EPOCH = epoch of elements (TT MJD)
ORBINC = inclination i (radians)
ANODE = longitude of the ascending node, big omega (radians)
PERIH = longitude of perihelion, curly pi (radians)
AORQ = mean distance, a (AU)
E = eccentricity, e (range 0 to 1)
AORL = mean longitude L (radians)
DM = daily motion (radians)
Option JFORM = 2, suitable for minor planets:
EPOCH = epoch of elements (TT MJD)
ORBINC = inclination i (radians)
ANODE = longitude of the ascending node, big omega (radians)
PERIH = argument of perihelion, little omega (radians)
AORQ = mean distance, a (AU)
E = eccentricity, e (range 0 to 1)
AORL = mean anomaly M (radians)
Option JFORM = 3, suitable for comets:
EPOCH = epoch of elements and perihelion (TT MJD) ORBINC = inclination i (radians) ANODE = longitude of the ascending node, big omega (radians) PERIH = argument of perihelion, little omega (radians) AORQ = perihelion distance, q (AU) E = eccentricity, e (range 0 to 10)
Unused arguments (DM for JFORM=2, AORL and DM for JFORM=3) are not accessed.
. The epoch of observation: the moment in time for which the position of the body is to be predicted.
. The epoch defining the position of the body: the moment in time at which, in the absence of purturbations, the specified position (mean longitude, mean anomaly, or perihelion) is reached.
. The osculating epoch: the moment in time at which the given elements are correct.
For the major-planet and minor-planet cases it is usual to make the epoch that defines the position of the body the same as the epoch of osculation. Thus, only two different epochs are involved: the epoch of the elements and the epoch of observation.
For comets, the epoch of perihelion fixes the position in the orbit and in general a different epoch of osculation will be chosen. Thus, all three types of epoch are involved.
For the present routine:
. The epoch of observation is the argument DATE.
. The epoch defining the position of the body is the argument EPOCH.
. The osculating epoch is not used and is assumed to be close enough to the epoch of observation to deliver adequate accuracy. If not, a preliminary call to sla_PERTEL may be used to update the element-set (and its associated osculating epoch) by applying planetary perturbations.