By the time you read this report, stargazers in Patna would have already witnessed the once (at most twice) in a lifetime movement of the planet Venus across the halo of the sun.

The phenomenon called the “Transit of Venus”, which will occur early on Wednesday morning, is the second instance of the movement of the neighbouring planet in recent times. The last time it occurred was on June 8, 2004. It usually happens when Venus passes directly between the sun and the Earth. During a transit, Venus can be seen from the Earth as a small black dot moving across the face of the Sun.

“It was observed the first time by young Jeremiah Horrocks in 1639,” said P. Iyamperumal, executive director, Tamil Nadu Science and Technology Centre, Chennai. “The Transit of Venus occurs in pairs, with a difference of eight years between the two sightings. The difference between two pairs is 105.5 or 121.5 years.”

The next pair of transits will occur in 2117 and 2125 (see chart).

In the state capital, residents would be able to see the transit at Srikrishna Science Centre near Gandhi Maidan. Enthusiasts can observe the heavenly incident through telescopes fitted with sun-tracking GPS (global positioning system). The science centre will also collaborate with Mauna Loa Observatory, Hawaii, for the live telecast of the Venus’s transit in its auditorium, which can accommodate 150 people.

“Patna residents would be able to see the Transit of Venus right from sunrise (4.57am). Our centre would be opened for organised viewing from 6.30am. We have made several arrangements for safe viewing of Venus’s transit, including protected telescopes and live telecast from Hawaii. Experts, including L.M. Ram, the former head of the department of geography of Patna University, and S.P. Ram, the former head of department of physics, would be present here to answer the queries of the visitors,” said Satish Ranjan, education officer, Srikrishna Science Centre.

Explaining the reason for the long gap of time between two Transits of Venus, Iyamperumal said it is caused by the difference between the orbital planes of Earth and Venus.

He said: “If Venus and the Earth orbited the Sun in the same plane, then Transits would happen frequently. However, the orbit of Venus is inclined to the orbit of Earth by nearly 3.4 degrees. So, when Venus passes between the Sun and the Earth every 1.6 years, it is usually a bit above or below the Sun. Thus, it is invisible to people on Earth.”