Seasons (Fr. saisons), the quarters of the year, spring, summer, autumn, and winter. These periods are determined astronomically by the apparent movements of the sun (the real movements of the earth) in the ecliptic. The passage of the sun across the equator, bringing on days of greater length than the nights, marks the vernal or spring equinox, and occurs about March 21 for the northern hemisphere and Sept. 23 for the southern. These dates also mark the autumnal equinox or commencement of the autumn, the hemispheres being reversed. The summer solstice, when the day is of greatest length and the astronomical summer begins in the northern hemisphere, is about June 21, and the winter solstice about Dec. 21. In the figure, S represents the position of the sun, A the position of the earth at the vernal equinox in the northern hemisphere (about March 21), and N the north pole. Both poles just catch the light of the sun, and in all other parts of the world the days and nights are equal; but at the north pole the sun is just rising, at the south pole he is just setting. B represents the position of the earth at the summer solstice (about June 21). The north pole has been continually exposed to the sun for three months, and it is the noon of the north polar day.
The direct light of the sun reaches about 23° 28' beyond the pole, and consequently anywhere within that distance of the pole the sun is visible during the whole 24 hours. Within the same distance of the south pole it is continual night. (See Polar Circles.) 0 is the position of the earth at the autumnal equinox of the northern hemisphere (about Sept. 23), but the sun is just setting at the north pole and just rising at the south, and again everywhere else the days and nights are equal. D is the position of the earth at the winter solstice of the northern hemisphere (about Dec. 21); it is the noon of the south polar day, the midnight of the north polar night. It will be seen that at the poles the day and the year are coincident, if we consider a day as made up of one period during which the sun is visible and one during which he is invisible. - The popular divisions of the year do not correspond with those of the astronomer, and are not the same in different countries. In England the spring begins with February, summer with May, autumn with August, and winter with November; but in the United States the seasons begin respectively with the months succeeding those named.
The marked changes in the amount of heat and light imparted by the sun in the different seasons upon those portions of the earth outside the tropics are not experienced in the equatorial regions. The sun as it passes twice each year over these regions sends down its rays so directly upon them, that the variations of temperature are comparatively inconsiderable; but the regular winds and rains and dry periods consequent on the movement of the sun in the ecliptic are the most marked periodic phenomena, and by these the year is divided into two dry and two wet seasons.
Positions of the Earth in its Orbit.