The present invention relates to a system for detection of forest fires, based on the chemical analysis of the atmosphere through optic spectroscopy. The smoke originated from a fire has a chemical composition different from that of a normal atmosphere. This chemical composition is determined by the analysis of light absorption, which passes through the smoke, in its different wavelengths, carried out by a spectrometer. In this case, the spectrometer is associated to a telescope and solar light is used as the light source allowing the detection of smoke originated from a fire in a specific area of the horizon. The maximum distance from which the smoke can be detected depends only on the potency of the telescope and may be of many kilometers. The installation of the system on a rotating support and the use of computational logarithms makes the detection in any point of the horizon possible, a completely autonomous way.