Passive solar heating is defined as using solar energy incident on windows, skylights, greenhouses, clerestories, mass walls, and thermosyphoning collectors in order to provide heating for a house. Generally such solar collection occurs passively, without the extensive use of pumps or fans typically used in active solar collector systems. However, the distribution of the energy collected sometimes uses small fans to redistribute the energy throughout the house. Because heating is needed only over the colder part of the year (September to March), passive solar design must also eliminate unwanted solar heat gains during the summer. The use of techniques to eliminate solar gains and to cool a house with the use of active systems is often referred to as passive cooling.