From the book "The Riddle of Gravity" by Einstein's colleague at
Princeton, Peter G. Bergmann:

"The advance of the perihelion of Mercury was considered a settled
matter in 1915 when Einstein derived from hs thoery ... "

"The excellence of this zgreement has been called into question by
Dicke" (Robert H. Dicke of Princeton).

"According to Dicke, the scaler-tensor theory leads to a slightly
different rate of perihelion advance than Einstein's original theory,
the difference amounting to about one tenth of Einstein's value."

WHOOPS!

The book goes on to say that possible oblateness of the Sun might be
responsible for part of Mercury's perihelion advance. Studies have
been done, but with no conclusiive results.

This just goes to show that nothing is nailed down, as the textbook
bangers would say.

I still like to pin my hopes on the exixtence of Vulcan as a partial
explanation. There is no one better poised to discover Vulcan than
David Tholen in his current survey of objects inside of Mercury's
orbit. He might be the first in 100 years to view Vulcan as it comes
out of the glare of the Sun! And that will prove it is not an
invisible planet as the astrologers claim. (Though it would be
interesting if they were right and it turns out to be a dark matter
planet!)

Live long and prosper, Dave!

Double-A