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Himangshu Ranjan Ghosh

1RefAn introduction to solar radiation-Muhammad Iqbal

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Energy is produced in the core through nuclear fusion of hydrogen atomsinto helium

Temperature of the core is around 16 106 K

Temperature of Photosphere is around 6000K

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The sun revolves around the sun in anelliptical orbit with the sun at one foci.

The amount of solar radiation reachingthe earth is inversely proportional to thesquare of its distance from the sun.

The mean sun-earth distance r 0 iscalled 1AU

The minimum sun-earth distance isabout 0.983AU

The maximum sun-earth distance isabout 1.017AU

The sun-earth distance r for any day ofany year can be found with maximumerror of 0.0001

31Astronomical Unit (AU)=1.496108km

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According to Spencer

(r 0/r)2=1.000110+0.034221cos +0.001280 sin +0.000719cos2 +0.000077sin2

(r 0/r)2 is called eccentricity correction factor E 0

is called the day angle, in radians =2 (d n-1)/365

is the day number of the year, ranging from 1 on 1 January to 365 on31 December

According to Duffie and Beckman, a simple expression isE0=1+0.033cos[(2 dn/365)]

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RefJ W Spencer, Fourier series representation o the position of the Sun. Search 2(5), 172,1971J A Duffie and W A Beckman, Solar Engineering of Thermal Processes, Wiley New York, 1980

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Calculate the sun-earth distance on 8 January

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The angle between a line joining the centres of the sun and the earth to theequatorial plane changes every day, in fact every instant. This angle is calledthe solar declination

In 24h maximum change is 0.5degree

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According to Spencer

=(0.006918-0.399912cos +0.070257sin -0.006758cos2 +0.000907sin2 -0.002697cos3 +0.00148sin3 )(180/2 )

The equation estimates the declination with maximum error of 0.0006rad

Two other simple and commonly used formulas are (in degrees)

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Solar time is based on the rotation of the earth about its polar axisand on its revolution around the sun

The solar day varies in length through the year.

The two principal factors for this variance are- The earth sweeps out unequal areas on the ecliptic plane as it revolves around the sun The earths axis is tilted with respect to the ecliptic plane

If an observer facing the equator today sets a clock at 12 noon, whenthe sun is directly over the local meridian; then after a month or so

on at 12 noon, clock time, the sun may not appear exactly over thelocal meridian

A discrepancy of as much as 16minutes is possible. Thisdiscrepancy is called the equation of time

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According to Spencer

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In minutes. The maximumerror is about 35sec

Refhttp://en.wikipedia.org/wiki/Equation_of_time

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Solar radiation data are often recorded in terms of local apparent time (LAT),also called true solar time (TST)

Solar meteorological data such as temperature and wind speed are oftenrecorded in terms of clock time/local standard time/daylight saving time

Local apparent time=local mean time + equation of time=local standard time + longitude

correction + equation of time=local standard time+4(L s-Lc)+E t

The longitude correction is positive if the local meridian is east of the standardand is negative if west of the standard meridian

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Determine the solar time (LAT) on 8 Jan at Jessore (23 deg E, 89degN) when the standard time is 10:00.

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