Indianapolis ? With concerts and sleepovers, astronomy buffs are giddily preparing to watch Venus’ silhouette cross the face of the sun Tuesday — the first time that sight has appeared in 122 years.

“Some people have been waiting all their lives for this. Nobody alive has seen it,” said astronomer Peter Boyce, who plans to watch the “transit of Venus” from Massachusetts’ Nantucket Island.

People across much of the Earth will be able to see it, but it won’t be visible in the western United States. In the Midwest and East, viewers can watch its final stages for a couple of hours starting at sunrise, if skies aren’t clouded over.

Transits of Venus occur twice — eight years apart — about every century, when the sun, Venus and Earth precisely line up. Past transits — the last pair were in 1874 and 1882 — helped astronomers calculate Earth’s distance from the sun.

In New York City, the Hayden Planetarium at the American Museum of Natural History plans to set up a bank of telescopes in Central Park to give people a view once the sun rises above Manhattan’s skyline. The sun’s image will be projected onto white screens so that dozens of people can watch the passage of Venus, which will appear as a small black dot.