Mercury can be seen crossing in front of the Sun about 13 times each Century. Though Mercury orbits the Sun every 88 days, the orbits of Mercury and Earth around the Sun are tilted with respect to each other. Therefore, transits are rare events that occur only when Mercury comes exactly in between the Sun and us.
The last transit was on 9 May 2016 and the next one will be on 13 November 2032.
There have been many published historic observations notably from the Royal Observatory of Cape of Good Hope on 5 November 1868, by William Mann who worked under Sir Thomas Maclear, the Astronomer Royal from 1833-1870.
Thereafter some observations were done on 10 November 1894 from Grahamstown in Eastern Cape as well as on 14 November 1907 from Johannesburg, Gauteng.
Two centuries ago, famous European astronomers traversed the world chasing transits of Venus and Mercury. Initially, this was a quest to measure the Universe sparked by Edmund Halley.
Using the laws of Kepler, “astronomers in the 18th and 19th Century could measure the relative distances between the Sun and all its planets. Meaning, all distances were known only as ratios of each other,” says Dr Niruj Mohan Ramanujam.
It was only by careful timing of the transits of Mercury and Venus from widely separated places on the Earth, that astronomers could measure the actual distances between the planets and the Sun.
According to Sivuyile Manxoyi, the SAAO outreach manager, “There will be a few public viewing locations around the country, the public are encouraged to regularly check our interactive map for locations near them.”
He continued to say, in the Garden Route, the Southern Cape Astronomy Club will set-up an event at Pearly Beach. The Cape Centre of the Astronomical Society of South Africa will be at the Eden on the Bay in Bloubergstrand. The public in Pretoria should go to the UNISA Science Engagement Centre, then in Senekal, Free State, where the Senekal Astronomy Club will coordinate the viewing. In Cape Town, the South African Astronomical Observatory will have a few telescopes on the lawn and webcasting in the auditorium.