The seasons
Fall and spring are triggered at the equinox, when the earth is closer to the sun than at any other time of the year, and when the sun crosses directly over the equator, resulting in equal day and night lengths. Winter and summer are triggered when the sun is at its furthest from the earth, during the winter and summer solstice. Daylength is either at its shortest (winter solstice) or longest (summer solstice) of the year. This diagram applies to the northern hemisphere.
