Starry Skies Above Santa Monica
April 25 - May 2, 2000

Mirek Plavec
Emeritus Professor of Astronomy, 
UCLA

   Sun, Moon, and Planets
   As usual, I will first report on the Sun. On Sunday, April 29, it will be already almost 15 degrees above the equator. Thus, it will shine from the zenith on the people living at such diverse places as the republics of Guatemala and Honduras, the African cities Dakar and Al-Khartum, Manila in the Philippines, and the Pacific island of Guam.
   For us, the Sun will pass 19 degrees south of the zenith, but you may very well feel that the Sun is virtually overhead. On Sunday, April 29, the Sun will rise at 6:06 (a gain of 7 minutes), and set at 7:35 p.m. (a gain of 5 minutes). The Sun now moves in front of the stars of the constellation Aries.    The troublesome problem for the astrologers continues: According to them, people born between April 22 and May 21 have Taurus as their “sun sign,” but the Sun is one constellation behind, and is peacefully traveling through Aries. Take your choice! I explained the problem a little bit in the preceding issue.
   The Moon was New on Monday, April 23. On the evening of April 25, Wednesday, a very thin crescent of the Moon will form a very nice trio with Saturn (to the west) and Jupiter (higher above the Moon, to the east). The star cluster of the Pleiades will be near to, and above Saturn, and the reddish first magnitude star Aldebaran will be below Jupiter and to the east of the Moon. It will be a very nice configuration, but you should look for it fairly early after sunset, before 8 p.m. After 9 p.m., all these objects, and Orion as well, will disappear below the western horizon. 
   After its encounter with the mighty planets in Taurus, the Moon will move to the east and grow fairly fast. The First Quarter Moon will shine, on April 30, between the bright stars Castor and Pollux to the west, and Regulus in Leo to the east of the Moon. On Wednesday evening, May 2, the Moon will be leaving Leo for Virgo, and will not be far from Full; actual Full Moon comes on May 7, when the Moon will be projected between the inconspicuous stars of Libra (the Scales) and the prominent stars of Scorpius.
   The “red planet” Mars now rises already before midnight. It is getting quite bright -– astronomically speaking, its magnitude is -1, which means that only Sirius among the stars, and Jupiter and Venus among the planets are brighter. However, Mars is projected well down below the celestial equator, at -24 degrees, which is a bit lower than the Sun can ever be. It shines, from the local zenith, on the people in Sao Paolo, Brazil! For us, Mars reaches its highest altitude above the southern horizon shortly after 4 a.m., and it is only 32 degrees above the horizon (one third the angular distance from the southern horizon to the zenith). 
   The morning sky is adorned by Venus. On Sunday, April 29, Venus rises in the east at about the time when Mars culminates, that is, shortly after 4 a.m. Venus now happens to be projected fairly close to the (imaginary) point in the sky where the Sun stands at the very beginning of spring, in Pisces. 

   Meteors and Meteorites
   In the preceding issue, I called your attention to the meteor shower of the Lyrids. This shower might be fairly conspicuous, were it not for the unpleasant circumstance that it appears to be active after midnight. Actually, we encounter these meteors for some 2 or 3 days, but they are visible only at night (naturally), and predominantly after midnight, since only then the “radiant point” is sufficiently high above our northwestern horizon. The meteors are coming in parallel orbits, but they appear to “radiate” from one point as a consequence of perspective. And, in the case of the Lyrids, the radiant is quite close to the bright star Vega in the constellation of Lyra, which is a small but fairly prominent constellation that will appear high in the sky in late summer.
   The fact that the radiant point rises late at night has an interesting consequence. The Earth orbits the Sun, and at the same time rotates on its axis. At noon, we face the Sun. Six hours later, we look into the direction from which the Earth is coming: we are, so to say, sitting at the stern of our space ship “Earth.” At midnight, we are looking into the universe exactly opposite to the Sun. And by the morning, we have changed our position completely and we are now the pilots of the space ship Earth, standing at the bow and watching the space regions into which we are moving!
   Meteors coming to us in the morning from the east therefore have a high speed with respect to us. We are moving, rather steadily, at 30 kilometers per second! Most meteors, moving in elongated cometary orbits, have a speed near 42 km/s at the point where we collide with them. Thus, in the evening, we are actually running away from meteors coming from behind, and their speed relative to us is only (42 - 30) = 12 km/s. However, in the morning, meteors coming from the east may have a relative velocity of (42 + 30) = 72 km/s! This difference shows in the appearance of the meteors. 
   Meteors are very small bodies: small stones, pebbles, often just “dirty snowballs” or “dirty ice balls”: fragile structures consisting of pieces of ice mixed with very small pieces of gravel. How can such a tiny body shine? It is just its very high speed that makes it possible: what shines are actually mainly molecules of air that were hit by the meteor, their electrons jumped to higher orbits, and when they fall back, the energy is released in the form of radiation. You will now understand that a fast-flying meteor will shine more brightly, but it will be pulverized sooner, and disappear quickly. 
   And a fast-flying stone, even if larger than usual, has virtually no chance of survival: it compresses the air molecules so much that their resistance will eventually completely disintegrate the intruder. During certain “meteor showers,” thousands upon thousands of meteors flashed above people’s heads, and not one came down to the ground! On the other hand, a heavier object weighing several pounds may partly survive the flight through our atmosphere – but on the condition that it is solid material (stone or iron), and that it comes at a slow relative speed – that is, from behind, in the afternoon or evening! There are many amusing stories about these meteorites – let’s talk about them soon!