A period from about 1645 to 1715 when the number of sunspots was unusually low. This solar activity minimum is attested also through the increased content of carbon-14 in tree rings in that period. The reason is that the cosmic rays which produce carbon-14 reach the Earth in a greater number when there is weak solar activity. The Maunder minimum occurred during a period of cooling of the Earth, called the Little Ice Age. The Maunder minimum is one of a number of periods of low solar activity, including the Dalton minimum, the Sporer minimum, the Wolf minimum, and the Oort minimum.