When Venus passed between Earth and the sun 251 years ago Tuesday, scientists scribbled down observations that helped calculate a rough estimate of the size of our solar system. Using crude telescopes, they watched the yellow planet move across the sun's face as a tiny black disk.

There is little more the same rare phenomenon, known as a transit of Venus, will reveal about our closest neighbors in space when it occurs again Tuesday. But astronomers will be watching nonetheless, hoping it will teach them to better discover and investigate planets that are much farther away and could sustain life.

That's not to mention that it will be the last transit of Venus visible in these parts for 113 years, offering a rare perspective of the heavens above. Scientists and backyard astronomers around the world will gather Tuesday to watch the celestial event.

"It's sort of an exciting thing to be able to see," said Dan Richman, a physics graduate student at the Johns Hopkins University. "It's nice to be reminded sometimes that we're part of this group of planets spinning around the sun."

Transits of Venus occur in pairs, with the last occurring in 2004. The next is set for 2117, but it will occur after sunset for the East Coast. In Baltimore, the next to be seen will occur Dec. 8, 2125.

That has many eager to see history overhead. The transit is only the eighth to occur since the invention of the telescope. One was first observed in 1639, but scientists didn't widely study a transit until 1761, at the urging of English astronomer Edmond Halley some 50 years earlier.

"It has a lot of historical significance because it was a critical event along the way of being able to measure distances in our solar system," said Lori Glaze, associate chief of the planetary geodynamics lab at NASA Goddard Space Flight Center.

Viewing opportunities have cropped up across the region to share the experience. Like any solar phenomenon, it is dangerous to view the transit of Venus with the naked eye. Looking at the sun won't cause pain, but it can cause blindness.

The silhouette of Venus will inch across the face of the sun starting at 6:04 p.m., completing its path nearly seven hours later. Of course, once the sun sets at 8:30 p.m., it won't be visible here, but NASA is broadcasting the event online from Hawaii, an ideal viewing location for this transit.

The Howard Astronomical League, a group of about 100 sky-watching enthusiasts, will have about 25 telescopes on hand at the Howard County Conservancy equipped for safe viewing, said Wayne Baggett, the group's first vice president. Events at the Maryland Science Center, Space Telescope Science Institute and various Carroll County libraries will also offer opportunities to safely watch the transit.

Baggett said his group is eager to share the phenomenon with others, but he encouraged viewers to keep their expectations in perspective.

"It's not going to be fireworks. We're going to see a little black dot moving across the face of the sun," he said. "It's not like it's an exciting thing like a football game, but it's exciting because they're rare."

For some, simply seeing a glimpse of evidence of outer space makes the experience worthwhile.

"They're actually observing something that's happening in the heavens, and that always makes it special," said Jim O'Leary, executive director of the science center.

Even though scientists have likely already learned all they can about our solar system from the transit, it will be an exciting opportunity for them, as well.

The way they once used the transit to learn more about Venus and this solar system is the same way they now discover and study faraway planets, when they pass in front of distant stars around which they orbit. The stars flicker and twinkle when planets pass by, giving scientists a sign of the planets' presence.

"The primary objective is to use this transit as an example of what's going on when we're trying to discover new planets," Glaze said.

Aside from that, scientists will also watch the transit's reflection on the moon via the Hubble Space Telescope to study more about Venus' atmosphere, and they will be collecting any other data they can for the good of future generations.

"We never know exactly what might be useful so we try to collect as much as possible," Glaze said.