Astronomical Tools:

Precession:  These routines probably have some history even before the
    authors quoted as original sources.  They have been collected and
    some rewritten into SPP by F. Valdes (NOAO), March 1986.  PRECES.F
    was used originally in V2.3-V2.5 IRAF by George Jacoby.  It has
    been replaced by ASTPRECESS.X which is the only procedure in the
    library.

    astprecess.x -- Precession written by F. Valdes based on Astronomical
	Almanac using new IAU system.
    precessmgb.x -- Precession + aberration + nutation based on the work of
	Manchester, Gordon, and Ball
    precessgj.x -- Originally written by G. Jacoby in Fortran and distributed
	with V2.3-V2.5 IRAF.  Transcribed to SPP by F. Valdes

    Notes:
	1. The differences between ASTPRECESS.X and PRECESSGJ.X (
	   and a routine written by D. Wells for the Cyber and later
	   used in other NOAO software) are on the order of a few
	   tenths of a second of arc.  I believe the differences are
	   due to using 1984 almanac methods in the former and much
	   earlier methods for the latter.
	3. PRECESSMGB.X differs considerably from the others to the order
	   of many seconds of arc.  It does include other effects not
	   present in the other routines.  It totally fails at DEC=+-90.
	   It is based on roughly 1970 almanac methods.
	4. See PRECESS.DOC for comparison.

Radial Velocity:  These formulas for these routines were partly obtained
    by inspection of the code for the subroutine DOP in the program DOPSET
    written by R. N. Manchester and M. A. Gordon of NRAO dated January 1970.
    They have been restructured, revised, and coded in SPP by F. Valdes.

    astvr.x -- Project a velocity vector in radial velocity along line of sight.
    astvbary.x -- Radial velocity component of center of the Earth relative to
	to the barycenter of the Earth-Moon system.
    astvrotate.x -- Radial velocity component of the observer relative to
	the center of the Earth due to the Earth's rotation.
    astvorbit.x -- Radial velocity component of the observer relative to
	the center of the Earth due to the Earth's rotation.
    astvsun.x -- Projection of the sun's velocity along the given direction.

Coordinates:

    astarcsep.x -- Arc distance (arcsec) between two spherical coordinates
	(hours, degrees).
    astcoord.x -- This procedure converts the longitude-latitude coordinates
	(a1, b1) of a point on a sphere into corresponding coordinates
	(a2, b2) in a different coordinate system that is specified by
	the coordinates of its origin (ao, bo) and its north pole (ap,
	bp) in the original coordinate system.  The range of a2 will be
	from -pi to pi.
    astgalactic.x -- Convert equatorial coordinates (1950) to galactic
	coordinates.
    astgaltoeq.x -- Convert galactic coordinates to equitorial (1950).

Dates and times:

    asttimes.x:
	AST_DATE_TO_EPOCH -- Convert Gregorian date and solar mean time to
	    a Julian epoch.  A Julian epoch has 365.25 days per year and 24
	    hours per day.
	AST_EPOCH_TO_DATE -- Convert a Julian epoch to year, month, day, and
	    time.
	AST_DAY_OF_YEAR -- The day number for the given year is returned.
	AST_DAY_OF_WEEK -- Return the day of the week for the given Julian day.
	    The integer day of the week is 0=Sunday - 6=Saturday.  The
	    character string is the three character abbreviation for the day
	    of the week.  Note that the day of the week is for Greenwich
	    if the standard UT is used.
	AST_JULDAY -- Convert epoch to Julian day.
	AST_DATE_TO_JULDAY -- Convert date to Julian day.
	AST_JULDAY_TO_DATE -- Convert Julian day to date.
	AST_MST -- Mean sidereal time of the epoch at the given longitude.
	    This procedure may be used to optain Greenwich Mean Sidereal Time
	    (GMST) by setting the longitude to 0.
    asthjd.x:
	AST_HJD -- Helocentic Julian Day for a direction of observation.
	AST_JD_TO_HJD -- Helocentic Julian Day for a direction of observation.

