By Ted Thurgur
It is well known that only two planets, Mercury and Venus, transit the Sun, as viewed from Earth because they are the two whose orbits around the Sun lie inside that of the Earth. This is clearly shown in the diagram (Image credit to Sky & Telescope) which shows the orbits of the planets and each of their movements during May 2016. The positions of the outer planets do not change significantly on this scale.
This was the second transit of Mercury that I have observed and attempted to photograph. My first attempt was on 2003 May 7th and was previously reported in Lyra. The transit of 2006 November 8th was not visible from the U.K.
Transits of Mercury occur much more frequently than those of Venus. About 13 or 14 for Mercury per century compared to those of Venus which occur in pairs eight years apart but only every 105 or 121 years.
Transits of Mercury can only occur when there is an alignment between the Earth and a node of Mercury’s orbit. Presently this occurs within a few days of May 8th for the descending node and November 10th for the ascending node.
Since the 2003 transit I have acquired a new camera but the maximum zoom of 300mm appeared to be insufficient to capture the very small dot of Mercury which, for May transits, has an angular diameter of only about 12 arcsec. I therefore resorted to using my Sony digital camera (DSC-P120) at the eyepiece of my TeleVue Ranger telescope (focal length 480mm and 70mm aperture) fitted with a 20mm TeleVue Plössl eyepiece. The result is shown below:
The next opportunity to observe a transit of Mercury will be on 2019 November 11th and this will be followed by a similar transit on 2032 November 13th.