This done with the two commands SOURCE and PLANET . They work in the same way, except that SOURCE uses objects from the catalogs, while PLANET takes Solar System objects.
SOURCE name will compute the apparent coordinates of an object from the current catalog. The epoch coordinates are reduced to current epoch (as defined by the last TIME command), and converted to horizontal coordinates azimuth and elevation relative to the observatory (as defined by the last OBSERVATORY command. Precession, nutation, aberration are taken into account. The motion of the Earth uses the ephemerides from the Bureau des Longitudes (BDL82). The source coordinates are printed on the screen, together with the line-of-sight components of the velocities of the Solar System barycenter G, relative to the observatory, and of the Local Standard of Rest (relative to G). All velocities are in km/s, counted positively for increasing distance (redshift).
SOURCE with no arguments will compute positions and optionally draw all sources from the current catalog; /ALTERNATE will direct the search to the alternate catalog.
PLANET name will do the same thing, but the object must be a known Solar System object: the Sun, the Moon, and planets from Mercury through Neptune. Pluto is not yet available. With no name, all Solar System Objects are used. In addition information is given on the planet's apparent size and osition angle, its brightness temperature and flux at millimeter wavelengths, at the current frequency. The current frequency (in GHz) is updated either through command LINE or by modifying the value of the variable FREQUENCY.
With option /DRAW arguments the position of the object are plotted in the current plotting frame. Arguments to /DRAW may be LINE (to plot a line from the previously computed position), MARKER (to plot the current GreG marker at the computed position, with a size corresponding to the flux or magnitude in the catalog), SYMBOL (to draw the source name in capitals), or FULL (to get both the marker and name; for planets, FULL gives the symbol, centered on the object's position).
The coordinates (in radians) of the last computed position are stored in the SIC variables RA, DEC, AZ, EL.
The plotting frame is defined by command FRAME .
The frame may be in Equatorial coordinates or in Horizontal coordinates.
The plotting may be done in rectangular coordinates (using
FRAME HO or FRAME EQ, with no other argument); or in a
stereographic projection, which gives a better representation of the sky
(distorsion of shapes is not too serious, but the scale of the map changes
from the center of the projection to the edges). The projection is defined
by its center coordinates and its size; for instance
FRAME EQ 12:00 45 20
will use a projection centered in the direction of 12 hours, 45 degrees
in 1950 equatorial coordinates, and of size 20 degrees. The size defaults
to 180 degrees (a half hemisphere). A few abbreviations exist, like
FRAME HO ZENITH, which gives the whole sky above the horizon, or
FRAME HO SOUTH, for the visible sky above the southern horizon.
Use of SIC variables permits to select frames such as
FRAME EQ RA*12|pi DEC*180|pi 45
which selects a frame of size 45 degrees, in equatorial coordinates,
centered on the last computed object position.
Command HEADER will label the frame with the times (LST, UTC) and date.