text: Since the sun is a variable star, its solar output varies slightly over short time periods. Solar disturbances such as flares, prominences, and sunspots increase the amount of radiation from the sun. However, no long-term variations in the intensity of solar radiation have been measured at altitudes above Earth’s atmosphere. Solar radiation, at about 1,400 W/m2, varies by no more than 2%. In contrast, the geothermal heat flow from Earth averages only about 0.061 W/m2. Some theories have related climatic changes to sunspot cycles (11 years) and magnetic cycles (22 years), but no clear connection has yet been established between climate and sunspots or magnetic cycles. \n\nThe sun emits energy as electromagnetic radiation. Unlike sound, such radiation requires no medium, therefore this energy is able to travel through the near vacuum of space from the sun to the earth. \n\nMost radiant energy from the sun is concentrated in the visible and near-visible parts of the EM spectrum, and it peaks at about 500 nm, very near the 555 nm visibility peak for Homo sapiens, a result which surely is not a coincidence (and is connected to evolution). Less than 1% of solar radiation is emitted as X-rays, gamma rays, and radio waves. \n\nOnly about 25%, or about 350 W/m2, of incoming solar radiation penetrates the transparent atmosphere of the earth. The remainder is either absorbed by the atmosphere or scattered back into space. There are some latitudinal differences found on Earth. These variations are determined by time of year, by the wavelength of the energy being transmitted, and by the depth and nature of the intervening material. \n\nTerrestrial radiation is produced mainly as a result of reradiation of solar radiation. The energy is absorbed from the sun as the sun’s spectrum but reradiated from Earth at the much longer wavelengths associated with the temperature of Earth. The Wien displacement law, in which the wavelength is inversely proportional to the absolute temperature, provides a comparison of those peak wavelengths. \n\nSince the sun has a surface temperature 20 times greater than that of Earth, the reradiated energy has a spectrum that peaks at a wavelength 20 times longer than the 500 nm for the sun, or at about 10,000 nm, well into the infrared. Since the atmosphere is more absorptive to such long-wavelength terrestrial radiation, the atmosphere is heated from the ground up instead of vice versa. \n\nIn addition, water vapor and carbon dioxide absorb the long wavelengths of radiation from the earth especially well, leading to the so-called "greenhouse" effect. (The temperature in real greenhouses rises mainly because the glass prevents the heated air from rising, thereby increasing the temperature inside.) Ozone, on the other hand, absorbs only very short wavelengths, mainly in the ultraviolet range, and therefore forms a shield of sorts, absorbing much of the ultraviolet radiation before it can reach the earth. \n\nRadiation from the sun follows an inverse square law, so that comparisons of the solar constant can be made for other planets if we know their mean distances from the sun in AU. For example, Mars is 1.93 times further from the sun than is Earth. Its solar constant should therefore be about 27% that of Earth, about 374 W/m2. Because there is little atmosphere to absorb the energy, this is about the same as what we receive at the surface of Earth. \n\nSpacecraft have provided us with temperature measurements for both Mars and Venus. Mars has an average temperature of about 5 oC, whereas Venus has a temperature of about 500 oC. Earth is about 1.4 times further away from the sun than is Venus, and we would expect from the inverse square law that it would receive about twice as much radiation. The fact that its temperature is so much greater can be attributed only to greenhouse warming. \n\nWinter and summer seasons on Earth are a consequence not just of the obliquity factor associated with the sun’s r