Position and orbit

Mars orbits the Sun in an elliptical orbit that has the third highest eccentricity of all the planets' orbits. Its mean distance from the Sun is about 228 million kilometres, placing it about one and a half times further from the Sun than is Earth. At perihelion, Mars is 208 million kilometres from the Sun, while at aphelion it is 249 million kilometres. The difference between the two is about 46 million kilometres, whereas for Earth the difference between perihelion and aphelion is only 5 million kilometres. This has a major influence on Mars's climate and results in a wide range of seasonal temperatures.

Mars orbits the Sun with a velocity of about 86 868 km/h and takes 687 Earth days to complete the trip. The planet takes 24.6 hours (1.029 Earth days) to rotate once on its axis, which is tilted at an angle of 25.2° to the vertical.

About every 780 days Mars passes through a point in its orbit where from Earth it appears opposite the Sun in the sky (opposition). Because of

Figure 7.7 Orbit of Mars around the Sun (distance circles are in astronomical units, AU)

its eccentric orbit, Mars's distance at opposition varies, so its apparent size and brightness also change. The most favourable opposition for observation is when Mars is closest to both the Earth and the Sun (this occurs about once every 17 years). Future oppositions for Mars are 29 January 2010, 3 March 2012, 8 April 2014, and 22 May 2016.

Mars can be studied easily from Earth using a telescope of moderate power. Dark markings and the white polar ice-caps may be seen on the surface, depending on the distance of Mars from Earth.

Even though Mars is more than half the diameter of Earth, it has only about one-tenth of its mass. Detailed knowledge about the interior of Mars is limited because of the lack of seismic data.

The average density of Mars is the lowest of the terrestrial planets (3.95 g/cm3 compared with Earth's 5.52 g/cm3). This suggests the iron-bearing core of Mars is smaller than Earth's core. In fact the core is thought to have a radius of only 1100 km (but some estimates have it as high as 2000 km). The core makes up only about 6 per cent of the planet's mass,

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