ay45c3-page28 - transmission Optical window 1 Atmosphere is...

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Unformatted text preview: transmission Optical window 1 Atmosphere is opaque 0 0.1 nm 1 nm 2.2 µ 3 - 5µ 8 - 12 µ as that of sunlight. Long-exposure color pictures by moonlight appear in normal colors, despite the inability of your eye's cones to function at moonlight levels. Brightness Temperature. The Planck spectrum is monotonic with temperature at each and every frequency  (see previous gure). Therefore a measurement of I at any single frequency  determines a temperature, called the \brightness temperature." Of course, identifying this temperature with a real physical temperature of the emitting object depends on knowing the the rest of the spectrum also ts a black body shape. The Planck spectrum by de nition has the same brightness temperature at all frequencies. The brightness temperature of the night sky is  200 300 K where the atmosphere is opaque, and as low as 3 K (cosmic microwave background temperature) in atmospheric \windows" of transparency. Radio window Atmosphere is opaque 10 nm 100 nm 1 µm 10 µm 100 µm Atmosphere is opaque 1 cm 10 cm 1m 10 m 100 m wavelength Note the \infrared windows" at 2:2; 3 5; 8 12, as well as the large optical and radio windows. UV and X-ray astronomy can only be done from space. 3.5.6 Radiative temperature balance of the Earth In the most naive model, the Earth maintains that temperature at which it radiates (in the infrared) exactly the same average power as it receives from the Sun. Note that the Sun illuminates R2 of projected Earth area, while  52 ...
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