Starry Skies Above Santa Monica

August 1-8, 2002

Mirek Plavec
Emeritus Professor of Astronomy,
UCLA

   Our Beautiful Evening Star
   – Still With Us
   The Sun will set by 7:52 p.m. (on Saturday), and rise at 6:06 on Sunday, so the days are getting just a bit shorter, and the nights a bit longer. However, these nights are the warmest of the year – a kind gesture towards those of us who love to watch meteors! However, we must be patient, since the night comes slowly. The brightest stars will be visible shortly before 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 into the constellation of Virgo, just one degree above the celestial equator. This implies that it passes through the zenith for people who live that close to the terrestrial equator. For us, Venus shines low above the western horizon, is quite bright, but sets already shortly before 10 p.m. Look for it by or after 8:40 p.m.

   Our Kindly Friend, the Moon
   As an ardent observer of stars, comets, and meteors (for 64 years!), I have always hated the Moon, which makes the sky background so bright that hardly any stars are visible about the time of Full Moon. That time is now past, and the Moon very kindly recedes in the wee hours of the morning – when the winter stars begin to show above the eastern horizon. On August 3, the Moon will pass below the famous star cluster of the Pleiades, then it can be seen above Saturn late on the night of August 4/5, and at the end of our week, it will shine below the “stellar twins,” Castor and Pollux in Gemini. By that time, the Moon will be a very thin crescent, since it will be New on August 8.

   The Story of Meteor Streams
   Last time, I called your attention to the famous meteor stream of the Perseids, which is active now, but will reach its maximum activity in the late hours of the night of August 12/13. And then I started to tell you the story of how meteors were eventually “admitted” into astronomy, after the fantastic “meteor storm” of November 12/13, 1833.
   That storm came again in 1866, and astronomers also “discovered” a report by the then famous traveler Alexander Humboldt, who saw a similar meteor storm in South America in 1799. This indicated that the meteors moved in an orbital ellipse with a period between 33 and 34 years. (I will not bother you, dear reader, with the fact that this period was not the only possible solution – the meteors could also have a period of 0.97, 0.49, 0.51, or 1.03 years – but 33.25 years happened to be the right guess).
   And then, astronomer Tempel discovered a comet in 1866, and when its orbit was calculated, its orbital period was found to be 33.25 years, and the orbit was so oriented that it nearly intersected the orbit of the Earth at a point where the Earth arrives every mid-November!
   Actually, the first evidence of a close relation between comets and meteors came a few years earlier, and was related to the August meteor shower of the Perseids. The Perseids never appeared as a true “meteor storm”, but, on the other hand, they come in good numbers quite regularly every August, and the first reliable report about them dates from the year 1762.
   When we want to calculate the orbit of a body in our solar system, we need to know six parameters: three of them represent its position (say x,y,z –- to refresh your memory of your beloved high-school math), and three corresponding components of velocity that the object has at the same point. When a meteor collides with the Earth, we know its position in space – it is identical with that of the Earth at that time. When we know that meteors of a meteor stream radiate, say, from the direction of Perseus, we know the direction of their velocity – but this is not enough, we must know the speed, say, in kilometers per second. And that was the stumbling block for the astronomers of the 19th century – they had no means of determining the speed. A policeman stopping you for allegedly driving too fast can present no convincing evidence if he has no radar!
   The Perseids have no distinct periodicity. They arrive pretty much in the same number every August. Thus, the clue offered by the periodicity of the Leonid meteor storms is missing here. In spite of this obstacle, the relation between the Perseids and a comet was actually discovered four years before, in 1862.
   By that time it was known to astronomers that comets move in very elongated ellipses. They may pass close to the Earth and even penetrate deeper, into the region where Venus and Mercury orbit – but when farthest from the Sun, at aphelion, they are typically located well behind the orbit of Saturn. For such an elongated elliptical orbit, the comet’s speed in the vicinity of the Earth is fairly near the speed valid for a parabola, 42 kilometers per second.
   Italian astronomer Schiaparelli used this fact to estimate the orbits of the Perseids and the Leonids – and then he recognized that the orbit of the Perseids is strikingly similar to that of comet Tuttle - Simon - Swift, which was seen in 1862.
   In this way, astronomers discovered that the meteor showers are caused by particles that orbit in the orbits of comets, and are therefore essentially cometary material. This fact has been confirmed many times since, and has told astronomers much about the objects that had been completely mysterious for many centuries – namely, the comets.