SEASONS, CHANGE OF. The phenomena of the seasons may be divided into those which always recur every year and those which are different in different years. We have in every year the same succes sion of longer and shorter days, with a summer and winter ; while the summer of one year is of a higher temperature, and accompanied by finer days, than that of another. The unvarying phenomena can be explained by what we know of the sun's (or earth's) motion ; the vary ing phenomena belong to the science of meteorology, and depend upon atmospheric and other circumstances, with which we have little or no acquaintance. At any given moment, the light and heat received from the aun, at any given place, depend upon the altitude of that body in two ways. In the first place, the lower the sun is, the greater the thickness of the portion of the atmosphere which its rays have to traverse before reaching the spot; the greater then is the light and heat which is lost in the passage. In the second place, the less the altitude of the sun, the less the actual quantity of light and heat which falls upon any given spot. If be the diameter of a circular portion of the earth's surface, and if the sun be seen in the direction al, the light which falls on that circle is a cylinder of rays with the diameter e D ; but if the sun be seen vertically, or in the direction ns:, the cylinder of rays has E F for its diameter, besides which, the rays of the first cylinder are weaker than those of the second, as having "eased through more of the atmosphere. Neglecting this latter consideration, the quantities of light and heat received when the sun is at two dif ferent altitudes are as the sines of those altitudes. Thus the sine of 30° being 4 and that of 90° being 1, the quantity of light which falls on a given spot when the sun is vertical is double of that which falls when its altitude is The earth's axis preserves its direction throughout the whole of the yearly motion. The consequence is, that places which are distant from the equator have very unequal days at different times of the year. [Sue.] The accompanying figure, which is generally given in con nection with this subject, represents the earth in its four principal positions; the sun being at a, and a: being the north pole of the earth. A is at the vernal equinox, tiro intersection of the equator and ecliptic passes through the aim, and days and nights are equal all over the world. B is at the summer solstice, where the sun is most above the equator on the northern side; the diurnal circles north of the equator have more day than night, and have their longest days : and vice versel. c is the autumnal equinox, the phenomena of A being repeated. D is at the winter solstice, where the sun is farthest from the equator on the southern side; the phenomena of a are now reversed, the days being shortest on the north side of the equator. This figure very well explains the variation of days and the main reason for the phenomena of seasons in the extra-tropical parts of the earth. It is evident that,
speaking of the northern hemisphere, the sum, being above the equator, gives not only longer days, but greater altitudes; more powerful light and heat, and more of it in duration.
The average temperature being nearly the same in different .years, the northern side of the earth must be receiving more than it parts with during a portion of the year, and parting with more than it receives during the remainder. The summer half of its year is that half during which it gains, on the whole, more than it parts with ; the surplus being that which is lost during the winter half. The clay in winch most heat is received is the longest day ; but it is notorious that the hottest weather is generally some time after the longest day. This is easily explained, as follows :—The time of the greatest heat is not that at which meet heat is received, but that at which the quantity of heat is the greatest, namely, just before the daily receipts of heat begin to fall abort of the daily expenditures. So long as the receipt exceeds the expenditure, heat is daily added to the hemisphere, and the weather becomes hotter. The same reason may be given for the greatest cold generally following the ehorteat day, with a considerable interval. AU these circumstances, however, depend much on the at mospheric circumstances of the year.
The preceding explanation does not serve for the tropical climates, the days and nights are here so nearly equal throughout the year, that seasons are caused more by the effect of the winds (which are very regular, and depend mainly on the sun's position) than by changes in the direct action of the sun's light and heat. The seasons are not a summer and a winter, so much as recurrences of wet and dry periods, two in each year.
With regard to the quantity of heat received in a day, it might be expressed, so far as it is not modified by the atmosphere, in a formula depending on the latitude of the place and the sun's declination. It will be enough to say that this fommla shows that, the sun being in the equator, the day's heat in different places is a8 the cosine of latitude ; and that for all places at the equator, the day's heat for different days is as the cosine of the eula'e declination.
The different distances at which the earth is from the sun, at dif ferent times of the year, do not affect the heat received in a given portion of the orbit. The sun is nearest to the earth in our mid winter, but for that reason the winter is shorter, since the earth moves more rapidly when nearer to the sun. The compensation is exact, for the quantity of heat received at any one moment, while the radius of the earth's orbit moves through any small angle, is greater or lees in the inverse proportion of the square of that radius. But the time of describing that angle is less or greater in the direct proportion of the same square. Consequently the heat actually received by the earth in the two halves of its orbit is the same in both.
SEAT (in a church). [Pew.] S E ItTII1N ESS. [San's.]