SKY(7)                       (grigg)                       SKY(7)

     NAME
          sky - calculate 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
          accuracies 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

     SKY(7)                       (grigg)                       SKY(7)

          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
          know that it actually runs in Ephemeris time.

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

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