The rare "black drop" effect photographed during the transit of Venus on June 5, 2012. Amazingly, Venus's disk does not appear to just touch the inside of edge of the Sun's tangentially for an instant, as expected. Rather, a dark bridge appears between them, so Venus resembles a black ink drop hanging from the inside edge. The effect lasts about a minute. This effect was noticed by Captain James Cook and other 18th & 19th century observers sent around the world to observer previous transits of Venus to determine the distance to the Sun or the astronomical unit (a.u.).  It prevented them from measuring the transit time with any degree of certainty. 

This image was published in the September, 2014 issue of Sky & Telescope Magazine.