Position and orbit

Earth orbits the Sun in a slightly elliptical orbit. Its mean distance from the Sun is just over 149 million kilometres. At closest approach (perihelion) it is 147 million kilometres from the Sun, and its greatest distance (aphelion) is 152 million kilometres.

Earth takes just over 365 days, 6 hours and 9 minutes to orbit the Sun once; this length of time is called a sidereal year. Its axis is inclined at 23.5° off the perpendicular to the plane of its orbit. A single rotation on this axis takes 23 hours 56 minutes and 4 seconds; this length of time is called a

Figure 6.3 Orbit of Earth around the Sun (distance circles are in astronomical units, AU).

sidereal day. Earth rotates from west to east on its axis and this makes the Sun appear to move across the sky from east to west each day. The tilt of Earth's axis causes the seasons.

The interaction of Earth and the Moon slows Earth's rotation by about 2 milliseconds per century. Current research indicates that about 900 million years ago there were 481 eighteen-hour days in a year.

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