A GREAT DEAL OF the variation that exists in the Earth's physical phenomena is caused by the revolution and the ROTATION of the earth. The rotation of the Earth on its axis every 24 hours and the revolution of the earth around the sun every 365 days regulate the amount of electromagnetic energy received by the Earth's surface. Another factor which influences the amount of energy upon the earth's surface is the tilt of the earth's axis. The tile of the Earth's axis in conjunction with the earth's revolution about the sun results in seasonal and diurnal variations. These variations affect the circle of illumination and duration of daylight and darkness at different points on the globe.
These relationships result in the seasons: summer, winter, spring, and fall. The seasons reflect a change in temperature throughout the year: a full rotation around the sun. Temperatures reach their extremes in summer, the hottest months, and winter, the coldest months. Fall and spring temperatures fall somewhere in between these extremes of hot and cold temperatures. The seasons repeat themselves every year fairly regularly.
The seasons are due to the movement of the earth around the sun and the tilt of the Earth's axis. The Earth's orbit around the sun is an ellipse, so some times during the year the Earth is closer to the sun than at other times. However, the change in distance from the sun during the Earth's rotation is relatively small and does not account for seasonal changes in temperature. The tilt of the Earth's axis, 23.5 degrees with respect to the plane of the perpendicular of the Earth's orbit, is the primary cause of the seasons. As the earth circles the sun, the orientation of the axis remains the same. This means that as the Earth orbits the sun the Northern Hemisphere is at various times of the year oriented more toward the sun, and this is similarly true for the Southern Hemisphere. Summer occurs in the Northern Hemisphere when the Earth's Northern Hemisphere is oriented toward the sun. The sun is higher in the sky and is above the horizon longer, and the rays of the sun strike the ground more directly. This causes longer daylight hours and increased temperatures. Conversely, winter in the Northern Hemisphere occurs when the Earth's Northern Hemisphere is oriented away from the sun. The sun is lower in the sky and is above the horizon for a shorter period, and the rays of the sun strike more obliquely. This results in shorter days and cooler temperatures.
The duration of daylight and darkness vary seasonally everywhere except at the equator, which is always cut exactly in half by the circle of illumination. The inclination of the Earth's axis affects the angle of the sun above the horizon. The angle of incoming solar radiation is dependent upon the declination of the sun— how far north or south of the equator the sun's rays strike during a given time of the year.
The most direct and intense solar radiation occurs between the TROPIC OF CAPRICORN (23.5 degrees south) and the TROPIC OF CANCER (23.5 degrees north). The winter solstice is the day when the noon sun is directly overhead at the Tropic of Capricorn. In the Northern Hemisphere, this is the shortest day of the year. The summer solstice occurs when the noon sun is directly overhead at the Tropic of Cancer. In the Northern Hemisphere, this is the longest day of the year. Midway between the solstices—approximately March 21 and September 21—the noon sun is directly above the equator. At these two times of the year, the periods of daylight and darkness are of equal duration. The vernal equinox occurs in March; the autumnal equinox occurs in September. The exact dates vary according to the astronomical relationships between the Earth and sun.