Watch Live | Venus's Once in a Lifetime Trip

Over the course of about seven hours, Venus will pass between the Earth and the Sun on Tuesday. It’s an extremely rare celestial event ― one that won’t happen again until 2117.

That’s part of the reason for all the hype surrounding the planetary lineup that starts Tuesday around 6 p.m. EDT, according to Dr. Michael Shara, an astrophysicist at the American Museum of Natural History.

Shara said it's actually a two-part event.

“It happens twice, separated by eight years. And then you have to wait 105 years for it to happen again, so it’s pretty rare,” he said.

As long as the sun is above the horizon while it’s happening, people can see it. For the New York City area, that means from about 6 p.m. until the sun sets at around 8:30 p.m.

Shara notes that people should not look at the sun directly because it can do serious damage to the eyes. And in any event, you’re not likely to see much. During the transit, Venus is a tiny black dot, not always visible to the naked eye. Shara said it’s best viewed through a telescope that has a proper filter.

The Amateur Astronomers Association of New York plans to set up two sites with safe, solar-viewing telescopes: one on the High Line at 10th Ave. and 14th Street, the other at Riverside Park South on the pier next to Pier 1 Café.

(Photo: People wait for the weather to cooperate to view the transit  from the High Line. Annmarie Fertoli/WNYC)

There’s a lot to learn from the event. In the 1700s and 1800s astronomers used information from the transit of Venus and some trigonometry to figure out the distance between the Earth and Venus and Venus and the Sun — getting an idea of the size of the solar system.

We know the distance to Sun, so to Shara’s mind what makes the transit really exciting this year is using it to try a new technique to get information about Venus’s atmosphere. If you get information about the sun’s spectrum (Shara says it’s like the sun’s DNA) before and during the transit, when it also passes through Venus’s atmosphere, and then subtract one from the other you’re left with the spectrum of Venus – information about it's atmosphere.

Now imagine another planet transiting a different star.

“If we take a spectrum of the star before and during the transit, we can actually measure what the atmosphere of plants orbiting other stars are like,” Shara said.

And in doing so, look for signs of oxygen and possibly signs of life.

If the weather in the tri-state area is uncooperative for this once in a life time event, Shara says to hit the web to see a bit of the transit.

NASA TV will be offering a livestream of the event.

Starting at 5:45 EDT, NASA EDGE has a live stream of the transit of Venus below.

Live streaming by UstreamIf you want to learn more about the Transit of Venus, NASA did a nice explainer.