Subject: how do physicists calculate the advance of perihelion of Mercury?

According to Pr.Trefil, Astronomy Feb2005 p48, Mercury's ellipse rotates about 
the sun even in the absence of Einstein's relativistic corrections to the Sun's 
gravitational field because of Newton's classical theory, and its perihelion 
advance equals 531 seconds of arc per century. My question is which formula did 
Isaac Newton use to perform his classical evaluation? I couldn't find the 
formula. Einstein's relativistic formula however is 
delta phi = 24 pi^3 a^2/T^2 c^2(1-e^2) where delta phi is the advance of 
perihelion, a the semimajor axis of the ellipse, T the revolution about the Sun,
e the excentricity, c the speed of light.

Re: how do physicists calculate the advance of perihelion of Mercury?