    SKY(VI)                      9/22/73                      SKY(VI)

    NAME
         sky - obtain ephemerides

    SYNOPSIS
         sky [ -l ]

    DESCRIPTION
         Sky  predicts  the  apparent locations of the Sun, the Moon,
         the planets out to Saturn, stars of magnitude at least  2.5,
         and certain other celestial objects.  Sky reads the standard
         input to obtain a GMT time typed on  one  line  with  blanks
         separating year, month number, day, hour, and minute; if the
         year is missing the current year is used.  If a  blank  line
         is  typed  the current time is used.  The program prints the
         azimuth, elevation, and magnitude of objects which are above
         the  horizon at the ephemeris location of Murray Hill at the
         indicated time.  The `-l' flag causes it to ask for  another
         location.

         Placing a ``1'' input after the minute entry causes the pro-
         gram to print out the Greenwich Sidereal Time at  the  indi-
         cated  moment  and  to  print  for each body its topographic
         right ascension and declination as well as its  azimuth  and
         elevation.  Also, instead of the magnitude, the semidiameter
         of the body, in seconds of arc, is reported.

         A ``2'' after the minute entry makes the  coordinate  system
         geocentric.

         The  effects of atmospheric extinction on magnitudes are not
         included; the brightest magnitudes  of  variable  stars  are
         marked with ``*''.

         For  all  bodies,  the program takes into account precession
         and nutation of the equinox, annual (but not diurnal)  aber-
         ration,  diurnal  parallax,  and the proper motion of stars.
         In no case is refraction included.

         The program takes into account perturbations  of  the  Earth
         due to the Moon, Venus, Mars, and Jupiter.  The expected ac-
         curacies are: for the Sun and other  stellar  bodies  a  few
         tenths  of seconds of arc; for the Moon (on which particular
         care is lavished) likewise a few tenths of seconds.  For the
         Sun,  Moon  and  stars the accuracy is sufficient to predict
         the circumstances of eclipses and occultations to  within  a
         few seconds of time.  The planets may be off by several min-
         utes of arc.

         There are lots of special options not described here,  which
         do  things  like substituting named star catalogs, smoothing
         nutation and aberration to aid generation of mean places  of
         stars,  and  making  conventional adjustments to the Moon to
         improve eclipse predictions.

         For the most accurate use of the program it is necessary  to

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    SKY(VI)                      9/22/73                      SKY(VI)

         know that it actually runs in Ephemeris time.

    FILES
         /usr/lib/startab, /usr/lib/moontab

    SEE ALSO
         azel(VI)
         American Ephemeris and Nautical Almanac, for the appropriate
         years; also, the  Explanatory Supplement to the American
         Ephemeris and Nautical Almanac.

    AUTHOR
         R. Morris

    BUGS

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