The mean distance of Mars from the sun is about 141 million mi (228 million km); its period of revolution is about 687 days, almost twice that of the earth. At those times when the sun, earth, and Mars are aligned (i.e., in opposition) and Mars is at its closest point to the sun (perihelion), its distance from the earth is about 35 million mi (56 million km); this occurs every 15 to 17 years. At oppositions when Mars is at its greatest distance from the sun (aphelion) it is about 63 million mi (101 million km) from the earth. It rotates on its axis with a period of about 24 hr 37 min, a little more than one earth day.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.