This Tuesday, Venus will move between the Earth and the sun—and you may want to catch it now, because you probably won't be around when it happens again in 2117. The event will be visible throughout the US shortly before sunset. On the East Coast, you'll want to be up high and facing west, because the sun will be low. Don't look directly at the sun, the New York Times reminds us: Instead, use eclipse-viewing glasses or read about projection techniques.
In observing the transit, you'll be following in the footsteps of historical luminaries like Captain James Cook. He and other 18th-century investigators used the event—which happens approximately twice every century, eight years apart—to figure the distance between the sun and the Earth: about 93 million miles. "Historically speaking," the 1769 observations that took place around the globe were "the beginning of big international science," says an expert. These days, it's more of a "tourist occasion." But NASA will be watching closely, and you can, too—with the aid of a smartphone app that lets you document the transit's timing and contribute to an international database.