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→Precession of the orbit of Mercury: Who did it, and when.
===Precession of the orbit of Mercury===
===Precession of the orbit of Mercury===
The second "classical" test of general relativity was the advance of the perihelion of the orbit of Mercury. There are many complex effects contributing to this, including gravitational perturbations from other planets and the effect of the oblateness of the Sun. These are hard to calculate accurately, but, by 1860 it was known quite accurately that there was an "anomalous" precession, that is, a precession beyond all other known effects, of 43 arc seconds per century. This is a very tiny effect, but astronomical measurements were sufficiently accurate by that time to show it clearly. This was based on observations going all the way back to about 1700.<ref>http://www.math.toronto.edu/~colliand/426_03/Papers03/C_Pollock.pdf</ref>
The second "classical" test of general relativity was the advance of the perihelion of the orbit of Mercury. There are many complex effects contributing to this, including gravitational perturbations from other planets and the effect of the oblateness of the Sun. These are hard to calculate accurately, but, by 1860 it was known quite accurately that there was an "anomalous" precession, that is, a precession beyond all other known effects, of 43 arc seconds per century. This is a very tiny effect, but astronomical measurements were sufficiently accurate by that time to show it clearly. This was based on observations going all the way back to about 1700.<ref>http://www.math.toronto.edu/~colliand/426_03/Papers03/C_Pollock.pdf</ref> It was calculated with great accuracy, and reported in 1859, by Urbain Leverrier. This was long before relativity was formulated.
::One might wonder how astronomical observations accurate enough to measure such a tiny effect could have been made with the technology of that era. A clever trick was used: Solar transits of Mercury were observed and timed. Some of these transits were timed with an accuracy of about 15 seconds.
::One might wonder how astronomical observations accurate enough to measure such a tiny effect could have been made with the technology of that era. A clever trick was used: Solar transits of Mercury were observed and timed. Some of these transits were timed with an accuracy of about 15 seconds.