Helocentric Parameters:

    astdsun.x:
	AST_DSUN - Distance to Sun in AU.

Misc:

    astlvac.x:
	AST_LVAC - Convert air wavelengths to vacuum wavelengths (Angstroms)

Y2K:
    Most routines work in Julian days or epochs.  If they have an input
    year it is converted to one of these forms by calling
    ast_date_to_julday.  This is the only routine that has a Y2K
    connection.  It assumes two digit years are 20th century.  These
    routines are Y2K correct.


The following are the comments and references from the DOPSET program
noted above.  The HJD routine was also derived from this code.

C     MODIFIED FOR IBM 360 BY R.N.MANCHESTER AND M.A.GORDON
C     JANUARY 1970
C
C
C  DOP CALCULATES THE VELOCITY COMPONENT OF THE OBSERVER WITH RESPECT
C  TO THE LOCAL STANDARD OF REST AS PROJECTED ONTO A LINE SPECIFIED BY T
C  ASCENSION AND DECLINATION (RAHRS, RAMIN, RASEC, DDEG, DMIN, DSEC) EPO
C  DATE, FOR A TIME SPECIFIED AS FOLLOWS:  NYR = LAST TWO DIGITS OF THE
C  (FOR 19XX A.D.), NDAY = DAY NUMBER (GMT), NHUT, NMUT, NSUT = HRS, MIN
C  (GMT).  THE LOCATION OF THE OBSERVER IS SPECIFIED BY THE LATITUDE (AL
C  LONGITUDE (OLONG) (GEODETIC) (IN DEGREES) AND ELEVATION (ELEV) (IN ME
C  ABOVE MEAN SEA LEVEL.  THE SUBROUTINE OUTPUTS THE LOCAL MEAN SIDEREAL
C  (XLST IN DAYS), THE COMPONENT OF THE SUN*S MOTION WITH RESPECT TO THE
C  STANDARD OF REST AS PROJECTED ONTO THE LINE OF SIGHT TO THE SOURCE (V
C  KM/SEC) AS WELL AS THE TOTAL VELOCITY COMPONENT V1 (KM/SEC).  POSITIV
C  VELOCITY CORRESPONDS TO INCREASING DISTANCE BETWEEN SOURCE AND OBSERV
C
C  THIS VERSION OF DOP TAKES INTO ACCOUNT COMPONENTS OF THE OBSERVER*S
C  MOTION DUE TO THE ROTATION OF THE EARTH, THE REVOLUTION OF THE EARTH-
C  BARYCENTER ABOUT THE SUN, AND THE MOTION OF THE EARTH*S CENTER ABOUT
C  EARTH-MOON BARYCENTER.  THE PERTURBATIONS OF THE EARTH*S ORBIT DUE TO
C  PLANETS ARE NEGLECTED.  THE ABSOLUTE PRECISION OF THIS VERSION OF DOP
C  ABOUT 0.004 KM/SEC, BUT SINCE THE DOMINANT ERROR TERM IS SLOWLY VARYI
C  RELATIVE ERROR WILL BE CONSIDERABLY LESS FOR TIMES UP TO A WEEK OR SO
C
C  REFERENCES:  MCRAE, D. A., WESTERHOUT, G., TABLE FOR THE REDUCTION OF
C                  VELOCITIES TO THE LOCAL STANDARD OF REST, THE OBSERVAY=2.
C                  LUND, SWEDEN, 1956.
C               SMART, W. M., TEXT-BOOK ON SPHERICAL ASTRONOMY, CAMBRIDG
C                  UNIV. PRESS, 1962.
C               THE AMERICAN EPHEMERIS AND NAUTICAL ALMANAC
C               THE SUPPLEMENT TO THE ABOVE
C
C     VERSION OF JUNE 1969

