Seasons

Seasons. If the earth were to rotate about an axis perpendicular to the plane of her path round the sun, at any position which she might occupy the whole of one hemisphere stretching from pole to pole would remain for twelve hours under the sun's inflnence, and would be turned away to the darkness of space for the other twelve hours. Every day would be exactly like every other day; the equator would always be the hottest portion of the earth, since it would always have the sun vertically overhead; and the poles would always be the coldest spots, as they would receive the most oblique rays. But the axis of the earth's rotation is not perpendicular to the plane of her orbit (the ecliptic), but makes an angle with it of 66-1/2° and remains pointing in the same direction, or parallel to itself, all through the year. The above diagram may be taken to illustrate the position of the earth on March 21st, June 21st, September 23rd, and December 21st. In position 1 the radius S C of the earth's orbit is perpendicular to the axis, N S. Light from the sun then falls vertically upon the equator, e e, and every spot on the earth enjoys equal day and night. This is the vernal equinox. In position 2 the line S C is perpendicular not to n s but to a b, a line inclined at an angle of 23-1/2° to n s, and the sun is not vertically above the equator, but is over c c, the Tropic of Cancer, a circle parallel to the equator and 23-1/2° north of it. Every point in the northern hemisphere is now having a day more than twelve hours long, is receiving more heat from the sun, and the summer season is in progress. If a circle, a a', be drawn 23-1/2° from the pole n, it will be seen that no point north of this circle is ever in darkness, since every point rotates about the line n s. At the summer solstice, therefore, there is perpetua1 day within the arctic circle, as a a' is called. It is to be remembered that the sun is so far away that his rays light up a complete hemisphere, the external rays touching the earth round the circle a b. In position 3 the sun is again vertical to the equator, equal day and night prevail everywhere, and the earth is at the autumnal equinox. Position 4 represents the earth at the winter solstice, when we have the conditions of 2 reversed. What was then true of the northern hemisphere is now true of the southern, and vice versa. S C is perpendicular to a b, but every point in the arctic circle is now in permanenent darkness, while in the symmetrically-placed antarctic circle, b b, there is perpetual day. The sun is vertical to d d, the Tropic of Capricorn, 23-1/2° south of the equator. In the northern hemisphere, then, the rays are as oblique as possible, the least amount of heat is received, the nights are longest, and it is winter. It might be thonght that since the sun reaches its most northern limit on June 21st, the longest day, that that day would also be the hottest; but this is not so. For some time after this day the earth (in the northern hemisphere) is receiving more heat during the day than it loses during the night; there is, therefore, a gain of heat during each twenty-four hours, although the gain itself gets gradually less. Hence it is hotter in the months of July and August than in June. It is to be remembered also that the earth moves, not in a circle, but in an ellipse with the sun in one focus; at one part of the year therefore the earth is nearer the sun (in perihelion) and is moving faster, while at another part it is in aphelion and is moving most slowly. The latter occurs during our summer, so that this is longer to the extent of about eight days than our winter. Being then farther away from the sun, summer in the northern hemisphere is less extreme than in the southern. This tends to make both summer and winter more temperate in the northern than in the southern half of the world.