Orbit of Mars

by Jerry Coffey on June 4, 2008

The orbit of Mars is the second most eccentric in the Solar System. Only Mercury’s orbit is more eccentric. At perihelion Mars is 206,655,215 km from the Sun and at aphelion it is 249,232,432 km distant. That is a variation of of just under 42,600,000 km. The average distance from Mars to the Sun (called the semi-major axis) is 228 million km. It takes Mars approximately 687 Earth days to complete on orbit. The orbit of a planet varies in relation to the gravitational influences of the bodies around it, so the eccentricity can change over time. AS recently as 1.35 million years ago, Mars was in a nearly circular orbit.

Mars, like all planets in the Solar System, is tilted along its axis(axial tilt). For Mars, the axial tilt is about 25.19 degrees. This tilt is similar to Earth’s, so Mars has seasons like ours. The Martian seasons are longer because a year on Mars is nearly twice as long as an Earth year. The dramatically changing distances between Mars’ aphelion and perihelion means that the seasons aren’t balanced like Earth. Mars is at its closest when its southern hemisphere is tilted towards the Sun. So the southern hemisphere experiences hotter summers than the northern hemisphere.

The orbit of Mars allows it to approach Earth at varying distances. It is easiest to observe when it is at its closest approach. Opposition occurs when Mars’ geocentric longitude is 180° different from the Sun’s. Opposition can occur as much as 8½ days before or after closest approach. The distance at close approach varies between about 54 and 103 million km due to their position in their orbits. The last Mars opposition was on January 29, 2010. The next will be on March 3, 2012(about 100 million km). The average time between the successive oppositions(synodic period) of Mars is 780 days. Mars made its closest approach to Earth in nearly 60,000 years(55,758,006 km) on August 27, 2003. While this was a record, it was only slightly closer than other close approaches.

The orbit of Mars is well understood and has been observed, and documented, for thousands of years. The planet’s short period of apparent retrograde motion was noted as early as 1534 B.C. After reading and understanding the planet’s orbit, you should research more about its atmosphere, gravity, and exploration. Only then will you have a grasp of a few of the mysteries surrounding the Red Planet.

Here’s an article about Martian ice ages in the past, related to tilt, and another about mid-latitude glaciers on Mars.

Here’s more general information about Mars. And here are some pages from NASA about the Mars Phoenix Lander mission.

Finally, if you’d like to learn more about Mars in general, we have done several podcast episodes about the Red Planet at Astronomy Cast. Episode 52: Mars, and Episode 91: The Search for Water on Mars.


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