The transit of Mercury across the face of the Sun on Monday 9 May is the first for the planet since 2006 and the first to be visible from Britain since 2003. Our next occurs in 2019, but we must wait until 2049 for a more favourable one than this. One would think that the little innermost planet would cross in front of the Sun every 116 days or so, every time it swings around the near side of its orbit of the Sun. However, that orbit is tilted at 7° to that of the Earth, so Mercury usually sweeps unseen above or below the Sun when it is closest, at its so-called inferior conjunction. The next most inclined orbit is that of Venus at 3.4°.
Transits of Mercury prove of less scientific interest than those of Venus, and this is not just because Venus has a substantial atmosphere which shows up as a bright ring as it traverses the solar limb – Mercury, with the merest trace of an atmosphere, displays no such effect. It is rather that Venus is so close to us during a transit (43 million km in 2012) that its parallax is more noticeable as viewed across the Earth. If its precise position can be measured from different points on the Earth, for example by timing the moments when the transit begins and ends, then calculations can reveal Venus’ true distance and, hence, that of the Sun and the scale of the solar system. In practice, this is easier to say than do. Famously, James Cook’s first voyage was to Tahiti to observe the transit of Venus in 1769.
Credits:
http://www.space.com/32766-transit-of-mercury-complete-coverage.html
https://www.theguardian.com/science/2016/may/08/starwatch-transit-mercury-9-may
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