Basic planetary data

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The mean distance of Earth from the Sun is about 149,600,000 km (92,960,000 miles). The planet orbits the Sun in a path that is presently more nearly a circle (less eccentric) than are the orbits of all but two of the other planets, Venus and Neptune. Earth makes one revolution, or one complete orbit of the Sun, in about 365.25 days. The direction of revolution—counterclockwise as viewed down from the north—is in the same sense, or direction, as the rotation of the Sun; Earth’s spin, or rotation about its axis, is also in the same sense, which is called direct or prograde. The rotation period, or length of a sidereal day (see day; sidereal time)—23 hours, 56 minutes, and 4 seconds—is similar to that of Mars. Jupiter and most asteroids have days less than half as long, while Mercury and Venus have days more nearly comparable to their orbital periods. The 23.44° tilt, or inclination, of Earth’s axis to its orbital plane, also typical, results in greater heating and more hours of daylight in one hemisphere or the other over the course of a year and so is responsible for the cyclic change of seasons.

With an equatorial radius of 6,378 km (3,963 miles), Earth is the largest of the four inner, terrestrial (rocky) planets, but it is considerably smaller than the gas giants of the outer solar system. Earth has a single natural satellite, the Moon, which orbits the planet at a mean distance of about 384,400 km (238,900 miles). The Moon is one of the bigger natural satellites in the solar system; only the giant planets have moons comparable or larger in size. Some planetary astronomers consider the Earth-Moon system a double planet, with some similarity in that regard to the dwarf planet Pluto and its largest moon, Charon.

Earth’s gravitational field (see gravitation) is manifested as the attractive force acting on a free body at rest, causing it to accelerate in the general direction of the centre of the planet. Departures from the spherical shape and the effect of Earth’s rotation cause gravity to vary with latitude over the terrestrial surface. The average gravitational acceleration at sea level is about 980 cm/sec² (32.2 feet/sec²).

Earth’s gravity keeps the Moon in its orbit around the planet and also generates tides in the solid body of the Moon. Such deformations are manifested in the form of slight bulges at the lunar surface, detectable only by sensitive instruments. In turn, the Moon’s mass—relatively large for a natural satellite—exerts a gravitational force that causes tides on Earth. The Sun, much more distant but vastly more massive, also raises tides on Earth. (See celestial mechanics: Tidal evolution.) The tides are most apparent as the twice-daily and daily rises and falls of the ocean water, although tidal deformations occur in the solid Earth and in the atmosphere as well (see tide). The movement of the water throughout the ocean basins as a result of the tides (as well as, to a lesser extent, the tidal distortion of the solid Earth) dissipates orbital kinetic energy as heat, producing a gradual slowing of Earth’s rotation and a spiraling outward of the Moon’s orbit. Currently this slowing lengthens the day by a few thousandths of a second per century, but the rate of slowing varies with time as plate tectonics and sea-level changes alter the areas covered by inland bays and shallow seas. (For additional orbital and physical data, see the table.)

The atmosphere and hydrosphere

The blankets of volatile gases and liquids near and above the surface of Earth are, along with solar energy, of prime importance to the sustenance of life on Earth. They are distributed and recycled throughout the atmosphere and hydrosphere of the planet.