Your last chance to watch a truly rare cosmic event is coming to New Hampshire on Tuesday night.

It's called the “Transit of Venus” and to put it in perspective, this is only the eighth time it has occurred since Galileo first aimed his telescope at the heavens in 1609, according to Dave McDonald.

McDonald is director of education at McAuliffe-Shepard Discovery Center in Concord, which is hosting a Transit of Venus event Tuesday, starting at 5:30 p.m.

The last time a Transit of Venus occurred was in June of 2004, but the one before that was in 1882.

The next time it will happen? Dec. 11, 2117.

“This is the last transit for anyone alive today, and even kids should write notes to their grandkids,” McDonald said.

Here's why: “If you have a 10-year-old kid now, if they had a child when they're 25, then that child would have to live to be 90 to see it,” he explained. “So really, any kids that are alive today, it's going to be their grandkids that will have the next opportunity to see it.”

The Discovery Center plans a number of events the evening of the transit, including children's activities and special planetarium shows. Special glasses to help you watch the transit safely are included in the $9 admission ($8 for students and seniors; $6 for children), and visitors can also use the center's solar telescopes to get a closer view.

It's “really, really, really important,” McDonald stressed, that no one look directly at the sun, even with sunglasses. You'll need to use either special glasses designed for viewing an eclipse or transit, No. 14 welder's glass or a telescope with a special solar filter.

What you'll see starting at 6:03 p.m. is the tiny black sphere of Venus (which is about the same size as Earth) crossing the sun. It will take about 6-1/2 hours for the planet to finish crossing the much-larger sphere of the sun, but you'll only be able to view the event here until sunset, which is at 8:23 p.m., McDonald said.

And if it's too cloudy to watch the transit live in Concord, the Discovery Center will have a video link to another location where the event is visible, McDonald said. “It's bound to be clear somewhere.”

NASA will offer a webcast of the Transit of Venus on NASA.gov, which also has an abundance of historical and astronomical information. For example, there's this tidbit: At the request of the Smithsonian Institution, John Philip Sousa wrote his “Venus Transit March” after the Transit in 1882, to honor American physicist Joseph Henry.

Two things have to happen at the same time for the Transit of Venus to occur, McDonald explained. “Venus has to be at a position we call inferior conjunction, which means it's between Earth and the Sun,” he said. And “it has to happen when Venus' and Earth's orbital planes intersect.”

Venus and Mercury are the only planets that pass between the Earth and the Sun. If you like to plan ahead, mark May 9, 2016, on your calendar; that's when the next Transit of Mercury will occur.

There's something about such events that makes people want to gather together, McDonald said. He'll be watching at the Discovery Center with his wife and 13-year-old daughter.

And, he said, “I think they do cause us to wonder about the vastness and the awesomeness of our universe.

“I'm tremendously excited. I can't wait.”

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Viewing the transit

When: Tuesday, starting at 6:03 p.m.

Safety instructions: Visit venustransit.nasa.gov/2012/transit/viewing.php.

Special glasses: McAuliffe-Shepard Discovery Center in Concord has transit glasses available in its gift shop for $3.

Group viewing: For more on the center's Transit of Venus event, go to www.starhop.com.