Last chance to see Venus transit

It happens only four times every 243 years. If you want to see the famed transit of Venus, Tuesday is your last chance this century.

Kirsten Larson plans a pool party to watch the progress of Venus across the sun’s disk with her sons, 3 and 5, and their friends. “They probably won’t be alive the next time it happens,” the Lancaster, Calif., mom says. The next transit will be in 2117. This one is spawning interest in things astronomical. There are websites galore and even an app, VenusTransit, that will allow people to be part of an Astronomers Without Borders experiment to measure the size of the solar system.

During the transit, Venus will pass directly between Earth and the sun. A tiny dark circle will appear near the top of the sun and slowly move down and across it.

The nearly seven-hour transit will begin at 6:04 p.m. ET and will be visible until sunset where you are. The planet will edge out of the sun’s disk at 12:51 a.m. ET. Local times: transitofvenus.org.

Watching requires care. Looking directly at the sun can cause permanent blindness within a minute, says Ivan Schwab, a professor of ophthalmology at the University of California-Davis.

After a solar eclipse May 20, sun-viewing glasses are in short supply, says David Aguilar with the Harvard Smithsonian Center for Astrophysics in Cambridge, Mass.

One safe option is to use binoculars to project the sun’s image. Take the cap off one of the binoculars’ lenses, stand 2 or 3 feet away from a light-colored wall and point the binoculars at the sun — but don’t look into the eyepiece. Instead, look at the wall behind you, which will have an image of the sun projected onto it. Or find instructions online to make a pinhole projector with a box, scissors, tape and a pin.