Want to see something that no living person has ever seen before?

Well, if you do a little planning today and get up a little early tomorrow, you can.

Tomorrow morning, just as the sun rises, you'll be able to see Venus passing between the sun and Earth. This event is called the transit of Venus, and the last time it happened was 121.5 years ago. (We told you no one alive today has seen it!)

Now don't hop out of bed tomorrow and look up at the sun; that could really hurt your eyes. You can spend some time today making a device that will let you look safely at the sun (go to www.skyandtelescope.com and click on "observing"). You also can watch the transit on the Web or at one of several places in the area with special viewings (see box).

But what will you see?

Well, Washington Post reporter Guy Gugliotta says that Venus passing the sun will look like "a bug walking across a lightbulb."

Venus is about 1/30th the diameter of the sun, and the transit (or bug's walk across the lightbulb) will take about six hours across the bottom third of the sun. People in parts of Africa and Europe will be able to see all of the transit, while people on the West Coast of the United States won't be able to see it at all. Folks on the East Coast (that's us) will only see about the last two hours of Venus's trip.

Okay, so the idea of a planet creeping its way across the path of the sun sounds pretty neat, but why should you care?

Well, almost 400 years ago, before space launches and telescopes, astronomers and mathematicians were just figuring out the size of the solar system and how planets moved.

By 1716, Edmund Halley (yeah, the comet guy) thought he could use a Venus transit to figure out just how far Earth was from the sun.

Mathematicians came up with really complicated trigonometric formulas for figuring it out and sent observers all over the world to track two transits, in 1761 and 1769. But it didn't work. Oh the math was right enough, but it turns out that Venus's atmosphere is really dense, making it impossible to do the calculations.

Nowadays, experts don't need a Venus transit to figure out distances in the solar system, but scientists can use Venus's passing in front of the sun to possibly spot planets orbiting other stars.

And besides, don't you want to be able to say you saw something that hasn't happened in more than 120 years?

-- Tracy Grant