Transit of Mercury on 9th May 2016
On 9th May as the sun sets in the west, a small dark spot will be seen crossing across the face of the Sun. One of the rare astronomical events, Transit of Mercury is once again visible in India after 10 years.
When Earth, Mercury and Sun are aliened in a straight line, Mercury, one of the inner planets will obstruct the face of Sun. This is just like Moon obstructs the face of the Sun during a solar eclipse. However, as the Moon is quite near compared to Mercury, it is so big that it can cover even the entire disc of Sun resulting in Total Solar Eclipse. Although Moon and Mercury are almost same size, but as Mercury on the average is 77 million km from Earth, it would look just a small speck. Hence, when it comes in front of the Sun, it cannot cover sizeable disc of Sun, but only a small part of it.
The very first recorded observation of the transit of Mercury was by Pierrie Gassendi, on November 7, 1631, which had been Johannes Kepler earlier. Quite interestingly the next transit of Mercury was observed in Indian soil, by Jeremy Shakerly, astronomer and mathematician, in Surat, on November 3, 1651. Historical records are not clear as to why he came to India; was he part of the East India Company or travelled to India to witness this rare event, which he had predicted in his almanac? We never know.
While the technical expertise for predicting such events existed with the pre-modern Indian astronomy, there is no available record of actual viewing of the event. It should also be noted although during the sunrise or sunset, if the transit occur, one may be able to view it with naked eye, in all other times, one needs a telescopic projection to view Mercury transit. Hence, in all likelihood it would have been difficult for anyone, anywhere to have observed it in the absence of telescope during the pre-modern astronomy.
Every now and then Mercury would come in between Earth and Sun, but that would not result in transit for the same reason, every poornima is not a solar eclipse. It is because the orbit of Mercury is inclined to the orbit of Earth at about 7 degrees. That means, when it is between Earth and Sun, it may be seen just above the Sun or just below. Somewhere between May 8-9 and November 14-15 Earth is right on straight line with the node of Mercury. If Mercury happens to be at any of the nodes on these dates, then transit would take place.
On 9th May 2016, at about 4:30 pm IST, as the sun is in the west, the transit would commence. The first contact will be at 16:32. From then on we can see the dot moving across the disc of Sun towards its center. The central point occurs at 20:27 IST (14:57UT) and the sun would have set long before that for us in India. Therefore, the best opportunity is around 17:30 IST when the effect of atmospheric absorption is still not strong enough. Photographing the sunset would be the safest. The transit event will continue beyond sunset and hence can be safely observed at sunset. In places like Africa and Europe the transit would be visible at noon. Nevertheless as the Mercury being small, recognising it as a black dot requires extra effort. A magnification tool is definitely needed.
The eclipse goggles are a good option, but Mercury could be very small for detection without magnification. A projection with small telescopes is would be ideal. Never look at the Sun with a telescope or binocular. However, telescope even a small one can be used to project the image of the Sun and one can easily watch the small black dot traversing across the face of the Sun. Astronomy enthusiasts are setting up observation posts across the country in hundreds of places and you may like to go there.
During the transit of Mercury one could do simple experiments like estimating the velocity of the planet and its angular size. One can also compute reasonable estimate of the distance of the Sun from Earth. All these provide valuable opportunity for hands-on minds-on education.