Learn it: The Solar Constant (also called Solar Irradiance)
The sun radiates energy in all directions, but only a very small fraction of this energy falls on the earth. The amount reaching a particular point on the earth's atmosphere depends on the position of the earth relative to the sun.
The solar constant is the amount of energy that normally falls on a unit area (1 m2) of the earth's atmosphere per second when the earth is at its mean distance from the sun.
The value of the solar constant is found experimentally to be 1.35 kW m-2. The proportion of this reaching a particular point on the surface of the earth depends on weather conditions and the time of year. In Ireland the annual average value is approximately 115 W m-2.
Virtually all of the energy "used" on earth comes – or came – from the sun, most of it being accumulated over long periods, e.g. in coal and oil.
Energy from the sun may also be collected directly. In one form of solar heating, radiation from the sun falls on panels containing a network of pipes through which water is pumped.
Each time the water passes through the panel it becomes warmer. In this way the radiant energy from the sun is converted into internal energy in the water.
Solar panels can also be made of photocells that convert the radiant energy directly to electrical energy. Panels such as these are used to power orbiting spacecraft.
Solar panels are usually covered with glass to increase the efficiency with which they absorb the incident radiation. Most of the radiation reaching the earth's surface has wavelengths in the visible and near (short wavelength) infrared regions of the spectrum.
Ordinary glass transmits these wavelengths but absorbs longer (and shorter) wavelengths. Solar radiation passes through the glass and is absorbed by the solar panels.
The panels re-radiate some of this energy - but at much longer wavelengths, as their temperature is relatively low. The glass is opaque to these wavelengths, so the energy is effectively trapped. This is known as the "greenhouse effect", as it is the principle on which greenhouses operate.
Next: Test Yourself Crossword