Define i. radiance, ii. solar constant as used in solar systems

i. Radiance is the radiant power per unit area per unit solid angle emitted by a surface. In the context of solar systems, radiance refers to the amount of radiant power, or light, emitted by the Sun per unit area of the Sun’s surface. This value is typically expressed in units of watts per square meter per steradian (W/m^2/sr).

ii. The solar constant is the amount of radiant power received from the Sun per unit area of the Earth’s surface at the mean distance of the Earth from the Sun. This value is approximately constant over time, although it can vary slightly due to factors such as the Earth’s orbit around the Sun and the Sun’s activity. The solar constant is typically expressed in units of watts per square meter (W/m^2).