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Unformatted text preview: Apparent vs. Actual Motions
North Daily Motions - Actual
Actual motion = Earth spin with axis pointed in fixed direction - towards Polaris North Pole Local view West to East NY leads LA "omnipotent being" view Apparent view - path of International Space Station across the sky as seen from Boulder on Sat. 8th Sept. Patterns in the Night Sky What does the universe look like from Earth? Why do stars rise and set? How does the sky change with latitude and over the year? Read up to page 41
Actual view - ground track of International Space Station over US on Sat 8th Sept. This will be even clearer in the Fiske Planetarium at Friday's class - 9am as usual - please be on time! Apparent Motions - The Local Sky Zenith: The point directly overhead Horizon: All points 90 away from zenith Meridian: Line passing through zenith and connecting N and S points on horizon Altitude: Angle from the horizon Discussion: Motions in the Sky Orient yourself in the classroom, and visualize solar & stellar motions: Where's the north star? Which way do stars move around it? Where did the Sun rise today, and where will it set? Where is the Sun right now? Where is the Moon right now? Apparent Motion of Sun
Equinox Sun moves 360 in 24 hours 360/24 hr = 15/hr We measure the sky using angles Latitude & Longitude
Latitude: position north or south of equator Latitude Longitude: position east or west of prime meridian (runs through Greenwich, England) Visibility of Stars What are we looking at?
A. Stars viewed from Hawaii B. Apparent motion due to Earth's spin C. Stars seen from Chile D. Two nearby galaxies Longitude
But here in Boulder how do we know when the sun is overhead in London? Phone? Carry a clock set on London time? John Harrison (1693--1776) made his first prototype marine timekeeper in Barrow-on-Humber Royal Greenwich Observatory between 1730 and 1735 as a first step towards solving the longitude problem and winning the great 20,000 prize offered by the British government. Now known as H1, the timekeeper is unaffected by the motion of a ship owing to its two, interconnected swinging balances. It compensates for changes in temperature and, thanks to extensive anti-friction devices, runs without any lubrication. It was the first relatively successful marine timekeeper of any kind and was the toast of London when Harrison unveiled it in 1735. It is one of the great milestones in clockmaking history Longitude Annual Motions - Actual
Actual motion = Earth orbits the Sun. Plane of orbit = ecliptic Seasons Lab topic Lab topic Annual Motion: The sky varies as Earth orbits the Sun As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic. At midnight, the stars on our meridian are opposite the Sun in the sky. How does the orientation of Earth's axis change with time?
Although the axis seems fixed on human time scales, it actually precesses over ~26,000 years. Polaris won't always be the North Star. Dates that Earth is opposite constellations has shifted by about a month over 2000 years Earth's axis precesses like the axis of a spinning top Local vs. External Viewpoints Local vs. External Viewpoints Northern Hemisphere Southern Hemisphere ...
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- Fall '07