Without detailed study, we may note the aggregate insolation received by the earth, at various latitudes, and its general effect on terrestrial temperature. The sun emits a nearly constant stream of radiation, from year to year, which plays continuously upon the earth as a whole, with an intensity which varies but slightly from month to month, due to the slightly varying distances of the earth from the sun. Owing to the sun’s seasonal wandering across the equator, the insolation at any latitude varies considerably month by month, and the polar regions receive much more light than if no such wandering occurred. The total yearly insolation for every 5° of latitude is shown in the following table from Hann, in which the unit is the amount that the earth would receive in one day at the time of the equinox, if the sun were at its mean distance from the earth: TABLE V _Annual Amounts of Insolation_ -----------+---------+------------ Latitude. | Thermal | Difference. | Days. | -----------+---------+------------ 0° | 350.3 | 5° | 349.1 | 1.2 10° | 345.5 | 3.6 15° | 339.4 | 6.1 20° | 331.2 | 8.2 25° | 320.5 | 10.7 30° | 307.9 | 12.6 35° | 293.2 | 14.7 40° | 276.8 | 16.4 45° | 258.7 | 18.1 50° | 239.6 | 19.1 55° | 219.4 | 20.2 60° | 199.2 | 20.2 65° | 180.2 | 19.0 70° | 166.2 | 14.0 75° | 156.5 | 9.7 80° | 150.2 | 6.3 85° | 146.5 | 3.7 90° | 145.4 | 1.1 -----------+---------+------------