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
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
nutation and aberration to aid generation of mean places of
stars, and making conventional adjustments to the Moon to
improve eclipse predictions.
1
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
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
2