***** File OBSNTERP.TXT The Fortran source code OBSNTERP.FOR is an interactive, seven-point Lagrange interpolation program that will return ephemeris data for periodic comet Halley over the interval November 26, 1981 through June 23, 1989, when it is run on the table EPHEM.TAB (in the EPHEM directory). VAX/VMS and MS-DOS executables of OBSNTERP are called VAXNTERP.EXE and PCNTERP.EXE, respectively; like OBSNTERP.FOR, they are stored in the \SOFTWARE\OBSNTERP directory of this disc. To run the program, type PCNTERP (or VAXNTERP ) and respond to the prompts. You can speed up the interpolation time by shortening the data table to include only the interval that you require and by requesting output for dates in chronological order. The information in the data table was generated with IHW orbit number 61, which was based upon 7469 astrometric positions over the interval August 21, 1835 through January 9, 1989. (Each line of the table is a fixed width of 110 characters plus .) The definition of each output quantity is given below: Date: Calendar date and Julian date (times in these ephemerides are universal times). RA, DEC: Geocentric right ascension and declination referred to the mean equator and equinox of 1950.0 - light time corrections have been applied. DELTA: Geocentric distance of comet in AU. DELDOT: Geocentric velocity of comet in km/sec. R: Heliocentric distance of comet in AU. RDOT: Heliocentric velocity of comet in km/sec. THETA: Sun-Earth-Comet angle in degrees. BETA: Sun-Comet-Earth angle in degrees. MOON: Comet-Earth-Moon angle in degrees. PSANG: Position angle of extended Sun-Comet vector. PSAMV: Position angle of minus comet velocity vector. (Position angles are measured east from north.) Ravenel N. Wimberly Astrometry Team Member Jet Propulsion Laboratory 4800 Oak Grove Dr. Pasadena, CA 91109 [NOTE: this write-up by R. N. Wimberly was modified slightly to take account of the generation at NASA/GSFC of executable modules of the source code OBSNTERP.FOR. -- M. B. Niedner, Jr.]