Starry Skies Above Santa Monica
September 29-October 5

Mirek Plavec

Emeritus Professor of Astronomy, UCLA

Moon and Planets
   After the Full Moon on September 25, the Moon recedes -- one might say “a bit reluctantly” -- into the night sky, and will reach the Last Quarter on Saturday, October 2; by that time, it will be rising at about midnight, and still later on the following nights. As a rather thin crescent, it will shine close to Venus in the morning of Tuesday, October 5. Venus, near the top of its brilliancy, and almost as far in the sky from the Sun as it can be, will form a nice trio on that morning with the Moon and the (much fainter, but still of the first magnitude) star Regulus in Leo. Venus is so bright these mornings that not even the lights of Los Angeles can prevent you from noticing it -- provided you get up sufficiently early. Even when the sky is already quite bright before sunrise, Venus still remains visible in the east.
   At about 5 a.m., the two giant planets of the solar system, Jupiter and Saturn, are already to the west of the meridian, so that they are also easy to recognize, although they are fainter than Venus. In the evening, Jupiter is above the eastern horizon already by 8 p.m., and Saturn follows about one hour later. Mars still remains visible in the early hours of the evening, low in the southwest, between the constellations Scorpius and Sagittarius.

Stellar Magnitudes
   In the preceding edition, I may have tortured you a little bit, trying to remind you of your high school mathematics, specifically of logarithms. While they might have been difficult for you to comprehend and use, your eyes and ears use them very efficiently every day. You may have heard the phrase “a star of the first magnitude,” referring perhaps to a real star in the sky or perhaps to such a prominent actor as the just deceased George C. Scott. 
   The concept of stellar magnitudes was introduced long ago, by the ancient astronomer Hipparchus, who compiled the first catalog of stars. He seems to have been encouraged to do this by the appearance of a “new star” in the year 134 BC ; he felt that, in the future, it would be easier to identify such an intruder if such a catalog existed. He listed about 800 stars, and his successor, Ptolemy, added about 200 more. The position of each star was given by its location in the imaginary figure representing the constellation. Thus, the brightest star in Orion, Rigel, was “the left leg of the giant.” 
   It was necessary to identify the stars, at least crudely, also by their brightness. Thus, the concept of “stellar magnitude” was introduced. The Latin term “magnitudo” actually means “size.” Naturally, the ancient astronomers had no idea about the real sizes of stars (they had very poor ideas about what stars really are), so “magnitude” actually means “apparent brightness” or, more physically, “illumination” -- expressed in a special way, by way of logarithms! They simply introduced six “magnitude classes.” The brightest stars were called “stars of the first magnitude,” and the faintest stars, just barely visible to the naked eye under the best circumstances, were assigned magnitude class 6.
   With the advance of physics and technology, it was possible, in the past century, to measure the amount of light coming from stars by means of photometers. When Norman Pogson, in 1857, compared such measurements with the traditional “magnitude classes,” he was surprised to find that a difference of 5 classes (between 1 and 6) nearly corresponded to a ratio of 100 : 1 in the amount of light received! In order to preserve the usefulness of the ancient data, he codified this exact ratio. Thus stellar magnitudes are again logarithms. A difference of 1 magnitude corresponds to a light ratio of 2.512 : 1; this follows from the rule that a difference of 5 magnitudes corresponds to the light ratio of 100 : 1. 
   As soon as actual measurements replaced the previous estimates, it became obvious that it is necessary to express the magnitudes more accurately, and, moreover, the scale had to be expanded. The bright star Spica in Virgo, to which Mars was close in the spring, has a magnitude of 1.0 almost exactly. The bluish Vega, now almost overhead every evening, is brighter, and is usually listed as of magnitude 0.03. Sirius, the apparently brightest star, is of magnitude -1.46. This fall, the planet Jupiter is at its brightest over a decade, and shines at magnitude -2.9, while Venus, it its greatest brilliancy, reaches -4.6.
   These accurate numbers are of little value to you, of course. Yet I hope you will understand better when we say that Polaris, and most stars of the Big Dipper, are approximately of the second magnitude. And it is easy to remember that a difference of 5 magnitudes implies a light ratio of 100 : 1. Beteigeuze, the famous supergiant in the shoulder of the giant Orion, usually has a magnitude near 0.4. Thus, 100 such stars would be needed to match the current brightness of Venus -- another fact hard to perceive.
   The negative values for the few very brightest objects in the starry sky bring a rather funny aspect. Certainly, any actor or actress will be pleased to be told that they are “stars of the first magnitude.” However, if you tell any one of them that he/she is “a star of zero magnitude” or even “a star of magnitude minus one” -- actually, a big compliment! -- you are in danger of something heavy landing on your head -- keep in mind that not all of them read Santa Monica Mirror, although they certainly should!