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 EDG Ultimate Member oh, crumbs!!! Posts: 611 Gender: Re: Habitability Reply #2 - 11/02/10 at 11:34:59   I don't think you need axial tilt in the equation there, I've never seen it in this equation at least (I guess that calculates the base temperature at the subsolar latitude on the planet?). Here's something I wrote on the SFRPG forums about this:     The equation below shows how to determine the surface temperature of a planet:     Code:```Ts = 278.66 * { [(4th root of L) * (4th root of 1-A)] / (square root of D) } * GFX ```   where L is the Luminosity of the star in Sols, D is the distance of the planet from the star in AU, A is the planet's Bond Albedo, and GFX is a Greenhouse Effect factor.   The 278.66 is a simplified constant that comes from the original (more complicated) equation. It's essentially a conversion factor since in the equation we use here we turn the distances and luminosities into AU and Sols instead of metres and watts. If anyone is curious it is (4th root of 1/16 * pi *sigma) * [(4th root of 3.846e26) / (SQRT(1.49598e11)] where sigma = stefan-boltzmann constant (5.87e-8), 3.846e26 = luminosity of the sun in watts, 1.49598e11 = distance between Earth and sun (astronomical unit) in metres.     The Bond Albedo tells you the fraction of radiation that is reflected by the planet, so (1-Albedo) is the fraction of radiation that is absorbed (and therefore heating the planet and what we're interested in).  A Blackbody is an idealised perfect absorber of radiation (A=0). A perfect reflector (A=1) would have a temperature of Absolute Zero (0 K) since it's not absorbing anything at all, but nothing natural would perfectly reflect all radiation - the closest we know to that is Saturn's cryovolcanic moon Enceladus, which has a Bond albedo of 0.99 due to all that fresh ice on it (it absorbs only 1% of the incoming radiation). Earth's Bond Albedo is 0.309, Venus' is 0.9 (most incoming radiation is reflected by its clouds. But even the 10% that gets through is enough to raise its temperature significantly due to the atmosphere's massive greenhouse effect).   The GFX is a multiplier that increases the surface temperature of the planet. Essentiually, it's a fudge factor and is unfortunately almost impossible to actually calculate, and it just has to be guessed based on the atmosphere type and thickness. Its mininum value is 1.0, which corresponds to a planet with no atmosphere at all (with no atmosphere, the temperature doesn't increase). Earth's greenhouse factor is about 1.13, while Venus' is around 3.0.     There are three types of "temperature" that we can get from this equation, however:   Tb (Blackbody Temperature): If we set A=0 and GFX=1, this gives us the Blackbody Temperature - the temperature of a perfect absorber at that distance from the sun. This effectively removes the A and GFX terms, so we get:   Code:```Tb = 278.66 * [ (4th root of L)  / (square root of D) ] ```     Te (Effective Temperature): If we set GFX=1 and A=whatever the albedo of the planet is, we get the temperature of the planet's surface without the effect of an atmosphere, which is known as the Effective Temperature (this isn't really used much in practice):     Code:```Te = 278.66 * { [(4th root of L) * (4th root of 1-A)] / (square root of D) } ```     Ts (Surface Temperature): If we use the planet's actual values for A and GFX, we get the actual average Surface Temperature of the planet. For worlds with no atmosphere, Te=Ts.   Code:```Ts = 278.66 * { [(4th root of L) * (4th root of 1-A)] / (square root of D) } * GFX ``` Back to top (formerly known as Mal)   IP Logged