Mercury, as seen by NASA's MESSENGER spacecraft.
Mercury is the innermost planet in the solar system, orbiting the Sun at a distance of 0.387 AU once every 88 days.
At its brightest, it can reach magnitude -1.9, a little brighter than Sirius, the brightest star in the sky. However, it more normally hovers at around magnitude 0, becoming roughly the fifth brightest object in the sky.
Nonetheless, it is always a challenging object to observe because it never ventures far from the Sun – at most 28.3°. This means it must always be observed in twilight, either at dusk or dawn. Additionally, because it is always close to the horizon at these times, it must always be observed through thick layers of the atmosphere where seeing conditions are poor.
Because of these poor viewing conditions, very little detail can be made out on its surface. Much more has been learnt about Mercury from two spacecraft which have visited it: Mariner 10 (1974–1975) which made three close approaches to the planet and surveyed around 50% of its surface, and Mercury MESSENGER, which made three close approaches (2008, 2008, 2009) before finally entering orbit around the planet in 2011.
These have revealed a planet with very changeable surface temperature, ranging from well below 150°C on the night side of the planet to up to 400°C at midday. The next mission to visit Mercury will be the joint European and Japanese BepiColombo spacecraft, named after the engineer whose work made the Mariner 10 mission possible, and due to be launched in 2018 and to reach orbit around Mercury in around 2024.
Mercury is observable only for a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.
These apparitions take place alternately in the morning and evening skies, depending whether Mercury lies to the east of the Sun or to the west.
When it lies to the east, it rises and sets a short time after the Sun and is visible in early evening twilight. When it lies to the west of the Sun, it rises and sets a short time before the Sun and is visible shortly before sunrise.
These morning and evening apparitions repeat once every 116 days, Mercury's synodic period.
Never attempt to view Mercury through a telescope or binoculars if the Sun is still above the horizon. A momentary glance at the Sun through such an instrument is sufficient to cause permanent blindness.