It seldom happens. It will not be visible from Earth again for more than 100 years. But it will be possible to see it Tuesday. And it’s free.

Tuesday evening people in Washington and elsewhere on the East Coast will be able to see the planet Venus cross the face of the sun, starting shortly after 6 p.m., in a celestial phenomenon known as the Transit of Venus.

A NASA Web site called it a “a rare and striking phenomenon you won’t want to miss.”

Viewing will not be easy. Looking at the sun with unprotected eyes is dangerous, and experts warn against it. Sunglasses are inadequate. Eclipse glasses or extremely dark welders’ glasses are needed.

Beyond the special measures necessary to protect the eyes, keen vision will be a plus. The sun is relatively small in the sky, and Venus will be little more than a black dot crossing its face, starting at the sun’s upper left.

“The Transit of Venus is best viewed directly when magnified,” NASA said. But it warned that any telescope or binoculars used should be equipped with a filter to protect the viewer’s eyes.