Although the sun is 93 million miles away, we can tell what the temperature is on its surface because we can measure the frequencies of the light waves it sends us.
Things at high temperatures generate a lot of light of very high frequency (or short wavelength); the frequencies sent out by the sun are the “signature” of an object at about 11,000 degrees Fahrenheit.
Light consists of the regular motion of magnetic fields caused by the vibration of the negatively charged electrons in a substance such as the sun, or, for that matter, a light bulb or a chair.
The electrons vibrate because they are being excited by heat; to say that something is hot means that its molecules are colliding very frequently. The hotter the object, the faster its electrons are forced to vibrate by the collisions of the molecules containing them. The magnetic fields made by the electrons move back and forth very frequently, producing a large number of wave fluctuations in each second (cycles per second or cps).
The hotter the object, the higher the peak frequency of the energy it emits. Since light always travels at the same speed of 186,000 miles per second, the higher the frequency, the shorter the wave.
You can see the relation between heat and light frequency in everyday life: a poker left in a fire turns from black to dull red to bright orange as it heats up; the fire itself is even hotter and makes a higher frequency “yellow” light. Burning stove gas is hotter still, giving off a peak of “blue” light from the high frequency end of the visible spectrum.
The solar temperature is estimated by comparing the light given off by flames on Earth with the light of the sun. Sending light through a glass prism makes a long band or spectrum of light that looks different depending on the elements in the sample and their temperature: hydrogen, helium, and traces of other elements at a man made 11,000 degrees Fahrenheit make the same spectral pattern as sunlight. Thus have we concluded that that is the composition and temperature of the sun.
The latest and best estimates of the sun’s surface temperature have come from spectral readings taken by satellites and spacecraft, which can measure the sun’s energy output from above the atmosphere, avoiding any confusion caused by the atmosphere’s filtering effect.
Gases in the atmosphere absorb many of the lower and higher frequencies before they reach us on Earth; this is fortunate for us, since life probably could not have developed if it were constantly subjected to a deadly fusillade of cosmic, X, and gamma rays.
It is no coincidence that the sun’s peak frequency, the frequency at which something sends out the most energy, is the span of wavelengths we call visible light. Human eyes evolved under the light of the sun, and the most efficient way to collect the most information about the world is to “see” as much energy as possible bounding off it.
The most direct way for an evolving organism to do this is to develop a sensitivity to the most abundant frequency.