A satellite-based spectrometer can not only measure the spectrum of the earthshine but can also obtain a spectrum of the incident radiation, the solar irradiance. Because it contains no absorptions by the Earth's atmosphere, the solar irradiance spectrum can serve as a reference against which the earthshine spectrum can be compared.

Fig 3.1.1: Solar irradiance and earthshine.

Image courtesy of: NASA Johnson Space Center - Earth Sciences and Image Analysis (NASA-JSC-ES&IA)

This section discusses two aspects of the solar irradiance spectrum:

• Measurement
  How and when the solar irradiance spectrum is measured.
• Comparison
  A comparison of some features of the spectra of solar irradiance and earthshine.