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Unformatted text preview: AS 101 Lecture 5 AS Logistics • Problem Set 2: Last Class: Observing the Night Sky II Changing Sky with Seasons Earth’s orbit Observing the Sun Today’s Class: Observing the Night Sky III Coordinates and Times Precession of the Equinox Observing the Moon Eclipse – may be Web based: go to masteringastronomy.com Course ID: “AS101SP10OPPENHEIM” Read Chapter 2 of Bennett Time Out to Think Question Time
In Boston, how high does the sun appear above the horizon on winter solstice at solar noon? 1. 24.50 2. 420 hori zon 42 +23.5 =65.6 0 3. 48 φ 0 4. 56 =90 65.5=24.5 5. 72.50 42
o o o 23.5 Algebraic Solution: Sun 48o above horizon at noon on equinox 48o 23.50 =24.5o at noon on winter solstice Coordinate Systems Coordinate Dec. is measured in degrees R.A. is measured in hours (024hrs) Sangmin Jung: What is the DEC of a star seen at zenith in night lab? How do you do angles in hours minutes and seconds?
• Don’t they usually measure time? • But, you see, the sky (the celestial sphere) rotates 360° in 24 hours, so each hour represents 360/24 or 15° of RA • One minute RA, therefore is 15/60 or 0.25° or 15 arcminute • One sec RA,…. 1/60 minute or 15 arcsec The Sky Changes with Latitude Sep 15, 2009 Sep 15, 2009 AS101 F09 Class 4 5 Determining Latitude Determining Determining Longitude is much more difficult Thought Question
The North Star (Polaris) is 50° above your horizon, due north. Where are you? 1. 2. 3. 4. 5. You are on the equator. You are at the North Pole. You are at latitude 50°N. You are at longitude 40°E. You are at latitude 40°N Determining Longitude
Harrison's amazing chronometers Harrison's In 1762, at the end of a 147day sea voyage, H4 (below) had lost only 1 minute and 55 seconds. Go to Greenwich and see them still working! Also, read the book Longitude by Dava Sobel Where are you if • If the Sun is at it’s zenith (in South):
your watch reads 12:00 GMT? You watch reads 13:00 GMT? Your watch reads 7:00 GMT? Longitude Longitude Sidereal Time Sidereal How long is a day? How long does it take the Earth to spin 360o?
• • • 24 hours exactly – solar day • From the time the sun crests until it’s next crest 23:56 Earth Day from point of view of Stars (celestial sphere) • Sidereal Day Now with an exciting demo! Now Two different kinds of Days
Solar day: Day by the Sun 24 hours (time between two consecutive noons) Sidereal day: Day by the stars 23 hours and 56 min (time to rotate 360°)
Courtesy Chaisson & McMillan, Astronomy Today The reason there is a difference is that the Earth rotates as well as revolves. Solar and Sidereal Days Solar Precession of the Equinox Precession Movement of N. Celestial Equator Movement Observing the Moon Observing Phases of the Moon Orbit of the Moon Eclipses Phases of the Moon Phases Full Waning Gibbous Waning Last First Waxing Quarter Crescent Crescent Quarter Waxing Gibbous New Moons occur approximately once per 29.5 days ...
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- Spring '11