The variable

Integrated over the whole of its radiation spectrum, the sun emits about 74 million watts of electromagnetic energy per square metre. At the mean distance of the earth from the sun, the energy received from the sun at the outer limits of the earth's atmosphere, at right angles to the solar beam, is about 1353 watts per square metre (W m–2) and is known as the solar constant. In fact the energy received is not quite constant but varies over the year by about 3%, because the earth is in an orbit around the sun that is actually elliptical. The actual output of the sun itself also varies with time, the most familiar regular rhythm being the 11-year sunspot cycle, although the variations due to this are less than 0.1%. There are other, longer, cycles such as the 22-year double sunspot cycle, and the 80–90-year cycles (Burroughs 1994). It is useful to define some terms.

Units and terms

Radiant flux is the amount of electromagnetic energy emitted or received in unit time, usually expressed in watts (1 watt = 1 joule per second).

Radiant flux density is the radiant flux per unit area expressed in watts per square metre (W m–2), although other units such as mW cm–2 and cal cm–2 min–1 may be used (1 W m–2 = 0.1 mW cm–2 = 10–3 kW m–2 = 1.433 × 10–3 cal cm–2 min–1). No longer in wide use is a unit called the langley, which is equivalent to 1 cal cm–2.