Mercury, the innermost planet, is 0.4 A.U. from the Sun on the average. It revolves about the Sun once every 88 days in an orbit that is the most elliptical of any planet except Pluto. The adjacent image shows the November, 1996, locations of the planets in the inner solar system and their orbits. (The image is constructed with Solar System Live; it is to scale for the orbits; the images of the planets are not to scale since they would be tiny dots at this magnification.)
The orbits of Venus and Earth appear to be circles on this scale, but the orbit of Mercury is clearly not circular. As a result of its highly elliptical path the distance between Mercury and the Sun varies a large amount, from 46 million kilometers at perihelion to 70 million kilometers at aphelion.
Its mass is 5.5% of the Earth's mass, which is 5 times the mass of the Moon (the best determination of the mass came from detailed tracking of the spacecraft Mariner 10). The density is 5.5 g/cc, the same as Earth, and it has a magnetic field that is about 1% of the strength of Earth's field. These facts suggest a differentiated interior with a large iron core, with the weakness of the magnetic field related in some way to the slow rotational period relative to the Earth.
Its rotational period is 59 days, as determined by radar measurements from the Earth using the Doppler effect. The ratio of the rotational period to the orbital period is 2/3, and is an example of more complicated tidal locking than for the Earth-Moon system. The tidal interaction between the Sun and Mercury leads to a 2/3 ratio (instead of 1/1 as for our Moon) because the orbit of the Moon about the Earth is almost a circle, but the orbit of Mercury around the Sun is rather elliptical.
The surface conditions are among the harshest in the Solar System. During the long Mercurian day the temperature rises to about 425 degrees Celsius, hot enough to melt lead and hotter than any planet except Venus. Because there is no substantial atmosphere to retain heat, during the equally long nights, the temperature drops quickly to around -180 degrees Celsius, which is among the coldest found in the Solar System. This range of -180 Celsius at night to 425 Celsius in the day is the largest surface temperature variation in the Solar System.