Astronomical Unit density

Some simple rearrangement of Equation 3.1 leads to the concepts of transmission T = Io/1 and absorbance A = — log T, with the quantity s c l called the optical density. The choice of units here for the extinction coefficient (M-1 cm-1) is appropriate for measurement of the absorbance of a solution in the laboratory but not so appropriate for a distance Z of astronomical proportions. The two terms and c are contracted to form the absorption per centimetre, a, or, more conveniently (confusingly) in astronomy, per parsec. The intrinsic ability of a molecule or atom to absorb light is described by the extinction coefficient s, and this can be calculated directly from the wavefunction using quantum mechanics, although the calculation is hard. 

Planetary radio astronomers estimate the radio power emitted by the planets by measuring with a radio telescope the power flux density received at the Earth. Figure 2 illustrates the geometry involved in the measurement of power from an ideal blackbody radiator. The spectral power (per unit frequency) emitted by an elemental surface element of the blackbody of area dA into a solid angle dQ is given by B cos(0) dQ dA, where 0 is the angle between the normal to the surface and the direction of the solid angle dQ. The total power (per unit frequency interval) radiated by... 

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