Mars is in the vicinity of the aphelion of its orbit when it is spring in the northern hemisphere. At this time, the planet is farthest from the Sun, and hence it is traveling at its lowest speed. Because Mars is then moving relatively slowly in its orbit, the time between the spring equinox and the summer solstice is exceptionally long, 199 days. Summer in the northern hemisphere is also long, 182 days, because Mars is still in the vicinity of its aphelion. Conversely, during autumn and winter in the northern hemisphere, Mars is near its perihelion and its orbital speed is near the maximum. These seasons are consequently much shorter. The situation is, of course, reversed in the southern hemisphere.
The considerably longer spring and summer in the northern hemisphere of Mars might be expected to make the summers hotter than in the southern hemisphere. This is not so, however, because of the considerable difference in the distances from the Sun at perihelion and aphelion. The ratio of the amounts of solar heat radiation received by Mars at these two locations is inversely proportional to the square of the ratio of the respective distances. The square of the ratio of the distances is (206.66/249.22)2, and the inverse of this is found to be 1.44. Consequently, at perihelion Mars receives about 44 percent more radiation than at aphelion.
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