Describing the transit of Mercury as rare is not altogether correct. It is rather its observation, more precisely its observation from our country that can be described as rare. Mercure passes between the Earth and the Sun every three months or so! However, the orbital planes of the Earth and Mercury mean that the phenomenon is not visible every time. In order for the passage of Mercury to be visible, the Sun, Mercury and the Earth must be aligned. And even when this occurs, not everybody avails of the opportunity. Therefore, on May 9, it is the western part of Europe, the East of North America and a large part of South America that will be able to fully avail of the spectacle (see map). Elsewhere it will be partially or totally absent. Using the word spectacle is a slight exaggeration. The phenomenon is not observable with the naked eye (Mercure is too small) and an instrument capable of 50 times magnification is required to be able to distinguish the small black dot (the planet) passing in front of the Sun! When will the next transits visible from Europe occur? May 7 2049 and November 9 2052…All the more reason to this transit which will undoubtedly be exceptional (weather permitting….):  in our region, Mercury will begin its “crossing” of the solar disc a little after 11H00, which will last more than 7 hours which is a full day’s observation.

A planetary transit in our solar system is therefore very rare because, apart from the conditions of alignment and visibility from certain parts of the globe, it is only possible for Venus and Mercury (the other planets will never be located between the Sun and the Earth!). And the transits of Venus are even rarer, not more than two per century (13 times for Mercury): the last one occurred in 2012, we will have to wait until December 11 2117 for the next transit of Venus… but it will not be visible from our region!
It should be noted that this transits method is mostly used outside the solar system to detect exoplanets and deduce information about them.

Visible zones of the transit of Mercury on May 9 2016 (P.Rocher/IMCCE)