The seasons are caused by a change in the length of daylight hours. As the seasons progress, the days get shorter or longer and noon sun changes its altitude in the sky. In summer the sun is high above the horizon as it makes its way across the sky. As winter approaches, the noon sun is lower in the sky and sunsets occur earlier each evening. The seasonal variation in the angle of the sun affects the amount of solar radiation the earth receives. When the sun is overhead, its rays pass through a thickness of one atmosphere. But when the rays enter at an angle of 30 degrees, they pass through a thickness of two atmospheres and at 5 degrees they pass through the equivalent of eleven atmospheres! The longer the path, the more change for scattering, absorption and reflection, which rob the sun's rays for their intensity. The annual changes in the sun's angle and the length of the day are created by the earth's angle of orientation toward the sun. The earth's axis is not perpendicular to the plane of its orbit around the sun. It is tilted 23 degrees from the perpendicular. Remember your globes? They are tilted on an angle and that angle approximates the earth's attitude toward the sun. The tilt is called the inclination of the axis, but that inclination wobbles a little. Right now the earth's axis is pointed at the North Star, Polaris. In a few thousand years it will be pointed at Vega, which will be the new North Star. That's because of that wobble and that wobble can affect the earth's orientation toward the sun and, as a result, its climate. On the day of the summer solstice (June Cool cycle fault), the north pole is inclined 23 degrees toward the sun. This is the beginning of summer in the northern hemisphere and the longest day of the year. In the southern hemisphere it's just the opposite - the beginning of winter and the shortest day of the year. Six months later on the day of the winter solstice (in the northern hemisphere), the north pole is inclined 23 degrees away from the sun. This is the beginning of winter, the shortest day of the year, the day in which the people in the northern hemisphere receive the least light and energy from the sun. The summer and winter solstices occur on opposite days in the southern hemisphere. The earth moves around the sun, but its orbit isn't exactly circular. Rather than a perfect circle, sometimes its orbit is oblong. This is called eccentricity of orbit and changes the distance from the earth to the sun at different times of the year. Currently the earth is closest to the sun in January and furthest away in July. However, we are in a period of low eccentricity - the earth's orbit around the sun is rounder - so the difference in the distance to the sun between January and July is slight, merely 3 percent. That variance doesn't have much effect on the amount of the sun's energy the earth receives. But when the eccentricity of the orbit is high, the difference can be as much as 20 percent and can have a pronounced affect on the atmosphere. |
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