Tonight, Tuesday, June 5, as the sun set on the East Coast, the planet Venus began its "transit" across the face of the sun. Just for your information, this is your only chance to witness such a crossing. The next one will take place in 2117.
Why only once a century?
That's because the transit of Venus is basically an eclipse, just like when the moon passes across the face of the sun. It requires a close alignment of the sun, Venus and Earth. The moon is not nearly as large as Venus but it is so close to Earth that its disk covers nearly the entire solar disk. Venus, on the other hand, is so far away that it appears as a small black dot covering about 3% of the solar disk.
Venus orbits the sun every 226 Earth-days, compared to our 365 days, so the two planets sort of pass each other relatively often. But their orbits lie in slightly canted planes, so the alignment needed for an eclipse happens only once a century or so.
Nearly 500 years ago, Johannes Kepler figured out the orbits of the planets using Newton's law of gravity. He understood that Venus was 30% closer to the sun than Earth, and that Mars was 50% more distant. But he didn't know how to measure the distance from any planet to the sun.
A century after Kepler, Edmund Halley, of comet fame, proposed measuring the distance from Earth to the sun using a series of observations of the transit of Venus from widely separated points on Earth.
Venus is the second closest planet to the sun, after Mercury; Earth is third; Mars is fourth. That means only Mercury and Venus (and our moon) can cause eclipses from a vantage point on the Earth. But if we lived on Jupiter, Earth and Mars could also be seen (rarely!) transiting across the sun.