This evening or tomorrow morning, depending on their locations, sky-watchers around the world will be able to witness a transit of Venus—a celestial event that won't be seen again for more than a century.

The transit will offer astronomers a chance to refine our understanding of Venus as well as to tweak models for searching for planets around other stars. (Pictures: See what the Venus transit will look like.)

Transits happen when a planet crosses between Earth and the sun. Only Mercury and Venus, which are closer to the sun than our planet, can undergo this unusual alignment.

With its relatively tight orbit, Mercury circles the sun fast enough that we see the innermost planet transit every 13 to 14 years. But transits of Venus are exceedingly rare, due to that world's tilted orbit: After the 2012 Venus transit, we won't see another until 2117.

During the upcoming transit, Venus will look like a black dot gliding across the face of the sun over the course of about six hours.

"Venus's diameter will appear only about a 30th the diameter of the sun, so it will be ... like a pea in front of a watermelon," said Jay Pasachoff, an astronomer at Williams College in Massachusetts. (Read a Q&A with Pasachoff about Venus transits.)

"The effect won't be visually impressive, but that black dot against the sun is a remarkable thing to see."