carxeš-e zamin (#)
Fr.: rotation de la Terre
The natural motion of the Earth around its own axis, which takes place once in a → sidereal day. The Earth rotates toward the → east, in the same direction as it revolves around the Sun. If viewed from the north celestial pole, the Earth turns → counterclockwise. The opposite is true when the Earth is viewed from the south celestial pole. The Earth's rotation is responsible for the diurnal cycles of day and night, and also causes the apparent movement of the Sun across the sky. The Earth's rotation velocity at the → equator is 1,673 km h-1 or about 465 m s-1. More generally, at the → latitude φ it is given by: vφ = veq cos φ, where veq is the rotation velocity at the equator. The Earth's rotation is gradually slowing down under the action of the → tides, which are generated by the → gravitational attraction of the → Moon. As the result of this → tidal friction, the day is becoming longer at a rate of about 2 milliseconds, or 0.002 seconds, per century (or one second every 50,000 years). Moreover, the loss of the Earth's → rotational angular momentum increases the Moon's → orbital angular momentum, because the angular momentum of the → Earth-Moon system is conserved. In consequence, the Moon slowly recedes from the Earth by about 4 cm per year, which leads to increasing its orbital period and the length of a month as well.