One Mercury solar day (one complete day-night cycle) equals 176 Earth days, or little more than two years on Mercury. Mercury's axis of rotation is just 2 degrees inclined in relation to the plane of its orbit around the Sun. Thus, all points on the surface experience both day and night, but they do so at different times.
Mercury has no global magnetic field, but it does have a magnetosphere that reacts to the flow of electric current through it from the sunward facing side. The magnetosphere gets thinner as you go farther from Mercury's center because there are fewer particles in the atmosphere for electrons to collide with and create a magnetic field. So far, everything we know about Mercury's interior agrees with this prediction, which means that it has no core of its own. It gets its magnetic field from the Sun.
The magnetosphere also contains many high-energy particles that come from the sun. Some of these particles reach the surface and may cause air showers when they hit it, but most are deflected by the planet's magnetic field back into space. Only about 10% or less of those particles that make it through the magnetosphere reach our planet's atmosphere.
The best time to observe Mercury is around midnight at the point where the sun is directly above the horizon, when it will be fully illuminated from one end to the other.
A year on Mercury lasts just 88 days, yet because to its sluggish rotation, a day lasts twice as long! That implies that if you could stand on the surface of Mercury, it would take 176 Earth days for the Sun to rise, set, and rise again in the identical location in the sky only once! The reason for this extreme difference between the two planets' periods is that while Earth's axis keeps pointing towards the Sun, causing its day to get longer over time, Mercury's orbit carries it away from the Sun at about 50,000 miles per hour, so it never gets closer than 29 million miles.
Mercury has one of the most unusual environments in the Solar System. It is surrounded by a cloud of particles known as the exosphere which extends out from the planet's surface almost as far as Earth's Moon. This exosphere reflects about 95% of the sunlight that hits it back into space, which is why Mercury is always dark except for its polar caps. The reflection from its exosphere also accounts for the planet's extremely low average temperature of -275 degrees F.
The Sun causes Mercury's orbit to precess, or change direction, every 878 days or so. But because its orbit is so elliptical, with an eccentricity of 0.206, even though Mercury orbits the Sun in approximately 87 days, it takes 74 years, 6 months, and 4 days to complete one revolution.
Mercury takes approximately 59 Earth days to spin once on its axis (the rotation period), and approximately 88 Earth days to complete one orbit around the Sun. Mercury, on the other hand, has a day that lasts 176 Earth days (from dawn to sunrise). This means that there are 8 Earth months in a Mercury year, which is almost twice as long as the Earth year (due to Mercury's elliptical orbit, it spends more time closer to the Sun than farther away).
A Mercuryian year is about 485 Earth years long, but due to the precession of the equinoxes, Mercury's perihelion shifts slowly relative to the Earth, so its seasons change over time. The average temperature at the planet's surface is 180° F (82° C), but it can reach 500° F (260° C) at the north pole and -180° F (-292° C) at the south pole.
Its atmosphere is 10-20 km thick and made up of carbon dioxide, oxygen, nitrogen, hydrogen, helium, and traces of other gases. The surface is mostly iron with some rock types including granite, gneiss, and basalt.
The average density of Mercury is 1130 kg/m3, but it can range from 740 to 1600 kg/m3.