A sun scorched world Mercury bears the mark of the proxmity of the sun in
the darkness of its rocks as measured by its low albedo of .11. It lies
.39AU, 58 million km, from the sun and in its highly elliptical orbit
varies from 46 million km at perihelion to 70 million km at aphelion. Its
perihelion advances by 574"/century of which 531"/century can be explained
by Newtonian gravitational attraction of planets. But the remaining
43"/century was not consistent with Newtonian gravity and for over a
century indicated that Newton was wrong. At first, an attempt to save
Newtonian gravity from the trash heap was attempted by postulating the
presence of an nobserved inner planet Vulcan, the gravity of which could
account for the unexplained 43"/centurey. Eclipse after eclipse, Vulcan
was pursued but was never found. By the end of the 1900s sufficiently
careful eclipse measurements had been made to eliminate any planet, large
enough to cause the perturbation of Mercurys orbit.
The solution to this puzzle was supplied by Einstein in 1915 when he
showed that such movement of its orbit is exactly predicted by curved
space-time. This was the first of a series of tests applied to Newton and
Einstein, in which Einstein always emerged the victor.
Mercury rotates with a period of 58.65 days in a 3:2 resonance with its 88
day period of revolution. The planet rotates three times for every
revolution, such that at every perihelion passage its equatorial bulge is
in line with the sun.