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| Solar Constant (S) |
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| The solar constant is defined as the
amount of heat energy received per second per unit area by a perfect black
body placed at the surface of the Earth with its surface being held
perpendicular to the direction of the sun's rays. |
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| The value of solar constant is 1388 Wm-2 or 2 cal cm-2 min-1. |
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| The instrument used for measuring the solar constant is called Pyroheliometer. The simplest apparatus used for this purpose, Angstrom's compensation pyroheliometer, is described below. |
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| The apparatus consists of two identical blackened strips S1 and S2. These are made of manganin or constantan. These are mounted side by side inside a tube. Either of the two is exposed to solar radiation and the other is completely shielded by a screen. |
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| A copper-constantan thermocouple is used between S1 and S2. |
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| When sun's rays fall on S1, its temperature rises. The temperature of the junction (of the thermocouple) near S1 increases. The galvanometer gives deflection. Now, current is passed through S2, so that temperature of S2 becomes equal to S1. The galvanometer now gives 'no deflection'. Hence, we can conclude that the heat energy received by S1 from sun is equal to the heat energy supplied electrically to S2. |
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| Heat energy absorbed |
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| per second by S1 from the sun = a A S -----(i) |
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| where a is the absorptive power of S1, A is the area of the exposed surface and S is the solar constant. |
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| Heat energy supplied per second to S2, by electrical means |
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| Equating (i) and (ii), we get |
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 or  |
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| All the quantities on the right hand side are known. So, S can be calculated. |
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| We know that energy falling in one second on the unit area of the earth's surface held normal to sun's rays is called solar constant S. Experimentally, S has been found to be equal to 1388 Wm-2. Let R be the radius of the sun and r be the radius of earth's orbit around the sun. Let E be the energy emitted by the sun per second per unit area. Then, the total energy emitted by the sun is one second =4pR2x E. This energy is falling on a sphere of radius equal to the radius of the Earth's orbit around the sun i.e., on a sphere of surface area
4pr2. |
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| By definition, this is the solar constant S |
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| Now S = 1388 Wm-2, R = 6.96 x 108 m, r = 1.496 x 1011m, |
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| On substituting these values above, we get T, the surface temperature of the sun. It is found to be equal to 5791 K. In this way, the surface temperature of sun can be estimated. |
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