 To calculate the temperature of the sun from Earth, we must first find out what light the sun emits. The sun emits a polychromatic light that is infrared light, visible light, and even x-rays and gamma rays. You need to figure out the wavelengths lambda of each of these radiations and with what intensity they are emitted. If we plot all wavelengths with their intensities in a diagram, we get a spectral intensity distribution of the sun. From this distribution, we have to read off the wavelength lambda max, which has the highest intensity. So we have to read off the maximum of the function. Then you can calculate the temperature of the sun with the Wien's displacement law. To do this, we divide 2897.8 times 10 to the power of minus 6 meter Kelvin by lambda max. In the case of the sun, the maximum is within the blue light spectrum at the wavelength 490 nanometers, or 490 times 10 to the power of minus 9 meters. This results in a temperature of 5,900 Kelvin or 5,630 degrees Celsius.