The peak wavelength of radiation emitted from an object is dependent upon the temperature of the object and can be calculated using the Wien Displacement Law when the temperature of the object is known. The temperature of the object radiating energy Using this law, the peak wavelength of radiation emitted from an object is inversely proportional to the temperature of the object. The irradiance or radiation output of an object can be calculated using the Stefan-Boltzman Law when the temperature is known. Emissivity numbers range from 0 to 1. Very black objects such as charcoal have an emissivity near 1 while shiny objects have an emissivity near 0.
Black bodies are capable of absorbing and emitting radiation at all wavelengths.
The fact that the Sun is not a perfect black body is especially important when studying solar cycles. Once the surface irradiance of the Sun is determined the amount of energy reaching the top of the Earth's atmosphere can be calculated using the Inverse Square Law. The average amount of energy received on a surface perpendicular to incoming radiation at the top of the atmosphere is the solar constant. The Inverse Square Law is used to calculate the decrease in radiation intensity due to an increase in distance from the radiation source. Satellite irradiance data is available from 1978 to the present on the internet.