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Equation (10.2) can be rearranged to produce the orbital radius, and by substituting the higher and lower temperature conditions and suitable figures for the star’s effective temperature and radius, the inner and outer edges of the habitable zone can be found. In the case of the Sun, using an effective temperature T* = 5,778 K and a radius of 1.9 X 106 km places the inner edge of the habitable zone at 0.74 AU and the outer edge at 1.39 AU. Earth, at 1 AU, sits comfortably in the Sun’s habitable zone (phew!). Bearing in mind that this is a fairly crude estimate, we can say that Venus, at 0.72 AU, sits on the inner edge of the zone and Mars, at 1.5 AU sits in the outer edge. However, Venus is considerably hotter than the 373 K predicted by this method; ***spaceprobe measurements*** place the surface temperature at approximately 750 K. On Mars, the situation is reversed. The minimum surface temperature that the planet experiences is 133 K, although because of the eccentric orbit it can reach a maximum of 288 K (14˚ C) at certain times of its year – comparable with parts of the temperate regions of the Earth.