Solar constant
Topic: Solar Radiation, Heat Balance and Temperature · NCERT: Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"
Meaning
The solar constant is the amount of solar energy that falls on a surface facing the Sun at the top of the atmosphere, with the Earth at its average distance from the Sun. NCERT gives it as 1.94 calories per sq cm per minute, which is about 1,353 W/sq m. The modern satellite value is about 1,361 W/sq m (2011). Despite its name, it is not truly constant. The Sun's output changes by about 0.1% over the 11-year sunspot cycle (sunspots are dark patches on the Sun).
Example
The Earth is round and turns through day and night. So the energy spread over the whole globe averages only about 340 W/sq m, which is one quarter of the solar constant.
Don't confuse with
- Insolation at the top of the atmosphere: this is the energy the Earth actually gets at a given time. It changes by about 7% between perihelion (the Earth's nearest point to the Sun, about 3 January) and aphelion (the farthest point, about 4 July). The solar constant is fixed at the average distance.