|MadSci Network: Earth Sciences|
Hi Marino, Your intuition is right on. The Sun will always be at its highest above the horizon for that day and that latitude at local noon anywhere in the world, except where timekeeping conventions arbitrarily cause noon to occur at some other time. But... This is not to say that the Sun will be directly overhead, which for most of the world never actually happens because the Earth is tilted on its axis. The altitude of the Sun over the southern horizon, at noon, is caused by the position of the Earth in its orbit around the Sun, and is the cause of the seasons. The lines of latitude farthest from the equator at which the Sun can appear directly overhead are the Tropic of Cancer (about 23 degrees N) and the Tropic of Capricorn (about 23 degrees S). This occurs at noon on the solstices, Northern Hemisphere Summer and Southern Hemisphere Summer solstices, respectively. North or south of these parallels the sun can never appear directly overhead. The Sun can appear directly overhead at the geographic equator at noon on two days a year, the vernal and autumnal equinoxes. According to tradition, Eratosthenes used this observation to make a fairly accurate estimate of the diameter of the Earth (1). Also, navigators have used this principle for centuries to estimate their latitude at sea using a sextant, the date of the year, and astronomical tables. Calculating their longitude was much harder because they had to develop accurate timekeeping at sea, but was done by measuring the time local noon occured and comparing to the time of local noon at the prime meridian. Hope this helps. --Gene Marlin (1) MadSci Archive http://www.madsci.org/posts/archives/sep99/937394188.As.r.html
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