Mercury is the innermost planet in the Solar System. It is also the smallest, and its orbit is the most eccentric (that is, the least perfectly circular) of the eight planets. It orbits the Sun once in about 88 Earth days, completing three rotations about its axis for every two orbits. The planet is named after the Roman god Mercury, the messenger to the gods.
Mercury’s surface is heavily cratered and similar in appearance to Earth’s Moon, indicating that it has been geologically inactive for billions of years. Due to its near lack of an atmosphere to retain heat, Mercury’s surface experiences the steepest temperature gradient of all the planets, ranging from a very cold 100 K at night to a very hot 700 K during the day.
Mercury’s axis has the smallest tilt of any of the Solar System’s planets, but Mercury’s orbital eccentricity is the largest. The seasons on the planet’s surface are caused by the variation of its distance from the Sun rather than by the axial tilt, which is the main cause of seasons on Earth and other planets. At perihelion, the intensity of sunlight on Mercury’s surface is more than twice the intensity at aphelion. Because the seasons of the planet are produced by the orbital eccentricity instead of the axial tilt, the season does not differ between its two hemispheres.
Because Mercury’s orbit lies within Earth’s orbit (as does Venus’), it can appear in Earth’s sky either as a morning star or an evening star. While Mercury can appear as a very bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus.
The perihelion of Mercury’s orbit precesses around the Sun at an excess of 43 arcseconds per century; a phenomenon that was explained in the 20th century by Albert Einstein’s General Theory of Relativity.
Mercury is bright when viewed from Earth, ranging from -2.3 to 5.7 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3ก. Since Mercury is normally lost in the glare of the Sun, unless there is a solar eclipse it can be viewed from Earth’s Northern Hemisphere only in morning or evening twilight, while its extreme elongations occur in declinations south of the celestial equator, such that it can be seen at favorable apparitions from moderate latitudes in the Southern Hemisphere in a fully dark sky.
