Basic Astronomical Data

Mars moves around the Sun at a mean distance of 228 million km (140 million miles), or about 1.5 times the distance of Earth from the Sun. Because of Mars's relatively elongated orbit, the distance between Mars and the Sun varies from 206.6 million to 249.2 million km (128.4 million to 154.8 million miles). Mars orbits the Sun once in 687 Earth days, which means that its year is nearly twice as long as Earth's. At its closest approach, Mars is less than 56 million km (35 million miles) from Earth, but it recedes to almost 400 million km (250 million miles) when the two planets are on opposite sides of the solar system.

Mars is easiest to observe when it and the Sun are in opposite directions in the sky—i.e., at opposition—because it is then high in the sky and shows a fully lighted face. Successive oppositions occur about every 26 months. Oppositions can take place at different points in the Martian orbit. Those best for viewing occur when the planet is closest to the Sun, and so also to Earth, because Mars is then at its

PLANETARY DATA FOR MARS

mean distance from Sun

227,941,040 km (1.5 AU)

eccentricity of orbit

0.093 399

inclination of orbit to ecliptic

1.850 20°

Martian year (sidereal period of revolution)

686.980 Earth days

visual magnitude at mean opposition

-2.01

mean synodic period*

779.94 Earth days

mean orbital velocity

24.1 km/s

equatorial radius

3,396.2 km

north polar radius

3,376.2 km

south polar radius

3,382.6 km

surface area

1.44 x 108 km2

mass

6.418 x 1023 kg

mean density

3.94 g/cm3

mean surface gravity

372 cm/s2

escape velocity

5.022 km/s

rotation period (Martian sidereal day)

24 h 37 min 22.663 s

Martian mean solar day (sol)

24 h 39 min 36 s

inclination of equator to orbit

24.936°

mean surface temperature

210 K (-82 °F, -63 °C)

typical surface pressure

0.006 bar

number of known moons

2

*Time required for the planet to return to the same position in the sky relative to the Sun as seen from Earth.

brightest and largest. Close oppositions occur roughly every 15 years.

Mars spins on its axis once every 24 hours 37 minutes, making a day on Mars only a little longer than an Earth day. Its axis of rotation is inclined to its orbital plane by about 25°, and, as for Earth, the tilt gives rise to seasons on Mars. The Martian year consists of 668.6 Martian solar days, called sols. Because of the elliptical orbit, southern summers are shorter (154 Martian days) and warmer than those in the north (178 Martian days). The situation, however, is slowly changing such that 25,000 years from now the northern summers will be the shorter and warmer ones. In addition, the obliquity, or tilt, of the axis is slowly changing on a roughly one-million-year timescale. During the present epochs the obliquity may range from close to 0, at which times Mars has no seasons, to as high as 45°, when seasonal differences are extreme. Over hundred-million-year time-scales the obliquity may reach values as high as 80°.

Mars is a small planet, larger than only Mercury and slightly more than half the size of Earth. It has an equatorial radius of 3,396 km (2,110 miles) and a mean polar radius of 3,379 km (2,100 miles). The mass of Mars is only one-tenth the terrestrial value, and its gravitational acceleration of 3.72 metres/ sec2 (12.2 feet/sec2) at the surface means that objects on Mars weigh a little more than a third of their weight on Earth's surface. Mars has only 28 percent of the surface area of Earth, but, because more than two-thirds of Earth is covered by water, the land areas of the two planets are comparable.

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