Starry Skies Above Santa Monica
October 18 - 25, 2000

Mirek Plavec
Emeritus Professor of Astronomy, UCLA

   Evenings With Planets…
   Let us assume that it's already Sunday, October 22. (This enables me to give some definite times; remember that changes during the week are small – only the Moon moves and changes fast).
   In the evening, you can easily find Venus above the western horizon. The Sun will set at 6:10 p.m., and Venus sets at 7:54. The difference in setting times is now sufficiently large, thus when you look towards the southwest after 6:30, you will see the brilliant Venus without difficulty. Venus entered the constellation of Scorpius, and will be passing through the claws of the celestial scorpion; to the south-east of Venus is the bright star Antares. It is a first-magnitude star, easy to find, but its brightness is no match for Venus.
   Scorpius is one of the constellations of the zodiac, as is Taurus, the Bull, which is now coming up in the east. Its orange first-magnitude star, Aldebaran, competes in brightness with Antares, and although neither of the two planets now visible in Taurus can compete with Venus in brightness, they, together with the background stars, form quite a prominent grouping. Saturn rises already at 7:44 p.m., and the much brighter Jupiter follows at 8:18.
   Thus all three bright planets are now "evening stars," although this term is regularly used only for Venus.

   … Mornings With Meteors
   The Moon reaches its Last Quarter on Friday, October 20. Thus it will not interfere when you watch the stars in the evening, yet it remains a bit of a nuisance, because it will interfere with the observations of a meteor shower The regular meteor shower of the Orionids is active approximately between October 20 and 26. The name indicates that the meteors appear to be radiating from the constellation of Orion. In fact, the radiant point lies in the north-eastern part of the constellation, near its boundary with Gemini. 
   For any meteor stream the (fairly obvious) rule is valid that the number of meteors you can observe increases as its radiant rises higher in the sky. In the case of the Orionids, this means that the activity should be significantly higher after midnight. Orionids are not a very rich shower: at its best you can probably expect to see 10 - 20 meteors per hour.
   The meteor stream of the Orionids is especially interesting for two reasons. One is the fact that the meteors are encountered in the morning hours, when we are, so to say, on the bow of the cosmic ship called the Earth. The Earth orbits the Sun in an orbit that is very nearly a circle. In addition to this orbital motion, the Earth also revolves on its axis. At noon, we are facing the Sun. At 6 p.m., we are looking back in the direction from where we have been coming. At midnight, we are facing that part of the sky that lies exactly opposite to the Sun. And at 6 a.m., we are looking forward in the direction where the Earth is moving. If some meteors happen to be coming from about the same direction, they encounter the Earth almost head-on, and their velocity with respect to us is quite high, up to 72 km/sec. (How to get this number: the orbital speed of the Earth is close to 30 km/sec, and meteors following the very elongated orbits of their parental comets have speeds close to the parabolic limit, which is 42 km/sec.) As a consequence, the Orionids are very swift meteors, and very often they leave behind a luminous trail.
   The other interesting aspect of the Orionids is their parental comet: it is none other than the famous comet Halley! It is an old stream, the meteors were ejected from the head of the comet a long, long time ago, so they have been dispersed along the entire orbital ellipse almost uniformly, and it would come to us as a great surprise if they appeared this year in unusually large number. Nevertheless, surprises are always possible! 

   Meteors and Meteorites
   Meteors are small pebbles, gravel, or "dirty snowballs." They shine high up in our atmosphere because they collide, at a high speed, with the molecules of air, and part of their kinetic energy is absorbed by the electrons in the atoms of nitrogen and oxygen. As these electrons fall back into their original orbits near the atomic nuclei, energy is released in the form of radiation. The meteor itself is also heated by the collisions and contributes its light. Naturally, the faster moving meteors, such as the Orionids, produce more radiation for the same original mass of the meteor. On the other hand, the high-speed collisions disintegrate the meteor very rapidly, so that it ceases to shine and disappears some 60 - 80 kilometers above the ground. 
   In order to survive the flight through the atmosphere, the meteor must be much larger, and it must enter our atmosphere from behind, so that the speed of the Earth is subtracted from its speed, and the relative velocity of the meteor is approximately (42-30) km/sec.
   Even then, a large part of the meteorite is dissipated in the atmosphere. In order to catch up with the Earth from behind, the "meteoroid" must be moving in an orbit typical for asteroids, and, indeed, meteorites do originate from asteroids, not from comets. I will explain in greater detail in one of the future issues of the Santa Monica Mirror.
   From time to time, there occurs a real "meteor shower" when many thousands of meteors penetrate our atmosphere per hour. None of them ends on the surface of the Earth as a meteorite, all are destroyed (evaporated and pulverized) high up in the atmosphere.