Measure the temperature of surface of the sun.

All bodies emit and absorb radiation at all temperatures. A body hotter than its surroundings loses more energy by radiation than it absorbs while a cooler body absorbs more energy than it radiates.

A special kind of a body is the one which absorbs all kinds of radiations. This special body is called a black body.

Also, the radiation emitted by a black body comprises of radiation of all wavelengths. The wavelength at which maximum energy per unit time is emitted has a special relationship with the temperature of the blackbody. The product of wavelength and the temperature of the body is a constant. The equation is

λ_maxT=b

where,
λ_max is the wavelength corresponding to maximum intensity (energy emmited per unit time per unit area),
T is the temperature of the black body,
b is a constant with a value of 2.898 * 10^-3 m°K

This is called the Wien's displacement law.

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