8/25/2023 0 Comments Summer equinox solar radiation![]() It is obvious that the highest amount of solar energy will be received by a collector that is perpendicular to the sun’s rays. To calculate the amount of energy reaching a surface, we not only need to know the position of the collector plate, but also its tilt – the angle that the collector makes with a horizontal plane. In southern hemispheres, the earth’s tilt is in the other direction and the seasons are reversed ( b). In the northern hemisphere, the angle of the earth’s rotation tilts toward the sun in the summer and away from it in the winter. Why then are summers warmer than winters?Īnswer: The earth’s rotational axis is not perpendicular to its orbit around the sun, but tilts by 23.5 degrees. Question: The data collected on the motion of the earth around the sun shows that the distance between the earth and the sun is slightly shorter in the winter than in summer (Figure 2). The earth’s (terrestrial) emission is in the infrared region of the spectrum and averages at around 172 W/m2. Like the sun, the earth is also emitting radiation, but at a much lower rate. Because the actual orbit of the earth around the sun is elliptic, the average distance varies by about 3.4% during the year and the solar constant varies between 1,300-1,400 W/m2. The solar constant is not truly “constant”, however. This amount is called the solar constant and represents the energy from the sun, per second, that falls on a surface perpendicular to the sun’s rays and just outside the earth’s atmosphere. Of all the radiation emitted by the sun, only a very small fraction is intercepted by the earth’s atmosphere. Furthermore, because of the higher angle of incidence in summer, more of the sunlight is reflected off the glass than would be during the winter. For example, every square meter of the window of a house in Long Beach receives, on average, 220 watts of solar energy per hour of daylight in the winter, whereas the same window receives 40% less energy in the summer. In the winter, the lower sun strikes the windows more directly than in the summer when the sun is higher. The south-facing windows in this house receive sunshine for only seven hours (8:30 am to 3:30 pm) during summer and receive the full ten hours of sunshine during the winter.Ģ) The summer sun is higher in the sky than the winter sun. For example, a house in Long Beach, California located at 33° 48’ latitude in the northern hemisphere experiences about fourteen hours of daylight (5:30 am to 7:30 pm) on Summer Solstice (June 21) and ten hours of daylight (7:30 am to 5:30 pm) on Winter Solstice (December 21). In winter, the opposite is true - sun rises and sets south of the window therefore, south-facing windows are subject to incident sunlight for the entire length of the solar day. Two factors contribute to this:ġ) Although the sun rises earlier in the summer, it remains north of the window before it sets in the afternoon. Unlike what some might suspect, the total solar energy incident on a south-facing window is greater during the winter than in the summer in spite of the fact that days are longer in the summer. ![]() The opposite is true in the southern hemisphere ( a). As winter proceeds into spring and summer, the sun rises earlier in the morning, moves higher in the sky, reaches its highest point at noon, and sets later in the afternoon (See Figure 1). It may also be noted that, in the northern hemisphere, the sun rises south of due east in winter and north of due east in summer. It is a common experience that the sun rises higher in the sky in summer than winter. Figure 2 Earth travels around the sun along an elliptical orbit about once every 365 days.
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