Axial tilt, or the tilt of the Earth's axis, is the second of the Milanko-vitch cycles. This is the inclination of the Earth's axis in relation to its plane of orbit around the Sun. If the Earth were not tilted, there would be no seasons.
The seasons are created by the change in length of daylight hours. As the seasons progress, the daylight hours get shorter (in winter) or longer (in summer) and the noon Sun changes its altitude in the sky (high altitude in the summer, low in the winter). Because the angle of the Sun changes, it affects the amount of solar energy the Earth receives. When the Sun is directly overhead (summer months) it only has to pass through the thickness of one atmosphere. But when the rays enter at a lower angle (such as 20 degrees), they must pass through two atmospheres. If the Sun is only 5 degrees above the horizon (very low), it is the equivalent of having to pass through 11 atmospheres, meaning that very little solar energy reaches the Earth's surface. The more atmosphere the energy must pass through, the more likely it is that it will be scattered, reflected, and absorbed before reaching the Earth's surface.
The changes in the Sun's angle and the length of day are a direct function of the Earth's axial tilt. Currently, it is tilted 23.5 degrees from the perpendicular. The tilt is referred to as the inclination. On the summer solstice (June 21), the North Pole is inclined 23.5 degrees toward the Sun. This is the Northern Hemisphere's first day of summer (the Northern Hemisphere receives the most light and energy from the Sun) and the Southern Hemisphere's first day of winter. Six months later on the winter solstice (December 22), the North Pole is inclined 23.5 degrees away from the Sun. This is the Northern Hemi sphere's first day of winter (the Northern Hemisphere receives the least light and energy from the Sun) and the Southern Hemisphere's first day of summer.
The Earth's tilt does not remain constant at 23.5 degrees. It can vary from 21.5 to 24.5 degrees. When the tilt is less (toward 22.5 degrees), the seasons will vary less. Remember, if there were no tilt to the axis, there would be no seasons at all. Less of a tilt causes the Sun's radiation to be more evenly distributed between summer and winter. It also increases the difference in the amount of radiation reaching the equator versus the poles. Scientists propose that when the Earth's axial tilt is less, it could change the climate and promote the growth of ice sheets. They reason that winters would be warmer, allowing the air to hold more moisture, which would produce more snowfall. Summer temperatures would be cooler, so less of the increased winter snowfall would melt. Conversely, when the Earth's axial tilt moves in the other direction and reaches the 24.5 degree range, the opposite conditions would apply. The oscillations in the Earth's axial tilt occur on a periodicity of 41,000 years from 21.5 to 24.5 degrees.
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