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Orbit of Mercury
Mercury has an orbit with a semimajor axis of .387 astronomical units (58 Gigameters,(Gm), or  million kilometers), and an  eccentricity  of 0.2056. The distances range from .31 to .47 au. It orbits the Sun in 88 days and travels 2.41 au in doing so, making the average orbital speed 50 km/s.

Changes in the orbit
The eccentricity of its orbit is increasing. Although the changes will not be markedly different than today's, it could exceed 0.6 billions of years into the future.

Conjunctions and transits
When the geocentric ecliptic longitude of Mercury coincides with that of the Sun it is in conjunction with the Sun, inferior if Mercury is nearer and superior if farther. Its conjunction distances vary from about 94 to 205 Gm, and the average time between them is the synodic year of 116 days. The inclination of Mercury's orbit is large enough so that the vast majority of inferior conjunctions do not lead to it appearing to cross the Sun. Thirteen times per century, on average, it does.

Close approaches to Earth and Venus
Mercury approaches Venus nearer than it does any other planet, and that distance is slightly less than 39.5 Gm, but it is decreasing and will eventually be less than 36 Gm apart.

Earth comes next. Mercury sometimes passes as close as 82.13 Gm, and the distances are in the midst of a long-term decrease. The alignment of the orbits does not favor small minimum distances: at J2000, the difference in the longitudes of perihelion was fairly small, about 26 degrees. The gap is increasing, but still the most favorable cases—with Mercury near aphelion—come while Earth is near aphelion. The distances are declining because of a more favorable orientation of orbits. In just 5,000 years Earth's heliocentric distance at the close passes will be less than 1 au, and the minimum distances will be a full Gm less than current ones.

Historical importance
The longitude of perihelion is precessing, as it does for the other planets, and general relativity is required to calculate the rate accurately. The discrepancy between the observed and calculated rates of precession before the invocation of relativity was greatest in the case of Mercury.

Accuracy and predictably
Mercury has a very well observed and predictable orbit. From the perspective of all but the most demanding its orbit is simple, An equation in Astronomical Algorithms that assumes an unperturbed elliptical orbit predicts the perihelion and aphelion times with an error of a few hours. Using orbital elements to calculate those distances agrees to actual averages to at least five significant figures. Formulas for computing position straight from orbital elements typically do not provide or need corrections for the effects of other planets.

However, observations are much better now, and space age technology has replaced the older techniques. They match predictions very well, and show the the orbit is now known to sub-kilometer accuracy.

table
No more than five significant figures are presented here, and to this level of precision the numbers match very well the VSOP87 elements and calculations derived from them, Standish's (of JPL) 250-year best fit, Newcomb's elements, , and calculations using the actual positions of Mercury over time.

number
1142310 t0027754