Motion of the Earth
Earth’s axis is an imaginary line which runs right across and passes through the centre of the earth. The earth spins round its axis which always remains inclined at an angle of 66 1/2° to the plane of the Earth’s orbit.
Rotation : The spinning of the earth on its axis is called rotation. The earth rotates from west to east and takes 23 hours, 56 minutes and 4.091 seconds to complete one rotation. Days and nights are caused due to the rotation of the earth. At the equator there is a 12 hours day and 12 hours night.
Revolution : The movement of the earth around the sun, simultaneously, with its rotation is called revolution. The earth moves around the sun at a speed of about 100,000 kmper hour. It takes 365 days, 5hours, 48 minutes and 45.51 seconds to complete one revolution. Change of seasons is caused due to the revolution of the earth.
Galileo got into deep trouble by saying that the earth moves. The earth moves in a number of ways :
- Most obviously, the earth spins on its axis, once every 23 hr. 56 min. This causes day and night, and makes most extraterrestrial objects ( sun, moon, planets, stars… ) seem to move around the sky in about one day. We, on the surface of the earth, move with the earth. A person at the equator moves about a thousand miles per hour ( about 1600 km/hr ) toward the East. We in temperate latitudes move slower. The spin of the earth is slowing down, because of the pull of the tides. The spin also speeds up and slows down very slightly because of changes in large weather patterns.
- The earth’s spin axis wobbles. This is called the precession of the equinoxes. One complete wobble ( rotation ) of the axis takes about 26,000 years. The wobble is mostly caused by the gravitational pull of Jupiter and other planets.
- The earth moves around the sun, in an elliptical ( but nearly circular ) orbit, once a year. Because of this, and the tilt of the earth’s axis, we experience The Seasons. This orbit also causes a solar day to be 24 hours instead of 23 hr. 56 min. The earth is moving about 67,000 mi/hr ( 107,000 km/hr ) in its orbit.
- The earth moves around the center of the earth – moon system, once a month. See the addendum below.
- The solar system ( the sun, its planets, and smaller objects ) is part of the rotation of the Milky Way galaxy. The various nearby stars move about the galaxy at somewhat different speeds and directions. Our motion, around the galaxy, is currently about 550,000 mi/hr ( 900,000 km/hr ) toward the constellation Lyra. It takes the solar system about 200 to 250 million years to go around the galaxy once.
- The Milky Way is moving within the local cluster of galaxies ( known as the Local Group ). I do not know how fast or in which direction the Milky Way is moving.
- The Local Group is moving within the Local Supercluster. Superclusters are clusters of clusters of galaxies.
- The various superclusters of galaxies are moving away from each other. Each of these superclusters can be viewed as being at rest, while all the others are moving away from it. So there is no logical reason to choose a direction and speed for the Local Supercluster. The most distant galaxies and quasars appear to be moving away at nearly the speed of light.
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