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


     sky - obtain ephemerides


     sky [ -l ]


     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


     Placing a ``1'' input after  the  minute  entry  causes  the

     program  to  print  out  the  Greenwich Sidereal Time at the

     indicated 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


     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)

     aberration, 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

     minutes 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

     know that it actually runs in Ephemeris time.


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


     azel (VI)

     American Ephemeris and Nautical Almanac, for the appropriate

     years;  also,  the  Explanatory  Supplement  to the American

     Ephemeris and Nautical Almanac.


     R. Morris