The Dark Side

Two planets can be seen after dusk tonight: Venus and Jupiter. Venus has put on a great show all spring, but by next month it will be lost in the glare of the Sun, to emerge as a morning star in September. Jupiter is now low in the southern sky not too 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 over twenty miles each second. By contrast, Jupiter hasn't moved very far from where it was last year. Further from the Sun than Venus, it travels much more slowly, about eight miles a second. It hasn't moved very far from where it was last year.

It'ss the Sun's gravity that keep 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 out by 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, X-ray telescopes. They've searched for all the conceivable ways this material could be detected. And theyıve 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.

(07/11/07)