**ACTION:**- Heliocentric position and velocity of a planet,
asteroid or comet, starting from orbital elements.
**CALL:**`CALL sla_PLANEL ( DATE, JFORM, EPOCH, ORBINC, ANODE, PERIH, AORQ, E, AORL, DM, PV, JSTAT)`

**GIVEN:**-

*DATE***D**Modified Julian Date (JD-2400000.5) *JFORM***I**choice of element set (1-3, see Note 3, below) *EPOCH***D**epoch of elements ( *t*or_{0}*T*, TT MJD)*ORBINC***D**inclination ( *i*, radians)*ANODE***D**longitude of the ascending node (, radians) *PERIH***D**longitude or argument of perihelion ( or , radians) *AORQ***D**mean distance or perihelion distance ( *a*or*q*, AU)*E***D**eccentricity ( *e*)*AORL***D**mean anomaly or longitude ( *M*or*L*, radians,JFORM=1,2 only) *DM***D**daily motion ( *n*, radians, JFORM=1 only)

**RETURNED:**-

*PV***D(6)**heliocentric , equatorial, J2000 (AU, AU/s) *JSTAT***I**status: 0 = OK -1 = illegal JFORM -2 = illegal E -3 = illegal AORQ -4 = illegal DM -5 = numerical error

**NOTES:**- 1.
- DATE is the instant for which the prediction is required. It is in the TT timescale (formerly Ephemeris Time, ET) and is a Modified Julian Date (JD-2400000.5).
- 2.
- The elements are with respect to the J2000 ecliptic and equinox.
- 3.
- Three different element-format options are available, as
follows.

JFORM=1, suitable for the major planets:

`EPOCH = epoch of elements`*t*(TT MJD) ORBINC = inclination_{0}*i*(radians) ANODE = longitude of the ascending node (radians) PERIH = longitude of perihelion (radians) AORQ = mean distance*a*(AU) E = eccentricity*e*AORL = mean longitude*L*(radians) DM = daily motion*n*(radians)JFORM=2, suitable for minor planets:

`EPOCH = epoch of elements`*t*(TT MJD) ORBINC = inclination_{0}*i*(radians) ANODE = longitude of the ascending node (radians) PERIH = argument of perihelion (radians) AORQ = mean distance*a*(AU) E = eccentricity*e*AORL = mean anomaly*M*(radians)JFORM=3, suitable for comets:

`EPOCH = epoch of perihelion`*T*(TT MJD) ORBINC = inclination*i*(radians) ANODE = longitude of the ascending node (radians) PERIH = argument of perihelion (radians) AORQ = perihelion distance*q*(AU) E = eccentricity*e* - 4.
- Unused elements (DM for JFORM=2, AORL and DM for JFORM=3) are not accessed.
- 5.
- The reference frame for the result is equatorial and is with respect to the mean equinox and ecliptic of epoch J2000.
- 6.
- The algorithm was originally adapted from the EPHSLA program of D.H.P.Jones (private communication, 1996). The method is based on Stumpff's Universal Variables.

**REFERENCE:**- Everhart, E. & Pitkin, E.T., Am. J. Phys. 51, 712, 1983.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk