the Sun at radio waves, so it was natural to expect that many other stars would be detected on radio waves, too, if only a more sensitive receivers are developed. When this was accomplished, many hundreds of radio sources were indeed detected, but none of them coincided with an optically observable star! True, the first radio telescopes were fairly sensitive, but they had very poor spatial resolution so it was impossible to decide whether the observed radio source was a point source, like all stars are, or something spatially extended. This also implied that they were able to determine the position of a radio source only with a great uncertainty. To make a crude comparison, they were only able to tell you that a town is in California, but not whether it is located near Sacramento or near San Diego! I remember how we, the optical astronomers, laughed at the radio astronomers in the 1950's, pointing out that the gigantic radio telescopes had a worse resolution than Tycho Brahe had, around the year 1600, with his mural quadrant, without a telescope! However, we did not laugh for long. Radio astronomers quickly improved the accuracy by combining observations from two or more radio telescopes. Thus, by 1960, the radio astronomers in Cambridge, England, were able to compile several catalogs of radio sources with their fairly precise positions.

Radio Stars

Now the time came to find the optical counterparts to the radio sources, which were at that time often called radio stars. However, the number of potentially real radio stars began to diminish quickly. Many radio sources were quickly identified with distant stellar systems (galaxies), others with supernova remnants, that is, expanding clouds of gas ejected into space in the explosions of supernovae. A few radio objects, however, still looked like genuine radio stars, and then it was time for the optical astronomers to identify them with known objects. At that time, in the 1960's, the big optical telescopes at Mt. Palomar and Mt. Wilson hardly had any competition in the world, so California astronomers had to do the job. Two radio sources, known simply as 3C 48 and 3C 273, looked most promising (the names simply give their running numbers in the Third Cambridge Catalogue of Radio Sources). No obvious galaxy or any extended nebulosity was found at the position of either source; the only slightly unusual object seen at both positions was a tiny bluish star. Allan Sandage then used the 200-inch telescope at Mt Palomar to obtain the spectrum of 3C 48. The suspected bluish star was so faint that only a few spectral lines recorded on the plates, and their positions did not correspond to any known chemical element. Today, with the modern CCD's, the job would have been incomparably easier. Another distinguished Caltech astronomer, Maarten Schmidt, began, in 1962 May, to observe the other radio star, 3C 273. Again, the only suspect in the field was a bluish star, a bit brighter than that of 3C 48. Yet the position of the radio source was still not accurate enough to be sure that its identification with the bluish star was correct. Who helped was—the Moon! It was realized that during its travel through Virgo, the Moon should occult (cover) that radio source, but the occultation was visible only from Australia. And at the only radio observatory available the Moon was so low at the time of the occultation that they had to fell several trees to see it. But they succeeded, the observation confirmed that the radio source was indeed identical with the bluish star, and since in this case the bluish star is a bit brighter than in 3C 48, the spectrum obtained at Mt. Palomar by Maarten Schmidt was also a bit better. Even so, the effort to identify the mysterious spectral lines was very frustrating, until, on February 5, 1963, Schmidt suddenly realized what he was looking at! That day, Schmidt, who is Dutch, came home to tell his American wife: something awful happened to me today, and when he saw her alarm, quickly corrected himself: I mean awesome—I discovered a new type of cosmic bodies

Hydrogen Rising

And he was quite right! The mysterious spectral lines he saw belong to the most common element in the universe— hydrogen! A well-exposed spectrum of a star or of a gaseous nebula shows a very well defined sequence of hydrogen lines. However, Schmidt's spectrum was poor and showed only very few of these lines and, more importantly, they were displaced from their normal positions so much that it was necessary to assume that the object was receding from us at a speed of about 47,000 km/s! Such speeds occur only as a consequence of the general expansion of the universe, but then they require that the object must be at a distance of about 1.5 billion light years!

Black Holes

Yet, seen at that enormous distance, the object is still visible even in an amateur telescope (only a very good one, of course). Thus it cannot be anything like an ordinary star! The original name, quasi-stellar radio source, became quickly contracted into quasar. Today we know that quasars are very massive black holes in distant galaxies, which produce their enormous luminous energy in a luminous accretion disk, formed by material slowly spiraling into a very massive black hole.