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Lecture 2

Lecture 2 - Archaeoastronomy and the Sun (continued), Historical Astronomy and the Cycle of the Stars.docx

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Natural Science
NATS 1745
Robin Metcalfe

Lecture 2 –Archaeoastronomy (continued), HistoricalAstronomy and the Cycle of the Stars Thursday, January 09, 2014 1. Archaeoastronomy and the Sun (continued) - How could a prehistoric person have tracked the equinoxes/solstices? All they would have to do is watch the Sun’s motion every day for the duration of at least a year. Therefore, the fact that prehistoric people could track the Sun had no bearing on their ability, or lack of ability, to comprehend the universe - How do we explain the occurrence of the seasons?  The occurrence of the seasons can be explained by the tilt of the Earth (1) The Sun’s Daily Cycle - the direction where the Sun rises (east) and sets (west) is determined by the nature of Earth’s rotation -1) Due to the Earth’s 24-hour eastward spin around its polar axis, the Sun moves westward across the sky; it rises in the east, reaches its highest point at noon, then sets in the west -2) Northern hemisphere: Sun is mostly seen on the South side of the sky, reaching due South at noon -3) Southern hemisphere: Sun is mostly seen on the North side of the sky, reaching due North at noon (2) The Sun’s Annual Cycle -1) Fall and Spring Equinox (September 23 , March 21 ): Sun rises due east and sets due west, spending equal time above and below the horizon (equal day and night  “Equinox” is Latin for “equal nightnd -2) Winter Solstice (December 22 ): Sun rises and sets at its southernmost position, traversing a short, low arc in the sky (shortest day of year and lowest noon Sun)  “Solstice” is Latin for “solar-standstill” -3) Summer Solstice (June 22 ): Sun rises and sets at its northernmost position, traversing a long, high arc (longest day, highest noon Sun) -4) In the S-hemisphere, the Sun’s daily arcs point North rather than South -5) Finding the Sun’s rising and setting positions on the solstices simply requires finding its northernmost and southernmost positions -6) The seasonal changes in the Sun’s daily arc are due to Earth’s 235° tilt from its orbit around the Sun -7) Winter: when a hemisphere is tilted away from the Sun, sunlight hits indirectly (i.e. at an angle), causing low arcs and less light per area -8) Summer: when a hemisphere is titled toward the Sun, sunlight hits more directly causing high arcs and more light per area -9) When one hemisphere is tilted towards the Sun the other is tilted away. Therefore the seasons are experienced at opposite times in opposite hemispheres (3) The Sun and Latitude -1) Latitude: a location’s angle North or South of Earth’s equator -2) Only within the tropics (l<23.5°), can the Sun be seen directly overhead -3) Tropic of Cancer: at 23.5
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