Question

critically analyse the effect of the curvature of the Earth on the amount of solar energy received by Earth

Answer 1

The solar constant (ISC) is the average radiation intensity falling on an imaginary surface, perpendicular to the Sun’s rays and at the edge of the Earth’s atmosphere (figure 2.1). The word ‘constant’ is a little misleading since, because of the Earth’s elliptical orbit the intensity of the solar radiation falling on the Earth changes by about 7% between January 1st, when the Earth is nearest the Sun, and July 3rd, when the Earth is furthest from the Sun (figure 1.2). A yearly average value is thus taken and the solar constant equals 1367 W/m2. Even this value is inaccurate since the output of the sun changes by about ±0.25% due to Sun spot cycles.

The solar radiation intensity falling on a surface is called irradiance or insolation and is measured in W/m2 or kW/m2. The solar constant can be used to calculate the irradiance incident on a surface perpendicular to the Sun’s rays outside and the Earth’s atmosphere (figure 2.1) on any day of the year (i.e. as the distance between the Sun and Earth changes thought the year):