2.5: Application to the Earth-Sun System

In the prior section, the discussion turned rather technical. We went down a rabbit hole of radiation wavelengths, total emitted power per area, and the assumption of black bodies. We did this so that we could understand basic relationships about the energy balance in the Earth-Sun system. The Sun’s average temperature is above 5,000 K while the Earth’s average temperature is in the range 210-310 K (we will discuss this further in a later chapter). This means that the Sun and Earth radiate energy very differently.

The Sun emits solar radiation, also known as ultraviolet radiation or shortwave radiation. The Earth emits infrared radiation or longwave radiation. This follows directly from the electromagnetic energy spectrum and the respective temperatures of the Sun and Earth. The Sun emits radiation at a shorter wavelength than the Earth because it has a higher temperature, and Planck’s curve for higher temperatures peaks at shorter wavelengths. It is for this reason that Earth’s radiation is referred to as longwave, and the Sun’s radiation is called shortwave.

We learned that a black body absorbs all incoming radiation and emits the maximum possible radiation given its temperature. In the Earth-Sun system, this means that the Earth absorbs all incoming radiation from the Sun, and emits the maximum amount given its temperature. In practice it is a little bit more complicated than this.

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