Mumbai: Un-witnessed in the last 122 years, a rare celestial event-transit of planet Venus across the face of the Sun will take place soon, giving sky-gazers of the chance of a lifetime.

"No human alive has seen this phenomenon," Nehru Planetarium sources say adding on June 8, 2004, Venus will drift across the face of the sun, as it passes between sun and the earth.

Transits of Venus across the disk of the Sun are among the rarest of planetary alignments. Indeed, only six such events have occurred since the invention of the telescope in 1631, 1639, 1761, 1769, 1874 and 1882.

The next two transits of Venus will occur on June 8, 2004 and June 6, 2012.

"Simple as transits may seem, they are instrumental in defining our place in the cosmos and historically expeditions were launched at these times to quantify the size of the solar system," according to planetarium Director Piyush Pande.

Meanwhile, scientists of USA's space programme National Aerospace and Space Administration (NASA) say the transit takes place in stages - Contact I, which is instant when the planet's disk is externally tangent to the Sun, Contact II- is when the entire disc of Venus is seen and the planet is internally tangent with the Sun, and Contact III- when the planet reaches the opposite limb and is once again internally tangent and ends with Contact IV when its Venus' limb is externally tangent to the Sun.

A live Webcast "transitofvenus.org" will be available on June 8.