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The Motions of the Earth

© Copyright 1999, Jim Loy

Galileo got into deep trouble by saying that the earth moves. The earth moves in a number of ways, of course:

  1. 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.
  2. 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.
  3. 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.
  4. The earth moves around the center of the earth-moon system, once a month. See the addendum below.
  5. 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.
  6. 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.
  7. The Local Group is moving within the Local Supercluster. Superclusters are clusters of clusters of galaxies.
  8. 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.

Addendum:

In my article on The Moon, I have this paragraph:

Center of gravity of the Earth/Moon system

Earth/Moon on a balance

The Moon goes around the Earth, right? Actually, both the Earth and the moon go around the center of gravity of the Earth/Moon system. This center of gravity (see the fulcrum of the balance in the picture above) is actually inside the earth. It is closer to the surface of the Earth, than it is to the center of the Earth. But it is fairly accurate to say that the Moon goes around the Earth.

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