Since Mercury and Venus are the only planets that lie inside the Earth’s orbit they are the only planets that can pass between Earth and the Sun to produce a transit.
The orbital plane of Venus is not exactly aligned with that of Earth, such that transits occur very rarely, in pairs eight years apart but separated by more than a century. The last was in 2004, but after next week’s event there won’t be another until 2117 – so make the most of this twice-in-a-lifetime event!
There are more chances to see transits of Mercury, however, with an average of 13 transits of Mercury per century.
Mercury has a highly eccentric orbit, varying in distance from the Sun from 46-70 million kilometres. In addition, its orbit is inclined by 7 degrees to that of Earth.
Mercury's orbit crosses Earth's orbital plane in early May and early November each year, but only if it passes between Earth and the Sun will a transit be seen. Because of Mercury’s eccentric orbit, for transits occurring in May Mercury appears 158 times smaller than the diameter of the Sun, while for November transits it appears 194 times smaller. For comparison, Venus’ diameter is approximately 32 times smaller than that of the Sun, which is right at the limit of good human eyesight.
The small size means you need telescopic equipment to see a Mercury transit but remember – NEVER look directly at the Sun with unprotected eyes, and NEVER look through a telescope or binoculars at the Sun as this will cause permanent blindness.
Mercury’s eccentric and inclined orbit also means that transits do not occur every year. So, some dates for your diary for the remainder of this century:
09 May 2016
11 November 2019
13 November 2032
07 November 2039
07 May 2049
09 November 2052
10 May 2062
11 November 2065
14 November 2078
07 November 2085
08 May 2095
10 November 2098
In the meantime, don’t forget the last transit of Venus of the 21st century, occurring next week, on 5-6 June.