The Times asked Krupp for some tips on watching this astronomical show Tuesday. Below is a Q&A, based mostly on Krupp’s answers.

Tuesday’s Transit of Venus offers an exquisite opportunity to view one of the rarest planetary alignments –- kind of like Venus eclipsing the Sun from our view, except the planet is so far away that all we see is a tiny dot on the sun’s surface. The next time the Transit of Venus will happen will be in 105 years.

When the cosmic show begins after about 3 p.m. PDT Tuesday, “the sun will be fairly high. But the fun part is following it to sunset and seeing Venus gradually move across the disc of the sun,” Krupp said. “You really want a view of the horizon that is not blocked by buildings or trees.”

What’s the best place to see the Transit of Venus? In the United States, find a perch on high ground with a view of the northwestern horizon, so you can see the tiny black dot of Venus scoot across the sun as it sets Tuesday afternoon, says Ed Krupp, director of the Griffith Observatory in Los Angeles.

A: To watch the sun safely means viewing the show through a solar filter. The best view is through a magnified image of the sun – and that means through a filtered telescope. In Southern California, the Griffith Observatory Columbia Memorial Space Center in Downey and UCLA Planetarium will offer free access to outfitted telescopes.

Venus will cross its midpoint on the sun’s disc around 6:25 p.m. PDT. In the continental United States, the show will end at sunset, but Venus will continue to be visible on the sun’s disc until roughly 10 p.m. PDT westward. That means places like Alaska, Hawaii, Asia, Australia, eastern Africa and all but the western edge of Europe will get to see the planet exit the sun’s disc.

You can also buy special solar eclipse glasses that can be used to look directly at the sun safely, blocking at least all but 0.003% of visible sunlight. They’re still available at the Griffith Observatory gift shop for $2.99 a pair. A bargain deal, with pairs selling for just 85 cents, can be found at a wholesaler that manufactures them, Rainbow Symphony at 6860 Canby Ave., Suite 120, in Reseda. It will be open between 8:30 a.m. and 5:30 p.m. Tuesday.

TIPS ON VIEWING TRANSIT OF VENUS

You can also try to find so-called No. 14 welder's glasses that might be on sale at welder's shops or at home improvement stores. Or use a pair of binoculars to project the sun's light onto the sidewalk or a piece of paper, and see if you can spot a small dot that is Venus.

Q: How can I photograph the Transit of Venus?

If you have a solar eclipse glasses, you can cover the lens of a simple point-and-shoot camera and point it at the sun to take a photo. Folks were able to take snapshots of the recent solar eclipse that way. But take care that you don’t accidentally look at the sun with your bare eyes or fry your camera’s lenses without covering them up with protection.

Those with larger SLR cameras, however, will probably find that the solar eclipse glasses aren’t large enough to cover their lenses. At this late hour, it will probably be hard to purchase special solar filters that can cap SLR camera lenses.

If you do photograph the Transit of Venus, send it to the Times via Twitter by posting the hashtag #LAVenus and tweeting it to @LANow.

MORE ON PHOTOGRAPHING TRANSIT OF VENUS

Q: What is the significance of the Transit of Venus?

Transits of Venus have not been known to humans for very long. The theory behind the transit goes back to the 17th century, when German astronomer Johannes Kepler predicted that both Mercury and Venus would cross over the Sun’s disc from Earth’s viewpoint. But Kepler’s prediction of a transit in 1631 was disappointing in that it wasn’t visible in Europe during that year.

So the credit to the person who first predicted –- and observed –- the Transit of Venus goes to an English astronomer, Jeremiah Horrocks, which he did in 1639.

It wasn’t long before scientists realized there would be more transits in the future, and there were: in 1761, 1769, 1874, 1882, 2004 and 2012.

Adding a little bit of adventure to Transit of Venus was an idea formulated by English astronomer Sir Edmond Halley, who lived from 1656 to 1742. Halley –- whose name graces the famous comet –- suggested that if you observed the Transit of Venus from multiple locations on Earth, and calculated the exact times of the planet’s move across the sun, scientists could use trigonometry to far more accurately understand the size of the solar system.

The idea prompted many international expeditions in the 1700s to get the data. In 1769, observations were made from 76 points across the Earth, according to NASA, including a trip made by British explorer James Cook to Tahiti. It was a dramatic effort, but scientifically, the efforts turned out to be a bust. Frustratingly, at the moment Venus should touch the edge of the sun, the black dot elongates into the shape of a tear drop – an optical result that stretches out the image of the planet and made it impossible to precisely calculate, to the second, when the transit actually began. It’s known as the “black drop effect.”

“You couldn’t get a sharp time for when Venus moves onto the disc of the sun,” Krupp said. The result ruined the effort to more accurately calculate the size of the solar system.

The journeys were more disastrous for other explorers. French astronomer Guillaume Le Gentil was to travel all the way to India to make observations at the French colony of Pondicherry during the 1761 transit.

But shortly before he was to make the calculations, the English were battling for control of the colony, and Le Gentil had to flee, according to an account written by David Coward, an assistant professor at the University of Western Australia. Le Gentil couldn’t take accurate measurements when the transit occurred, because he was out at sea.

Le Gentil decided to try viewing the Transit again in Pondicherry, eight years later in 1769. The night before the event, the skies above the colony were clear, Coward wrote.

Then, he wrote, “the wind changed.” And then came the clouds.

And then there was the case of another unlucky French astronomer, Jean-Baptiste Chappe d'Auteroche. According to Andrea Wulf, author of “Chasing Venus. The Race to Measure the Heavens,” Chappe was to travel all the way to the Americas to observe Venus’ entire transit.

He reached Baja California with a few days to spare, and in a rush, decided to make the observations at a Baja California mission that had been stricken with typhus.

Shortly after taking the measurements, he died.

Able to photograph the transit? Tweet it to us at @LANow and tag it with #LAVenus. You can upload photos to our website.

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-- Rong-Gong Lin II

Photo: In this June 8, 2004, photo, amateur astronomer Jody McGowen looks through a telescope to watch the Transit of Venus from Sydney's Observatory Hill. Venus will again cross the face of the sun Tuesday, a sight that will be visible from parts of Earth. This is the last transit for more than 100 years. Credit: Mark Baker / Associated Press