If the earth did not possess an atmosphere, or if this atmosphere were perfectly diathermal—_i.e._, pervious to heat radiations—we should be able to calculate the mean temperature of the earth’s surface, given the intensity of the solar radiation, from Stefan’s law of the dependence of heat radiation on its temperature. Starting from the not improbable assumption that, at a mean distance of the earth from the sun, the solar rays would send 2.5 gramme-calories per minute to a body of cross section of 1 sq. centimetre at right angles to the rays of the sun, Christiansen has calculated the mean temperatures of the surfaces of the various planets. The following table gives his figures, and also the mean distances of the planets from the sun, in units of the mean distance of the earth from the sun, 149.5 million km. (nearly 93 million miles): ─────────┬─────────┬───────────┬──────────┬───────────────┬────────── │ Radius │ Mass │ Mean │ Mean │ Density Planet ├─────────┴───────────┤ distance │ temperature │ according │ According to See │ │ │ to See ─────────┼─────────┬───────────┼──────────┼───────────────┼────────── Mercury │ 0.341 │ 0.0224 │ 0.39 │ + 178°(332°) │ 0.564 Venus │ 0.955 │ 0.815 │ 0.72 │ + 65° │ 0.936 Earth │ 1 │ 1 │ 1 │ + 6.5° │ 1 Moon │ 0.273 │ 0.01228│ 1 │ + 6.5°(105°)│ 0.604 Mars │ 0.53 │ 0.1077 │ 1.52 │ - 37° │ 0.729 Jupiter │ 11.13 │ 317.7 │ 5.2 │ - 147° │ 0.230 Saturn │ 9.35 │ 95.1 │ 9.55 │ - 180° │ 0.116 Uranus │ 3.35 │ 14.6 │ 19.22 │ - 207° │ 0.388 Neptune │ 3.43 │ 17.2 │ 30.12 │ - 221° │ 0.429 Sun │ 109.1 │ 332,750 │ 0 │ +6200° │ 0.256 ─────────┴─────────┴───────────┴──────────┴───────────────┴──────────