SAN DIEGO - People around the world will get a view Tuesday of a transit of Venus, a rare astronomical event when the planet's orbit carries it directly between the sun and the Earth.

During the pass, Venus appears as a small, dark round spot moving across the face of the sun, like a bug on a dinner plate.

Transits of Venus happen in pairs eight years apart, with more than a century between cycles. The next transit will not occur again until 2117.

The event allows viewers to see it (as long as they use proper viewing equipment for safety) as it appears to cross in front of the sun.  San Diego residents can purchase special glasses from the Reuben H. Fleet Science Center in Balboa Park for $2.

San Diego County residents can begin viewing the event at 3:06 p.m. and expect for it to last for about seven hours.

It's not all about pretty pictures. Several science experiments are planned, including studies that should help in the search for habitable planets beyond Earth.

If you don’t have special solar glasses to view the rare planetary alignment, what are you to do?

Can I put 10 pairs of sunglasses together to view the sun?

No. You’re risking temporary or permanent damage to your eye. Only two kinds of commercially available vision filters are safe – solar filters sold at science museums and planetariums or a No. 14 welder’s glass. These filters block all but 0.003% of visible light, and also protect your eyes from harmful invisible infrared light.

So if I don’t have solar glasses, what can I do?

If you have a pair of binoculars, try focusing the light from the sun onto a sidewalk, and see if you can see a little dot on the sun after 3 p.m.  through sunset. The planet will hit the center of its journey around 6:27 p.m. PDT.  (The transit continues westward beyond the continental United States in Hawaii, and Australia, Asia and Europe, and ends just before 10 p.m. PDT.)

Some experts also suggest making a homemade pinhole projector. But others warn that while they may work during solar eclipses, it may be difficult to see the tiny dot of Venus using this method.  Pinhole projection images are dim and small, according to the Exploratorium.

The best way, at this point, is to head to a Transit of Venus viewing party, because looking at the sight magnified through a safely-filtered telescope is the best option. You can find one at this NASA interactive map.

Or watch the event live online. NASA will anchor live Web coverage of the Transit of Venus from Hawaii.