May 9th 2016 sees one of the astronomical highlights of the year - a transit of Mercury across the sun, the best opportunity to observe this phenomenon until 2049. To mark the event, the Sky at Night attempts to explain the many mysteries of Mercury - a planet so bizarre that it is sometimes described as the 'problem child' of the solar system. Surface temperatures exceed 450 degrees but it also has patches of ice, its day is twice as long as its year, and it is a planet that appears to be shrinking.