Mercury is the nearest planet to the Sun and is normally difficult to spot because it is never seen in a dark sky. It is difficult to observe satisfactorily even with a telescope on account of its small size, and, unless it can be found during the day, the view is always marred by being seen through a thick wedge of atmosphere.
Mercury orbits the Sun in only 88 days, and its frequent shuttling east and west of the Sun is alluded to in its name, referring to the winged messenger of Greek mythology. It is a barren, airless, crater covered world, resembling our Moon in both appearance and size. It rotates in 59 days, so its proximity to the Sun causes it to suffer extreme heat during the day, and plunging temperatures during the long night. The difficulty in observing Mercury meant that even a simple fact like the rotation period was uncertain before it was visited by the Mariner 10 spacecraft in 1974.
Because the orbit of Mercury is inside that of the Earth it shows phases like that of the Moon. When it is on the far side of the Sun we see all of the sunlit side so it appears full, and as it comes towards the Earth on its orbit it presents more of its dark side towards us. Although it gets larger (by about a factor of 3) as it approaches us, unlike with Venus, this is not sufficient to counteract the shrinking phase, so maximum brightness occurs at maximum phase.
Powerful binoculars will distinguish Mercury from a star by showing a distinct size, and if you are lucky may just about reveal the phase. Mercury's orbit is the most elliptical and highly inclined of any planet - this puts the northern hemisphere of the Earth at a distinct disadvantage when trying to see it. The best occasions to see it easily with the naked eye from the UK are in the evening if it has an elongation east of the Sun in late April to early May, or before dawn if it elongates west of the Sun in late October / early November.
|Distance from Sun (Million km):||45.9 (min)||69.7 (max)|
|Min. Distance from Earth (Million km):||91|
|Orbital Period:||88 days|
|Mean Interval between Inferior Conjunctions:||116 days|
|Rotation Period:||59 days|
|Apparent Diameter (arc seconds):||12.9 (max), 4.5 (min)|
|Max. Apparent Brightness (magnitude):||-1.5|
|Jet Propulsion Laboratory Mercury pages|