Washington:
Astronomers around the world will train their telescopes on the skies Tuesday to watch Venus pass in front of the Sun, a once-in-a-lifetime event that will not be seen for another 105 years.
The nearly seven-hour passage of the planet between the Earth and the Sun will be seen as a black dot on the solar surface, but should only be viewed through approved solar filters to avoid the risk of blindness, experts warn.
On the evening of June 5, North America, Central America and the northern part of South America will get to see the start of the transit -- clear skies permitting -- beginning at 6:03 pm Eastern (2203 GMT).
All of the transit will be visible in East Asia and the Western Pacific.
Europe, the Middle East and South Asia will get to see the end stages of the eclipse as they go into sunrise on June 6.
Most of South America as well as west and southwest Africa will not get to see it except by broadcasts from other locations.
The US space agency NASA has promised "the best possible views of the event" through high-resolution images taken from its Solar Dynamics Observatory (SDO), in orbit around the Earth.
"A transit is a wonderful and rare sight; when you consider the vastness of the sky, for a planet to pass the disc of the Sun is pretty unusual - and you do have to wait until 2117 for the next one," said SDO co-investigator Richard Harrison.
The European space agency's Venus Express is the only spacecraft orbiting the hot planet at present and will be using light from the Sun to study Venus's atmosphere.
ESA and Japan's space agency also have satellites in low-Earth orbit to observe as Venus passes in front of the Sun.
And the NASA Hubble Space Telescope, which cannot view the Sun directly, will use the Moon as a mirror to capture reflected sunlight and learn more about Venus's atmosphere.
Many universities and observatories have also scheduled viewings and astronomy talks for the public.
The Smithsonian's National Air and Space Museum plans to place sun-safe telescopes outside its downtown Washington location so that tourists can catch a glimpse. A large screen will also show a broadcast of the transit.
For the smartphone set, there's an app for tracking the transit at http://tov2012.esri.com, according to Astronomers Without Borders.
Scientists say that studying the transit will boost future efforts to identify distant planets and learn more about their atmospheres.
Only six Transits of Venus have ever been recorded. There have been 53 transits of Venus across the Sun between 2000 BC and the last one in 2004.
Sunglasses are not strong enough to shield the eyes for viewing the transit, experts say.
Specialised solar filters and glasses will be offered for sale at some observatories, or people can make a pinhole camera or use a pair of binoculars -- not held to the eyes -- to project the Sun's image onto a light-colored surface.