Starry Skies Above Santa Monica

July 24-31, 2002

 Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   The Moon
   — Still Ruling Our Evenings
   The Moon reaches its Full phase in the morning hours of Wednesday, July 24. Since the Full Moon stands opposite to the Sun in the sky, it rises when the Sun sets, reaches the meridian at midnight, and sets at the time of sunrise. Slight deviations, in minutes, from this rule occur because the Moon is not located exactly opposite to the Sun, since it travels along a path that does not coincide with the ecliptic. Thus, this time, it is located well below the ecliptic – this is also the reason why we have no eclipse of the Moon, since the shadow of the Earth misses it. Who says that astronomy is a simple science?
   After its Full phase, the Moon will recede into the night, but rather reluctantly. The average interval between the successive times of moonrise is about 50 minutes. This week, it will be significantly shorter – the shortest interval will be only 25 minutes! And this will happen between July 28 and 29. So the Moon will still be present in the sky during the late evening hours, rising later and later only gradually. The Last Quarter comes late in the night of July 31/ August 1.
   Why this fairly close succession of the times of moonrise? The Moon will imitate the behavior of the Sun in the spring, and as it progresses eastward with respect to the stars, it also moves to the north: from Full Moon in Capricorn through Aquarius and Pisces into Aries, where it will arrive at the time of its Last Quarter. By the way, on July 29, the Moon will shine in the constellation of Cetus, the Whale, which is not one of the constellations of the zodiac. Again, the main reason is that the path of the Moon does not coincide with the ecliptic – and the other reason is that astronomers, when drawing boundaries between constellations, tried to preserve the traditional ancient and medieval pictures of the constellations. So they kept the head of the Whale with its body – although the real whale does not have a distinct head sitting on a fairly thin neck as the celestial Whale has – often the constellation was considered to represent “a sea monster” – please go to the beach and find one for me!

   Our Beautiful Evening Star
   – Still With Us
   The Sun is now setting just a few minutes before 8 p.m., and since it is still fairly high above the equator in the northern hemisphere, the night comes slowly. The brightest stars will be visible at about 9 p.m. The first “star” you will be able to see is not a real star – it is the planet Venus. It is currently projected between the constellations Leo and Virgo, and sets very shortly after 10 p.m. Look for it above the western horizon by 8:40 or so.
   Venus is still receding from the Sun along the ecliptic – it will reach its greatest “elongation,” 46 degrees east of the Sun, on August 21. However, Venus now moves along that part of the ecliptic through which the Sun will descend in the Fall, so the times of its setting come earlier and earlier.

   Meteors Are Coming!
   The nights are warm, the Moon will gradually disappear, and the time is coming for a beautiful display of the Perseid meteor shower. The moonless nights of August 10 - 14 promise us fairly many meteors coming from the northeast (where the constellation Perseus rises late at night), and the peak of the activity of this meteor shower will come in the late hours of the night of August 12/13. I am announcing it a bit in advance, since some of you may be planning to spend the early August weeks on vacation somewhere far from the city lights, so you should be aware of the meteors.
   Meteors – and meteorology, the science of weather. Isn’t it a very strange similarity? The root is in history – for centuries, astronomers paid no attention to meteors, since they were believed to be a phenomenon related to our atmosphere. And, in a sense, they are, since they flash typically some 80 - 120 kilometers above our heads, and therefore well inside our atmosphere. And they flash because of the atmosphere – if the Earth were not surrounded by a fairly thick and fairly dense atmosphere, its surface would be endlessly bombarded by those tiny specks of solid matter. The meteors come from the interplanetary space mostly at a speed not far from 42 kilometers per second. As they penetrate deeper and deeper into the atmosphere, they collide with millions of air molecules, and gradually disintegrate – actually pulverized into tiny specks of dust.
   Nevertheless, the meteors belong in astronomy, since they come from the interplanetary space. This was first demonstrated by two German students, Brandes and Benzenberg, in 1798. They plotted the apparent paths of meteors from two different places, and combining their plots, they established that meteors flash high up in our atmosphere, but are coming from outside.
   As sometimes happens, those few “professional” astronomers living then did not know about it or ignored it. A true awakening came on the night of November 12/13, 1833. Perhaps all the people living then in North America were awake and many of them in sheer panic. As the sky darkened, they saw dense streams of “falling stars” in the sky, and the shower of lights became even more impressive — or scary — after midnight. In the “bible belt,” church bells were ringing and people were preparing for the Day of Judgment.
   Then the night ended, the sun came up as usual, and the next night all the stars were in the sky as they had always been. The only lasting result was that a new branch of astronomy was born. A few keen sky watchers noticed that the meteors were not simply “falling down” – rather, they were “radiating” as if they were all coming from one spot in the sky, located in the constellation of Leo (the Lion). Thus those November meteors are known as Leonids, and they caused several exciting “meteor storms” every 33 - 34 years afterwards.
   Why always at intervals of 33 or 34 years, and virtually no show in the years between? Well, I will tell you after a much, much shorter time interval of only one week!