An evaluation of the radiation from the sun that reaches the observation point. The observing instrument is usually a pyrheliometer or pyranometer. Two types of such observations are taken. The more common consists of measurements of the radiation reaching a horizontal surface, consisting of both radiation from the sun (direct solar radiation) and that reaching the instrument indirectly by scattering in the atmosphere (diffuse sky radiation). The second type of observation involves the use of an equatorial mount that keeps the instrument pointed directly at the sun at all times. The sensitive surface of the instrument is normal to the path of solar radiation and is shielded from indirect radiation from the sky.