***** 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 <carriage return> (or VAXNTERP <cr>)  
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 <CR>  
<LF>.)                                                                        
                                                                              
     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.]