PLANET - An Orbit Simulation (c) 2009, David Newgas <dn271@cam.ac.uk> Usage: PLANET [-f infile] [-s timestep] [-t max_time] [-e | -l] PLANET simulates orbits for given initial conditions, which must be specified on infile, or stdin if unspecified. The format is each body has a line such as: body-name x-position y-position x-velocity y-velocity is-static those are hopefully self explanitory except for is-static. This allows you to "pin" objects in space. If it is "1" the body will not experience any forces, otherwise set it to "0". Making objects static saves a bit of cpu, so it is worth doing for any really massive bodies. The length of the simulation is specified with as max_time, the steps in time are timestep. The simulation is either euler or (default) "leapfrog" style iteration, depending on wheter -e or -l is set. The values should be in the appopriate SI units - seconds, meters and meters per second. body-name must not have spaces in it. You can specify large positions and velocities in the input file as understood by fscanf(3) - eg 3.44e4 .