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The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy , it usually applies to planets or asteroids orbiting the Sun , moons orbiting planets, exoplanets orbiting other stars , or binary stars .
One complete orbit takes 365.256 days (1 sidereal year), during which time Earth has traveled 940 million km (584 million mi). [2] Ignoring the influence of other Solar System bodies, Earth's orbit, also called Earth's revolution, is an ellipse with the Earth–Sun barycenter as one focus with a current eccentricity of 0.0167. Since this value ...
Earth's rotation period relative to the Sun (solar noon to solar noon) is its true solar day or apparent solar day. [26] It depends on Earth's orbital motion and is thus affected by changes in the eccentricity and inclination of Earth's orbit. Both vary over thousands of years, so the annual variation of the true solar day also varies.
As first explained by German engineer Maximilian Schuler in a 1923 paper, [1] a pendulum that has a period that equals the orbital period of a hypothetical satellite orbiting at the surface of Earth (about 84.4 minutes) will tend to remain pointing at the center of Earth when its support is suddenly displaced.
The rotation rate of the Earth (Ω = 7.2921 × 10 −5 rad/s) can be calculated as 2π / T radians per second, where T is the rotation period of the Earth which is one sidereal day (23 h 56 min 4.1 s). [2] In the midlatitudes, the typical value for is about 10 −4 rad/s.
The gravitational attraction of the Sun and Moon on the Earth's equatorial bulge cause the rotational axis of the Earth to precess in space similar to the action of a top. This is called precession. Nutation is the smaller amplitude shorter-period (< 18.6 years) wobble that is superposed on the precessional motion of the Celestial pole. It is ...
The ECI reference frame is not truly inertial because of the slow, 26,000 year precession of Earth's axis, so the reference frames defined by Earth's orientation at a standard astronomical epoch such as B1950 or J2000 are also commonly used.
At present, the rate of axial precession corresponds to a period of 25,772 years, [3] so sidereal year is longer than tropical year by 1,224.5 seconds (20 min 24.5 s, ~365.24219*86400/25772). Before the discovery of the precession of the equinoxes by Hipparchus in the Hellenistic period , the difference between sidereal and tropical year was ...