Define solar constant.

Answer

### Step-by-Step Solution:

1. **Definition of Solar Constant**: The solar constant is defined as the amount of solar energy received per unit area at a distance of one astronomical unit (the average distance from the Earth to the Sun) in the absence of the Earth's atmosphere.

2. **Measurement Units**: The solar constant is typically measured in terms of energy per unit area. The common units used are calories per square centimeter per minute (cal/cm²/min) or watts per square meter (W/m²).

3. **Orientation of the Surface**: The surface receiving this energy must be oriented perpendicular (normally) to the incoming solar rays. This means that the angle of incidence of the sunlight on the surface is 90 degrees.

4. **Mean Distance from the Sun**: The measurement of the solar constant is taken at the mean distance of the Earth from the Sun, which is approximately 149.6 million kilometers.

5. **Value of Solar Constant**: The value of the solar constant is approximately 1.937 calories per square centimeter per minute (cal/cm²/min) or about 1361 watts per square meter (W/m²).