03/07/2017
Earth, our home, is the third planet from the sun. It is the only planet known to have an atmosphere containing free oxygen, oceans of liquid water on its surface, and, of course, life.
Earth is the fifth largest of the planets in the solar system — smaller than the four gas giants, Jupiter, Saturn, Uranusand Neptune, but larger than the three other rocky planets, Mercury, Marsand Venus.
Earth has a diameter of roughly 8,000 miles (13,000 kilometers), and is round because gravity pulls matter into a ball, although it is not perfectly round, instead being more of an "oblate spheroid" whose spin causes it to be squashed at its poles and swollen at the equator.
Roughly 71 percent of Earth's surface is covered by water, most of it in the oceans. About a fifth of Earth's atmosphereis made up of oxygen, produced by plants. While scientists have been studying our planet for centuries, much has been learned in recent decades by studying pictures of Earth from space.
Orbital characteristics
Earth spins on an imaginary line called an axis that runs from the North Pole to the South Pole, while also orbiting the sun. It takes Earth 23.439 hours to complete a rotation on its axis, and roughly 365.26 days to complete an orbit around the sun.
Earth's axis of rotation is tilted in relation to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. This means the northern and southern hemispheres will sometimes point toward or away from the sun depending on the time of year, varying the amount of light they receive and causing the seasons.
Earth's orbit is not a perfect circle, but is rather an oval-shaped ellipse, like that of the orbits of all the other planets. Earth is a bit closer to the sun in early January and farther away in July, although this variation has a much smaller effect than the heating and cooling caused by the tilt of Earth's axis. Earth happens to lie within the so-called "Goldilocks zone" around its star, where temperatures are just right to maintain liquid water on its surface.
Orbit & rotation
Some statistics about Earth, according to NASA:
Average distance from the sun: 92,956,050 miles (149,598,262 km)
Perihelion (closest approach to the sun): 91,402,640 miles (147,098,291 km)
Aphelion (farthest distance from the sun): 94,509,460 miles (152,098,233 km)
Length of solar day (single rotation on its axis): 23.934 hours
Length of year (single revolution around the sun): 365.26 days
Equatorial inclination to orbit: 23.4393 degrees.
