Sun is the earth’s primary source of energy. Sun provides us extremely large amount of energy in the form of both heat and light. The energy given out by sun is called solar energy. This enormous amount of energy is produced in sun by the nuclear fusion reaction taking place in it. In nuclear fusion reaction two nuclei of heavy isotope of hydrogen (deuterium 1H2) atoms fuse together to produce a heavy nucleus of helium (2He4) molecule. During this process a large amount of energy is liberated in the form of heat and light.
1H2 + 1H2 
2He4 + Energy
All of this energy produced by sun does not reach on the earth. The amount of energy which reaches the surface of earth is only 47% because the remaining part of solar energy is either absorbed or reflected back into the space by the atmosphere. It is estimated that one square meter area of earth’s atmosphere receives about 1.4 kilojoules energy per second. This value is constant and is known as solar constant. Thus,
Solar constant = 1.4 kilojoules energy per second
Test Your Understanding and Answer These Questions:
- What is solar energy?
- What is solar constant?
- What is value of solar constant?