What is the extraterrestrial constant?
The extraterrestrial constant is the total amount of sunlight arriving at the top of the atmosphere. The extraterrestrial constant is often represented as Io in Sun photometer formulas. A Sun photometer at the top of the atmosphere would not need to observe the Sun with a narrow FIELD OF VIEW since outside the atmosphere there are no air molecules or aerosols to scatter sunlight. Since Sun photometers usually measure discrete colors or wavelengths of sunlight, the extraterrestrial constant they measure is that of the specific color or wavelength and not all the light emitted by the Sun. You can use the LANGLEY METHOD to measure the extraterrestrial constant for a Sun photometer such as the VHS-1.
