swift/README.j2

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		         README.J2
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The code assumes that to incorporate an oblateness to the first particle
that the grav. potential of a central body is given by

	V = -(GM/r) * (1 - j2rp2*P_2/(r^2) - j4rp4*P_4/(r^4) ) 

where j2rp2 = j2 * rp^2  and j4rp4 = j4 * rp^4, with rp the radius of
the central object.  The P_n are Legendre polys of order n and their
argument is (z^2/r^2).

N.B. THE MINUS SIGNS IN THE SUMMATION OVER MULTIPOLE MOMENTS ARE
TAKEN BY SOME AUTHORS TO BE PLUS SIGNS , SO THAT THE SIGNS OF J2 AND
J4 MUST BE CAREFULLY CHECKED. IN OUR NOTATION FOR EXAMPLE THE
J2 FOR AN OBLATE BODY IS USUALLY POSITIVE.  A POSITIVE J2 IN OUR
NOTATION LEADS TO THE ADVANCE OF THE ARGUMENT OF PERIHELION, AS WOULD
HAPPEN E.G. FOR MERCURY DUE ONLY TO AN OBLATE SUN. NOTE ALSO WE
REQUIRE THE USER TO INPUT J2RP2 and J4RP4, SO THE SATELLITE DISTANCE
MUST BE IN THE SAME LENGTH UNITS AS IS USED TO CALCULATE J2*(RP^2)
FOR EXAMPLE.