Mercury's transit of the Sun. The Sun with the planet Mercury seen in silhouette (small dot at lower left) as it transits across the solar disc on 9 May 2016. Mercury passes between Earth and the Sun only about thirteen times a century, with the previous transit taking place in 2006 and the next in 2019. Mercury is the innermost planet of the solar system and is 58 million kilometres (km) from the Sun. It is 6786 km in diameter compared to the Sun which is approximately 860,000 km in diameter. This image was obtained in Boyertown, Pennsylvania, USA.