Get up off the plane of the ecliptic by a couple of million miles. Look back at the Sun and watch the Earth's orbit in time-lapse for a few centuries. The orbit is an ellipse tilted at 23.5 degrees from the ecliptic. Hold station with the Sun as it rotates about the galactic center.

Question: Does that tilted ellipse -- Earth's orbit -- precess? Over millions of years, does the long axis rotate with the Sun's rotation in the galaxy? Or does it stay fixed in some larger frame of reference, such as the Local Group of galaxies? Or does it exhibit some more complex and subtle motion?

Does the long axis nutate?

Does the angle of the orbit to the ecliptic change in any periodic way?

Why? What are the forces in play?

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    $\begingroup$ I suppose it does, but slowly. The precession of Mercury's orbit is much more famous as a test of general relativity. en.wikipedia.org/wiki/Tests_of_general_relativity $\endgroup$ – Mark Eichenlaub Mar 13 '11 at 2:14
  • $\begingroup$ No one seems to have pointed this out: that 23.5 degree tilt is the Earth's spin axis compared to its own orbital axis. The orbit of the Earth is very much closer in line with the orbits of all the other planets. A table can be found here, showing that we're 7.2 or 1.6 degrees off, depending on which reference plane you take. $\endgroup$ – user10851 Nov 11 '12 at 7:57

The major axis of the Earth's orbit does precess. As Mark Eichenlaub points out, in the case of Mercury this precession provides a classic test of general relativity, but the precession due to general relativity is small compared to the precession due to other, Newtonian effects. In the case of the inner planets of the solar system, the dominant cause of the precession is the gravitational pull of other planets, mainly Jupiter. See the Table "Sources of the precession of perihelion for Mercury" in the Wikipedia page Mark linked to, for instance. Presumably you can follow the reference there for a more scholarly source.


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