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Lecture

chapter 2

10 Pages
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Department
Natural Science
Course Code
NATS 1745
Professor
Robin Metcalfe

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LECTURE 2:ARCHAEOASTRONOMY CONT’D January 9, 2014 NATS 1745: History ofAstronomy Instructions for the Term Project • Project in 3 parts • Play the role of a person on an advisory committee for a funding organization of astronomy o Make decisions about whether or not funds should be continued to these people • Choose 3 recent news articles • 1 report gives an opportunity to see what the TA’s are looking for o Choose an article published on or after December (?) 2013 o Stick to sites on the list o PartA– header info o Part B  Use headings The Sun’s Daily Cycle • Due to 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. o Understanding of sun’s hourly, daily, and annual path across the sky o Rises in the east and sets in the west; highest point at noon  Happens due to the rotation of the earth o Spins easterly  causes window of sky to move westerly • N-hemisphere: Sun is mostly seen on the South side of the sky, reaching due South at noon. • S-hemisphere: Sun is mostly seen on the North side of the sky, reaching due North at noon. o 2 opposite hemispheres seen opposite sides of the sun LECTURE 2:ARCHAEOASTRONOMY CONT’D January 9, 2014 The Sun’sAnnual Cycle • Fall and Spring Equinox (~Sept. 23, ~Mar. 21): Sun rises due E and sets due W, spending equal time above and below the horizon (equal day and night). (Equinox: Latin for “equal night”). o Shorter days; daytime arch is lower = colder  less time to warm the ground  Indirect light is hitting us instead of directly overhead • Is weaker, colder o Winter solstice is the shortest day  After, sun’s path shifts back Eastward • Daytime lengthens, temperature gets warmer, and sun again rises due east and then sets due west  path shifts more due North o Ark is tilted towards the south side of the sky  Only around sunset and sunrise that the sun will be seen to the North • Winter Solstice (~Dec. 22): Sun rises and sets at its southernmost position, traversing a short, low arc in the sky (shortest day of the year and lowest noon sun). (Solstice: Latin for “solar standstill”.) • Summer Solstice (~Jun 22): Sun rises and sets at its northernmost position, traversing a long, high arc (longest day, highest noon sun). • In the S-hemisphere, the Sun’s daily arcs point N rather than S. LECTURE 2:ARCHAEOASTRONOMY CONT’D January 9, 2014 • Identifying the Sun’s rising and setting positions on the solstices simply requires finding its northernmost and southernmost positions. o Mosaic photograph of rising sun behind this mountain o If prehistoric person were to follow the sun 365 days of the year, then after 365 days, SE most notch would point to direction of winter Solstice o NE most notch would point to sunrise on Summer Solstice o Solar alignments in prehistoric civilizations = recognize daily patterns of sun in the sky • The seasonal changes in the Sun’s daily arc are due to Earth’s 23.5 degree tilt from its orbit around the Sun. LECTURE 2:ARCHAEOASTRONOMY CONT’D January 9, 2014 o Axis is tilted 23.5 degrees from straight up and down • 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. • Summer: When a hemisphere is tilted toward the Sun, sunlight hits more directly, causing high arcs and more light per area. • When one hemisphere is tilted toward the Sun, the other is tilted away. Therefore, the seasons are experienced at opposite times in opposite hemispheres. • Animation: Earth's annual orbit The Sun and the Latitude • Latitude (/): a location’s angle N or S of Earth’s equator. • Only within the tropics (I < 23.5 degrees) can the Sun be seen directly overhead. • At 23.5 degrees N (Tropic of Cancer), the noon Sun is in the Cancer constellation when the Sun is direc
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