Mercury orbits the Sun at a mean distance of 0.387 astronomical units (AU). The high eccentricity of the planet's orbit (e = 0.206), however, dictates that it can be as far as 0.467 AU away from the Sun, and as close as 0.307 AU. The high eccentricity attributed to Mercury's orbit is the second largest in the solar system, only the planet Pluto has a more eccentric orbit. (Eccentricity in astronomy indicates that an orbit is not absolutely circular. The value of e = 1 indicates an orbit shaped as a parabola. An ellipse is less than one, and a circle has zero eccentricity.)
Constrained as it is in an orbit close to the Sun, Mercury is not an easy planet for naked-eye observers to locate. The greatest separation between the planet and the Sun, as seen from Earth, is 28° and consequently the planet is never visible against a truly dark sky. Even at its greatest angular separation from the Sun, Mercury will either set within two hours of sunset, or rise no earlier than two hours before the Sun. Nonetheless, Mercury has been known since the most ancient of times, with observations of the planet being reported as far back as several centuries B.C. The Greek philosopher Plato refers to the distinctive yellow color of Mercury in Book X of his Republic.
The sidereal period, or the time it takes Mercury to orbit the Sun, is 87.969 days. The planet's synodic period, which is the time required for Mercury to return to the same relative position with respect to the Sun and Earth, is 116 days. As seen from Earth, Mercury undergoes a change of phase as it moves around the Sun. These phase changes were first observed in the early seventeenth century by the Polish astronomer Johannes Hewelcke (1611-1687), who is perhaps better known today through his latinized name, Hevelius. Zero phase occurs when Earth, the Sun, and Mercury are directly in line, with Mercury on the opposite side of the Sun to Earth. At this phase, Mercury is said to be at superior conjunction. Half phase occurs when Earth, the Sun, and Mercury are once again in a line, but this time with Mercury being on the same side of the Sun as the Earth. Mercury is said to be at inferior conjunction when it exhibits a half phase. While moving from inferior to superior conjunction Mercury passes through a quarter phase, during which the disk of the planet is half illuminated as seen from the Earth. Mercury also passes through its greatest western elongation when moving from inferior to superior conjunction. Likewise, in moving from superior to inferior conjunction Mercury passes through greatest eastern elongation, and exhibits a second quarter phase, or half-disk illumination.
Because Mercury's orbit is quite elongated, so that its distance from the sun varies significantly, the maximum angular separation between Mercury and the Sun, as seen from the Earth, can vary from a minimum of 18 to maximum of 28°. The largest angular separation of 28° occurs when Mercury is at either greatest western, or greatest eastern elongation and near its aphelion (its greatest distance from the Sun). Irrespective of whether the planet is at aphelion or not, the best time to view Mercury in the evening is when the planet is near greatest eastern elongation. Because the synodic period of Mercury is about 116 days, the planet will be favorably placed for evening viewing three times each year. Similar conditions apply for viewing Mercury before sunrise.
The inclination of Mercury's orbit to that of the ecliptic plane (the plane of Earth's orbit about the Sun) is 7.0°. This slight orbital tilt dictates that when Mercury is at inferior conjunction it is only rarely silhouetted against the Sun's disk as seen from Earth. On those rare occasions when Earth, Mercury, and the Sun are in perfect alignment, however, a solar transit of Mercury can take place, and a terrestrial observer will see Mercury move in front of, and across the Sun's disk. A transit of Mercury can only occur when the planet is at inferior conjunction during the months of May and November. During these months Earth is near the line along which the orbit of Mercury intersects the ecliptic plane—this is the line of nodes for Mercury's orbit. Approximately a dozen solar transits of Mercury occur each century, and the final transit of the twentieth century occurred on 15 November 1999.