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.