STARRY SKY ABOVE SANTA MONICA

August 11–17

Mirek Plavec

Emeritus Professor of  Astronomy, UCLA

Meteors Above Santa Monica

   Meteors above Santa Monica would probably be a more fitting title this time. For this is the week when it is easiest to see a lot of meteors! No doubt many of you like to get out of the city and into the mountains or deserts as often as possible, and I hope you will do it this Thursday. But even if you are ready and willing to undertake such a trip just once per year, my recommendation is: do it on Thursday night, August 12/13! Or, if this is entirely impossible, the Wednesday night (11/12) or Friday night (13/14) might also do. 
   If you manage to escape artificial lights entirely, you will be treated to a very nice celestial show - a meteor shower. You may see some 60 to 80 meteors per hour! Only remember that the show is at its best after midnight! These August nights are typically among the warmest of the entire year, so sky watching after midnight is no great sacrifice. After all, you can (actually, should!) lie comfortably on a reclining chair, and since for the meteors you need only your eyes and no instruments, you can cover the rest of your body as much as you wish! Your chances of having clear skies out there in the desert are pretty high at this time of year. 

New Moon Tonight

   This year, even the Moon cooperates, by not showing at all! The Moon is new on August 11, and the very thin crescent you might see in the early evening of the following nights will set quite early after sunset, leaving the sky dark.
   Every year at this time, the Earth passes through a fairly thick swarm of meteors, which orbit the Sun in a very elongated ellipse. They follow the orbit of comet Swift-Tuttle. The comet itself visited us in 1992, and before that, in 1862, when it was discovered. The comet is nothing conspicuous. Perhaps it is an old, worn-down comet that has lost too much of its stuff—thereby creating a prominent meteor stream!

Bagfuls of Nothing

   It is funny to realize that for many centuries, comets were considered to be mighty and dangerous beasts, while in reality they are better described as “bagfuls of nothing.” When a comet is very far from the Sun, that is, somewhere among the outermost planets, it consists of only one part: the nucleus. And this nucleus is only a dirty iceberg, a fairly large block of ice (water ice and “dry ice” of carbon oxide, methane and ammonia), mixed with stones, boulders, gravel, and dust. When comet Halley, one of the larger comets, visited us in 1975, it was possible to observe its nucleus directly. It has the shape of a big peanut, or of a potato (spelt without an -e), with dimensions 16 km x 8 km x 8 km. When such a dirty iceberg is far from the Sun, it is in a kind of cosmic freezer. When it comes close to the Sun, it heats up, the ices evaporate and directly sublimate into gas. More volatile substances evaporate so fast that explosions occur, throwing out not only gas but also stones, gravel, and dust. The gas forms a nebulous coma about the nucleus. This, then, is the head of the comet, which can be as large as a big planet (and, in the comet seen in 1811, was larger than the Sun!). 

Comet’s Orbit

   The comet must follow an elliptical orbit about the Sun under the gravitational attraction of the Sun. However, two other forces emanate from the Sun: the radiation pressure, when the photons of radiation hit the tiny gas molecules and push them away from the Sun, and solar wind, which is a fast stream of solar atoms and molecules, driven outwards from the solar atmosphere. When these fast-moving particles hit the molecules of the comet, they push them away from the Sun. The famous tail of the comet is formed by the joint impact of these two forces. Gas molecules and dust are in this way driven out of the head of the comet and eventually disappear in space.
   More important for us here are the powerful explosions in the nucleus, which throw out larger pieces—the future meteors. These are often simply small stones. However, the nucleus, being a dirty iceberg, throws out plenty of dirty snowballs, mixtures of solid gravel and ice. Especially in some meteor streams, the meteors disintegrate in our atmosphere so fast that they must be snowballs or ice balls rather than small stones! In all cases, the meteors are thrown out from the comet’s head at speeds much lower than is the orbital speed of the comet. Thus, they follow essentially the orbit of the comet, but are gradually dispersed behind, as well as ahead, of the nucleus.

Meteor Showers

   It does happen now and then that the orbit of the comet intersects the orbit of the Earth. So far, in each such case, we did not meet the comet at the intersection. However, we do occasionally encounter the associated meteors. In young meteor streams, large numbers of meteors are found only in the vicinity of the comet. If we run into them, we may get a fantastic meteor shower, with thousands of meteors seen per hour. We may have this opportunity in the fall. If the meteors were ejected from the comet a long time ago, they have stretched out along the entire orbital ellipse. We encounter fewer of them, but we encounter them every year, at the orbital intersection. This is the case of the Perseids. In the vicinity of the parental comet, which crossed the Earthís orbit in 1992, there were more meteors, but not strikingly more. On the other hand, the annual encounter promises you to see quite a lot of meteors.

Big Planets of The Morning

   And as you keep looking for the meteors, the constellations will gradually flow across the sky: the summer constellations with Vega, Altair, Deneb, and Antares (the latter near Mars) will slowly slide towards the west, and the autumnal constellations will be coming up: Cassiopeia, Andromeda, Pegasus, Perseus, and with them, the two biggest planets of our solar system will rise in the east: first Jupiter, and shortly after, Saturn. There is no need to explain how to find Jupiter. After the demise of Venus, Jupiter is by far the brightest object in the sky, and begins its reign already shortly before midnight. Saturn lies to the east of Jupiter, rises by midnight, and shines like a star of the first magnitude. So Saturn does have competition in the sky as to the brightness, but it lies fairly close to the east of Jupiter, and no first-magnitude star is anywhere nearby, so the identification will be easy, too. While Saturn does have competition as to its brightness, in its appearance in a good telescope, it is definitely without competition!