Suite of functions in GNU Octave (they should also run in Matlab) to perform some basic calculations on direct solar illumination of a planar surface. The functions make a number of approximations, notably: the Earth is assumed to have no atmosphere (and no clouds), so that only direct insolation reaches the surface; and all years consist of 365 days (and the Sun’s position varies from day to day in exactly the same way every year). They can, however, allow for a non-flat horizon, to take account of shadowing effects. All functions are internally documented.