Post by Chicago Astronomer - Astro Joe on Jun 30, 2005 0:04:56 GMT -6
Venus and Jupiter are telling us something
This is from Errol Pomerance. He runs Skyshows of Vermont, dedicated to bringing planetarium shows to Vermont schools. It's quick, to the point, and also applies to Chicago skies as well.
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Two planets can be seen after dusk tonight: Venus and Jupiter. Venus has just emerged from the glare of the setting sun and will be easier and easier to see as the summer progresses. Jupiter is gleaming high in the southern sky.
Together, these two planets tell us something very strange.
In the early spring, Venus was a morning star, visible in the east before dawn, west of the sun. Now it is east of the sun — an evening star, blazing in the western sky after sunset. By next year, it will be a morning star again. By contrast, Jupiter hasn't moved very far from where it was last year.
Venus changes so quickly (from morning star to evening star) because it is speeding around the sun at more than 20 miles each second. But Jupiter travels much more slowly, only about eight miles a second.
It's the sun's gravity that keeps Jupiter and Venus in orbit. Out by Jupiter, the sun's gravity is much weaker than it is by Venus. If a planet going as fast as Venus were near Jupiter, it would quickly escape from the solar system.
That's the general rule: The farther an object is from the center of attraction, the slower it goes.
But the stars circling the center of the galaxy are all going about the same speed — no matter how far they are from the center. They're moving too fast for the material (what we can see) at the center of the galaxy to hold them.
Astronomers have searched for whatever is holding the galaxies together with all sorts of telescopes, not just the telescopes we all know about. They've used radio telescopes, microwave telescopes and X-ray telescopes. They've searched for all the conceivable ways this material could be detected. And they've found nothing.
It just can't be seen. So they call this invisible material dark matter. We know it must be there, but we can't see it. What it is is one of the great mysteries of the universe.
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All stars are traveling at the same speed around the galaxy eh? A great mystery...
