# What did general relativity clarify about Mercury?

I frequently hear that Kepler, using his equations of orbital motion, could predict the orbits of all the planets to a high degree of accuracy -- except Mercury. I've heard that mercury's motion couldn't be properly predicted until general relativity came around. But what does general relativity have to do with Mercury's orbit?

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– Qmechanic Mar 24 at 12:26

This web page has a nice discussion on it: http://archive.ncsa.illinois.edu/Cyberia/NumRel/EinsteinTest.html

Basically the orbit's eccentricity would precess around the sun. Classical stellar mechanics (or Newtonian gravity) couldn't account for all of that. It basically had to do with (and forgive my crude wording) the sun dragging the fabric of space-time around with it.

Or as the web page says:

Mercury's Changing Orbit

In a second test, the theory explained slight alterations in Mercury's orbit around the Sun.

Daisy petal effect of precession

Since almost two centuries earlier astronomers had been aware of a small flaw in Mercury's orbit around the Sun, as predicted by Newton's laws. As the closest planet to the Sun, Mercury orbits a region in the solar system where spacetime is disturbed by t he Sun's mass. Mercury's elliptical path around the Sun shifts slightly with each orbit such that its closest point to the Sun (or "perihelion") shifts forward with each pass. Newton's theory had predicted an advance only half as large as the one actually observed. Einstein's predictions exactly matched the observation.

For more detail that goes beyond a simple layman answer, you can check this page out and even download an app that let's you play with the phenomenon: http://www.fourmilab.ch/gravitation/orbits/

And of course, the ever handy Wikipedia has this covered as well: http://en.wikipedia.org/wiki/Tests_of_general_relativity#Perihelion_precession_of_Mercury Although, truth be told, I think I said it better (i.e. more elegantly) than the Wiki page does. But then I may be biased.

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Mercury's orbit is elliptical. The orientation of this ellipse's long axis slowly rotates around the sun. This process is known as the "precession of the perihelion of Mercury" in astronomical jargon. It's a total of 5600 arcseconds of rotation per century.

The precession is mostly a result of totally classical behavior; almost all of the movement of the perihelion (about 5030 arcseconds per century) is present in a two-body system with point masses for the Sun and Mercury. Another 530 arcseconds per century are caused by gravitational effects of the other planets.

That leaves 40 arcseconds per century of unexplained movement. The observed value of 5599.7 arcseconds per century is measured very accurately, to within 0.04 arcseconds per century, so this is a significant deviation.

It turns out that 43 arcseconds per century are expected to result from general relativity. One hand-wavey way of explaining this is that the curvature of spacetime itself by the two bodies (Sun and Mercury) causes some changes to the gravitational potential, so it isn't really exactly $\frac{GMm}{r}$.

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Another way of looking at it is that the circumference of a circular orbit near the Sun is slightly less than 2*pi*r because of the positive curvature of space-time. – Pete Jackson Aug 3 '11 at 22:25
@PeteJackson: You can't downvote comments, but this is not a good way of looking at it, since this assumes that the effect is independent of velocity, which it isn't. – Ron Maimon Jun 3 '12 at 18:56
What is this 5030 arcseconds in the 2-body problem? There isn't any precession in two-body problem--- is this tidal effects of Mercury on the sun? – Ron Maimon Jun 3 '12 at 18:57
@RonMaimon No, it was due to the precession of the equinoxes. The precession of the perihelion of Mercury was being measured in a non-inertial reference system for historical reasons. – mmc Jun 3 '12 at 20:56
The 5030 arcseconds is not present in a two body system. Please remove this sentence. The 5030 arcseconds is only present because you are using an Earth-bound measuring device, because of the motion of the Earth, it has nothing to do with mercury. The total precession is 570 arcseconds per century, the rest is artifact, see discussion above. – Ron Maimon Jun 4 '12 at 4:18