METHOD RK4 ; much more accurate than the euler method
; 88 steps of one mean-earth-day
STARTTIME = 0
STOPTIME=88
DT=1

e = 0.2056306 ; eccentricity for MERCURY
P = 87.969 ; period in mean earth days
b = sqrt(1-e^2) ; semi-minor / semi-major
K = 2*PI/P ; mean radians per day

; now define the distance from the sun to the planet, rho,
; as a func of beta, the aperture from the center
rho = 1 + e * cos(beta)
; -------------
INIT beta = 0
beta' = K/b*rho

; next define nuprime as auxiliary func (we do not need nu) from eqn [7]
nuprime = 21600*K/(2*PI*rho^2)