term% cat index.txt SKY(7) Miscellaneous Information Manual 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 ce‐
lestial 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 hori‐
zon 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 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 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) 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 aberra‐
tion 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.
grigg SKY(7)