Many people are planning to watch the transit of Venus on Tuesday, but it’s extremely important that prospective viewers be warned to take special precautions (as with a solar eclipse) to view the silhouette of Venus against the brilliant disk of the sun.

For the United States and Canada the transit will begin when the dark disk of Venus first touches the outer edge of the sun, an event that astronomers call Contact I. From the Eastern U.S. and Eastern Canada, Contact I should occur around 6:03 p.m. EDT. From the Western U.S. and Western Canada, Contact I should occur around 3:06 p.m. PDT.

It will take about 18 minutes for the black disk of Venus to move completely onto the sun’s face; ultimately bringing its black disk just inside the sun’s upper left edge. If you imagine the sun’s disk as the face of a clock, Contact I will occur between the 11:30 and 12 o’clock position. Venus will then progress along a track that will run diagonally from the upper left to the lower right.

If you wish to generate predictions for the transit times from where you live, the Astronomical Applications Department of the US Naval Observatory has produced an online Transit Computer at: http://www.usno.navy.mil/USNO/astronomical-applications/data-services/transit-us

Unlike transits of the sun involving the planet Mercury, those of Venus are readily visible with the unaided eye; the planet appears as a distinct albeit tiny black spot with a diameter just 1/32 that of the sun. This size is large enough to readily perceive with the naked eye. [Venus Transit of 2004: 51 Amazing Photos]

But again . . . be careful! Eye safety is always a prime concern when dealing with the sun. Observing a transit is a lot like studying sunspots because, after all, you are looking at a dark spot on the sun.

But trying to see a transit is also like trying to view a solar eclipse. You have to be ready at a particular time, and you may have to travel far from home. For the transit of Venus, however, your exact location is much less critical than it is for a total solar eclipse.

In particular, observers in Eastern North America, where the transit will happen in the early evening, your observing site should have a low horizon to the east-northeast. It is a good precaution to check the sun’s setting point, to verify that trees or buildings do not block your view. As Venus moves across the face of the sun, it will appear absolutely jet black in contrast to the lighter gray of any sunspots that may also be present on the solar disk.

By far, the safest way to view the transit is to construct a so-called pinhole camera. A pinhole, or small opening, is used to form an image of the sun on a screen that is placed about three feet behind the opening. [Video: How to Make a Solar Eclipse Viewer]

Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card. Just be sure not to look through the binoculars or telescope when they are pointed directly at the sun! Venus should appear as a distinct, albeit tiny, dot on the projected image.

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