Starry Sky Above Santa Monica

AUGUST 4-10

Mirek Plavec

Emeritus Professor of Astronomy, UCLA

Meteors Are Coming

   Meteors are coming! You can see a meteor (or, as it is often called, a "shooting star") on any night of the year, if you watch the starry sky away from city lights. Your chances of seeing several meteors per hour increase after midnight. And you will see more meteors in the fall months than in the spring. I will be happy to explain "why" below. It may be a little bit heavy for many of you, so I will talk now about more exciting things in the sky. 
   Let me say here that on an ordinary night, you will hardly see more than 10 –12 meteors per hour, so you will probably fall asleep pretty soon. These meteors appear at random, move in random directions, and we call them sporadic meteors. During the first days of August, however, things change. Night after night (if the Moon and the clouds permit), you will be seeing more and more meteors, especially after midnight. And they do not fly at random; rather, they appear to radiate from a point located between the constellations Cassiopeia and Perseus, in the north-eastern sky. This radiation is a simple consequence of perspective: we are encountering a meteor stream, the Perseids, in which the meteors are moving in parallel orbits. You will not see them originate in that radiant point. Some meteors, appearing in the southwestern part of the sky, will have long light paths; others, seen closer to the radiant point, will have only short trajectories. However, when you mentally extend their trajectories backward, they will intersect at the radiant point.


Peak Activity

   The peak activity of the Perseids will come next week, on the night of August 12/13 (Thursday/Friday). Next week, I will try to persuade you to take your kids or friends on an outing, and spend that night in the desert, under the (hopefully!) starry sky. Since such a trip may need some arrangements and preparations, I advertise it a week ahead.
Mars is the only evening planet, after the departure of Venus. All the week August 4 through 11, it remains in the constellation of Libra (the Scales), actually in close vicinity of the star Alpha Librae, just a bit below it . Last time, I recommended that you take binoculars or any telescope at hand, and look at this star. It is a double star, with a fainter component easy to detect. And I also recommended to you to look at the only other tolerably bright star in Libra, Beta Librae, which lies above Alpha and somewhat to the northeast. Some people see that star as green (I do, too), which is rather puzzling.

More On Meteors

   And now, more on meteors!
   When something resembling a star suddenly flashes across the sky and disappears in almost no time, many people would call it a shooting star. Some of my students are occasionally willing to believe that this was a fast-flying comet, but I don't think you would accept this explanation. Comets do move with respect to stars, true, but they are very far from us in the planetary system, typically tens of millions of km, so their motion with respect to stars is very slow and often can be easily recognized only from one night to another. What you saw was, of course, no star either; bright or faint, we call it a meteor. A typical, ordinary meteor is actually an unbelievably small piece of rock, just like a small stone weighing a few ounces. And it shines high up in the atmosphere; starts to shine about 120 km above the earth, and disappears at 70 - 80 km above the earth. 
How can such a tiny stone shine as bright as a star? It penetrates our atmosphere at a high speed, often at some 50 km per second, and therefore has a fairly high kinetic energy. If a car hits a tree at a speed of, say, 70 km per hour, this is rather a big catastrophe. Now the tiny meteor has about the same amount of kinetic energy: it is much lighter than the car, but it has a much higher velocity, and it is velocity that largely determines the amount of kinetic energy a body has (it enters into the formula with the second power). As the meteor penetrates deeper into the atmosphere, it collides with more and more air molecules, slows down but heats up, shines for a very short time (and also makes the molecules of air shine), and then disintegrates— it is pulverized by the collisions with air molecules into a swarm of tiny dust particles.

Sporadic Meteors

   Let's now talk about the randomly moving sporadic meteors. Why should we see more of them in the morning than in the evening? To understand this, we should consider the situation in space. The Earth orbits the Sun counterclockwise (if observed, say, from Polaris), and it also rotates on its axis in the same sense, although the axis of rotation is tilted, not perpendicular, to the plane of orbit. When we have noon in California, we face the Sun. When midnight comes, we have turned by 180 degrees and are looking in the direction opposite to that of the Sun. Six hours later, we have turned an additional 90 degrees and are now looking in the direction in which the Earth is moving. On the contrary, in the evening, we are looking backward, as if through the rear window of our vehicle. Sporadic meteors are coming essentially from all directions. However, if they are coming from behind us, they have to catch up with us, since the Earth is running away from them at almost 30 km/sec. Thus their relative velocity with respect to us is small, say 40 - 30 = 10 km/sec; they have correspondingly lower kinetic energy, and shine much more feebly. This is why observe few sporadic meteors in the evening hours. Meteors coming towards us—those we can observe in the morning—hit our atmosphere more or less head-on, their relative speed can be as high as 40 + 30 = 70 km/sec, and are much more conspicuous. 
Obviously, those meteors will appear brightest that hit our atmosphere very nearly head-on. The point in the sky toward which our Earth is moving is called the apex. As we circle the Sun, the Sun appears to us to move around the ecliptic eastward, and the apex follows the Sun by 90 degrees. Everyone knows that the Sun is lowest in the sky in December, and highest in June. The apex, then, is lowest in March, and highest in September. The higher the apex appears to us, the more sporadic meteors can be seen: thus they are rare in the spring and most numerous in the fall. It would be, in any case, rather a pitiful show, if it were not for occasional meteor streams. Again, for good visibility of a meteor stream, its radiant point should be high in the sky. The radiant point of the Perseids is low in the north-east in the evening, and rises higher with every hour of the night. This is why we observe most Perseid meteors in the pre-dawn hours.