|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|||||||
|
|
|
|
|
|
|
|
|
||
Determining the Temperature of a Planet
--1st
Approximation
Let's now consider the factors determining the temperature of a
planet. The sun has a temperature of about 6,000 K and is located 149
million km (93 million miles) from the earth. The flux density of
solar energy reaching the earth from the sun is 1367 Watts/m2 or 430
Btu/hr/ft2, which we refer to as the solar constant, S (although closer
inspection has revealed that it is, in fact, not constant). If the
earth absorbed all this energy over its disk area (π R2)
and
re-radiated it back to outer space as a sphere (of area 4
π R2), the
energy balance given by the Stefan-Boltzmann
law gives us an estimate
of the effective radiating temperature of the Earth:
--> (π R2) S =σ T4 (4 π R2)
--> T = [(0.25*S)/( 4 σ)]1/4
= 279 K, or 6 °C (40° F)