# Twice in a lifetime, once in two generations

At 3.15 pm on a hot Friday afternoon, my taxi pulls up outside the Tata Institute of Fundamental Research (TIFR) on Homi Bhabha Lane in Navy Nagar, Colaba. After another 15 minutes spent at a thorough security check, I am told to go to the second floor to Room number 265. Professor Mayank Vahia, principal investigator, archeo astronomy project, greets me with a pleasant smile, ready to combat my questions. His weapon is a 24 year-old junior research fellow, Vinita Navalkar, an MSc in Physics with a passion for astronomy. Dressed in a lab coat and denims, her hair neatly plaited, she answers all my questions (including the silly ones) with a smile and a sparkle in her eye.

Vinita Navalkar explains the Transit of Venus at Tata Institute of Fundamental Research {TIFR} in Mumbai. Pic/ Pradeep Dhivar

What is the Transit of Venus?
Mercury and Venus are the two inner planets of the solar system, as they are placed between the Earth and the Sun. A transit occurs when Venus is positioned in between the Earth and the Sun.

This is called a ‘conjunction’ of Venus with Sun (as seen from the Earth) and happens every 1.6 years. But transit is not possible for every conjunction since a conjunction near the nodes (two points where the plane of Venus’ orbit intersects the plane of Earth’s orbit) is a necessary and sufficient condition for transits to occur.

How does the Transit of Venus occur?
The Transit of Venus occurs in a pair of eight years separated by either 105.5 or 121.5 years. So one complete cycle of transits goes as 8 — 105.5 — 8 — 121.5 (in years). This cycle of four transits repeats itself after every 243 years. Two TOVs which occur with a gap of eight years are seen in the same month either in June or in December. For example, in 2004 and 2012, we have both the transits in June. Whereas the next pair of transits occurring in the years 2117 and 2125 will be seen in December.

This pattern alternates itself over a 243 year cycle. Interestingly, when the transit occurs exactly at the node there will only be one transit instead of a pair (in eight years).

What is the significance of the Transit of Venus?
The TOV has helped determine the distance between the Earth and the Sun, the Venus and the Sun and also the size of Venus.

What precautions should be taken while viewing TOV?
The Sun is a very bright object and can have blinding effects on the naked eye. Filters or welder’s glasses (minimum power 6) can be used to view the TOV. Do not observe the TOV with naked eyes or regular sunglasses. It can be viewed using solar goggles or a pin hole projection. Kits available in the market contain a pair of convex lenses which help project the scene on a white screen. You can also project a magnified view of the Sun through a telescope or binoculars, on a white surface. The best way to view the TOV is to see directly through a telescope shielded with solar filters.

What will we get to see in Mumbai?
The total TOV will occur for six hours. You will see Venus as a tiny black spot moving across the Sun at a very slow pace. The Transit of Venus (TOV) will take place from 3.39 am to 10.29 am. In Mumbai, the sunrise is at 6.01 am, which is when it will be seen in the city. The maximum visibility will be at 6.59 am.

Diagram
This diagram explains when the Transit of Venus will be in a pair, that is June-June or December-December like in year 2004 and 2012 (June).
In the other two, the transit occurs exactly at the node and hence the next transit is automatically shifted out of the solar disc.

Trivia
> Prior to the 1639 transit, Edmund Halley, an English astronomer suggested that the Transit of Venus could be used to measure the distance between the Earth and the Sun

> However, it was Jeremiah Horrocks, an English astronomer, who first calculated the distance to around 90 million kilometres using transit method. He was the first one to witness the TOV

> It was Johannes Kepler, a mathematician, astronomer and astrologer, who predicted the TOV and gave relative distances between the Sun and each of the planets around 1618

> It was HC Russells who witnessed the TOV in 1874 and calculated the distance between the Earth and the Sun as 149 million kilometres. The actual distance is 149.9 million kilometres.

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