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Mars comes closer to Earth more than any other planet save Venus at its nearest—56 million km is the closest distance between Mars and Earth, whereas the closest Venus comes to Earth is 40 million km. Mars comes closest to Earth every other year, around the time of its opposition, when Earth is sweeping between the Sun and Mars. Extra-close ...
Mars's equator is defined by its rotation, but the location of its Prime Meridian was specified, as was Earth's (at Greenwich), by choice of an arbitrary point; Mädler and Beer selected a line for their first maps of Mars in 1830.
An 1877 map of Mars by Giovanni Schiaparelli. North is at the top of this map. In most maps of Mars drawn before space exploration the convention among astronomers was to put south at the top because the telescopic image of a planet is inverted.
A simulation of a 4-satellite constellation in areostationary orbit . An areostationary orbit, areosynchronous equatorial orbit (AEO), or Mars geostationary orbit is a circular areosynchronous orbit (ASO) approximately 17,032 km (10,583 mi) in altitude above the Mars equator and following the direction of Mars's rotation.
The maps below were produced by the Mars Global Surveyor ' s Mars Orbiter Laser Altimeter; redder colors indicate higher elevations.The maps of the equatorial quadrangles use a Mercator projection, while those of the mid-latitude quadrangles use a Lambert conformal conic projection, and the maps of the polar quadrangles use a polar stereographic projection.
Tracking Mars’ wobble, or nutation, enabled the team to measure the size of the core. The RISE data suggested the core has a radius of about 1,140 miles (1,835 kilometers).
On both Earth and Mars, these two precessions are in opposite directions, and therefore add, to make the precession cycle between the tropical and anomalistic years 21,000 years on Earth and 29,700 Martian years (55,900 Earth years) on Mars. As on Earth, the period of rotation of Mars (the length of its day) is slowing down.
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