Transits of Mercury occur in May or November. The last four transits occurred in 1999, 2003, 2006, and May 9, 2016. The next will occur on November 11, 2019, and then on November 13, 2032. A typical transit lasts several hours.
The interval between one November transit and the next November transit may be 7, 13, or 33 years; the interval between one May transit and the next May transit may be 13 or 33 years. May transits are less frequent than November transits because during a May transit, Mercury is near aphelion whereas during a November transit, it is near perihelion.
Transits of Mercury are gradually drifting later in the year; before 1585 they occurred in April and October.
On June 3, 2014, the Mars rover Curiosity observed the planet Mercury transiting the Sun, marking the first time a planetary transit has been observed from a celestial body besides Earth.
Once again, astronomers in Malta have actively viewed and recorded this occasional event despite the adverse weather we had on Monday making it near impossible for imaging. Nonetheless, for those who didn’t get to see Mercury’s transit can still appreciate this event in the images below from various Maltese astronomers!
The transit of Mercury could also be considered as a partial solar eclipse – still spectacular in spite of the stark difference in size between the solar system objects involved. Out of curiosity and with a bit of geometry Alexei Pace worked out the size which Mercury would need to grow to (170x its current diameter) for it to cause a total solar eclipse as seen from the Earth..!
Follow link below for the image:
At that size it would appear the same size as the full Moon in our skies, large enough to cover the Sun.
On a final note, well done to all observers especially to Josef Borg and Leonard Ellul-Mercer who provided real time images which were sent to the Instituto de Astrofísica de Canarias (IAC) who were monitoring this event.
Information obtained from: en.wikipedia.org/wiki/Transit_of_Mercury