The revolution of the earth around the sun causes the latter to appear to shift its position in the heavens from day to day. lu connection with the inclination of the earth's axis the revohition also causes the change of seasons. The earth travels around the sun in an elliptical orbit with the sun at one focus of the ellipse. At the nearest point (perihelion) it is about 91,500,000 miles distant; at the most remote point (aphelion) it is about 94,500,000 miles away.
The inclination of the earth's axis to that of the axis of the ecliptic or the earth's path around the sun is 23 degrees 27 minutes, which means that the plane of the earth 's equator is inclined at the same angle to that of the plane of the ecliptic. This inclination remains fixed (or nearly so) with reference to space and distant stars in the heavens, so that the axis of the earth with slight variations always points to the same star in the heavens, but it causes the earth to assume quite different positions with reference to the sun.
The Seasons - On December 21st the northern hemisphere reaches its maximum inclination from the sun, the vertical rays of the sun are on the Tropic of Capricorn, their southern limit, and it is then the winter solstice (Sun stands). The area around the north pole is in darkness, and it is winter in the northern hemisphere, while the area around the south pole is in continual sunshine and it is summer in the southern hemisphere.
The opposite condition prevails on June 21st, the summer solstice, wheni the northern hemisphere is inclined toward the sun and the rays are vertical on the Tropic of Cancer. It is then summer in the northern hemisphere and winter in the southern. On March 21st and September 23rd the axis of the earth is perpendicular to the line joining the center of the earth and the center of the sun and the sunshine extends from pole to pole when the days and nights are equal (the equinoxes). Name the corresponding seasons.
By consulting fig. 11 it may be seen that the winter in the northern hemisphere does not come when the earth is farthest from the sun, but when it is near perihelion, and the summer season when it is near aphelion, showing that the few degrees difference in the angle at which the sun's rays strike the earth have a greater influence on the temperature than the three millions of miles difference in distance.
The heat of summer and the cold of winter are increased by reason of long days and short nights in the summer and long nights and short days in the winter. It is thought that the space between the sun and the earth is exceedingly cold; and that the sun's rays or insolation are changed to heat only after entering the earth's atmosphere, and very little there until they strike the solid earth.
If the axis of the earth were perpendicular to the plane of the ecliptic what would be the effect on the seasons? What would be the effect on the winter and summer in New York State if the axis were inclined twice as much as at present? The axes of some of the other planets are inclined much more than that of the earth.
Related post : The Rotation of The Earth
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