A transit of Mercury over the disc of the sun will take place in the afternoon of May 9 and will be visible from India after a gap of about ten years. The transit of Mercury is a phenomenon in which the planet will be seen as a small black dot travelling from one end of the solar disc to the other, according to Sanjib Sen, Director, Positional Astronomy Centre. This phenomenon is seen when mercury passes between the sun and the earth and it happens only when the three are lined up, Sen said.
Mercury appears as a dot on the solar disc because its angular size is very small compared to that of the sun as seen from earth. Sen said the transit of mercury will be visible from most of Asia (except south eastern parts and Japan), Europe, Africa, Greenland, South America, North America, Arctic, North Atlantic Ocean and most of the Pacific Ocean. The transit will be visible in India after ten years and the next it will be visible in the country after a long gap of 16 years in 2032, he said. The transit of Mercury begins with contact-I, the instant when the disc of Mercury is externally tangent with the sun (ingress exterior) followed by contact-II, when Mercury is internally tangent with the Sun (ingress interior). Mercury will be seen as a black spot, travelling several hours over the face of the disc and will reach the opposite limb of the sun at contact-III, when the disc of Mercury is internally tangent with the sun (Egress interior). Finally the transit ends at contact-IV when the disc of Mercury is externally tangent with the sun (egress exterior). The transit of Mercury is relatively a rare phenomenon, occurs 13 or 14 times in a century, mostly in the months of May and November.
The interval between one November transit and next November transit may be seven, 13 or 33 years, whereas the interval between a May transit and the next May transit may be 13 or 33 years.