calculate_sky_radiance
                        Sample Prague sky radiance at a chosen
                        wavelength.
calculate_sky_values    Sample a direction from the Prague model.
calculate_sun_brightness
                        Sample sun luminance at the disk center.
clear_sky_data          Clear cached sky data files
download_sky_data       Download Prague Sky Model Coefficient Data
generate_moon_latlong   Generate the atmosphere with the moon
generate_planets        Generate a bright-planet image array
generate_sky            Generate a Hosek-Wilkie sky dome array
generate_sky_latlong    Generate a location and time-specific sky dome
                        (optionally with stars)
generate_stars          Generate a star-field array aligned with
                        'generate_sky()'
list_sky_data           List cached sky data files
