It's Venus Transit Day: A Twice-in-a-Lifetime Experience

Get ready for a rare astronomical and sky watching event Tuesday—the passage of the planet Venus across the face of the sun. 

Locally, the transit will begin at about 3:04 p.m. It will be visible in its entirety only from the western Pacific, eastern Asia, eastern Australia and at high northern latitudes.

Transits of Venus are rare, happening in pairs eight years apart, with each pair separated by more than a hundred years. This June's transit, the bookend of a 2004-2012 pair, won't be repeated until the year 2117, according to the NASA website.

NASA will webcast it from the summit of Mauna Kea, HI. Click here for Venus transit of 2012 tips on how to see Venus cross the face of the sun.

Over a seven-hour span, Venus will trek across the solar disk, appearing in silhouette as a slow-moving, tiny black dot—weather permitting—according to the NASA website.

"Only six such events have occurred since the invention of the telescope," said astrophysicist Sten Odenwald, of NASA's Goddard Space Flight Center in Greenbelt, MD, in a statement.

The key to watching it is to find an unobstructed view of the horizon. It is recommended observers watch from the top of a high building. The transit will occur near the lower rim of the sun, according to NASA.

How to View the Transit of Venus Safely

The transit will be visible to the naked eye but experts say it's . 

As with the recent solar eclipse, experts says it's crucial that you choose a safe way to view the transit. Looking directly at it would damage your eye's retina, the light-sensitive area at the back of the eye that provides central vision. 

Safe options include:

• Watch the transit at a planetarium or program by a university astronomy department. Because Venus will look quite tiny against the sun's vast surface, it will be best to watch this amazing event via professional projection on a large screen.
• Visit NASA's website for a live-streaming broadcast and enjoy a live chat with scientists, if you like.
• Make a simple "pinhole camera" using two sheets of paper: make a pinhole in the center of one sheet; then stand with your back to the sun, holding that sheet so that the sun shines through the pinhole onto the second piece of paper. You'll see an image of the transit of Venus projected on the second sheet.

The following devices will NOT protect your eyes: sunglasses, binoculars with filters, neutral density filters, or exposed photographic or radiographic film, according to the American Academy of Ophthalmology.

The transit will be shown on NASA TV, which is available on many cable and satellite transmission systems, and on NASA TV's website.