Observations of the Sky Observations of the Sky What patterns, motions, and events do we notice in the sky? What theories can we create to explain them? How can these theories be tested? How does the sky change over… A day? A night? A month? A year? A lifetime? A millenium? Why do we see these changes? How does the sky change over… A day? The Sun rises in the East, sets in the West It rises and sets at an angle It takes about 24 hours from sunrise to sunrise Good Science => Careful observations? How does the sky change over… A day? The Sun rises in the East, sets in the West … for *most* observers, but not all! … not always due East, nor due West How does the sky change over… A day? The Sun rises and sets at an angle … but angle isn’t the same everywhere! Oakland Canada Alaska Sunrise 9 February in the North America Venezuela Rio Sunrise 9 February in the Southern Hemisphere New Zealand How does the sky change over… A night? The Moon, planets, and most stars also rise in the East, set in the West, at an angle The sky seems to rotate around a fixed point – Polaris. How does the sky change over… A night? The sky rotates around Polaris (check out http://www.youtube.com/watch?v=3V3rmDG5J8A ) Did you know? The ANGLE of Polaris above the horizon indicates your latitude in the Northern Hemisphere Altitude angle How does the sky change over… A night? In the Southern Hemisphere, the sky seems to rotate the other way! (check out http://www.youtube.com/watch?v=XfRFCdYCcZA How does the sky change over… A night? Some constellations always visible (circumpolar) How does the sky change over… A “moonth?” The Moon goes through phases We see slow change in which constellations are visible at night The Sun’s slowly shifts “in front of” different zodiac constellations Good Science => Careful observations? How does the sky change over… A “moonth?” The Moon goes through phases How does the sky change over… A “moonth?” We see slow change in which constellations are visible at night How does the sky change over… A “moonth?” The Sun’s slowly shifts “in front of” different zodiac constellations H. A. Rey (Author of Curious George!) “The Stars” 1980, from Houghton Mifflin Co. What have we learned? Why do the constellations we see depend on latitude and time of year? — Your location determines which constellations are hidden by Earth. — Time of year determines the location of the Sun in the sky. How does the sky change over… A year? Sun’s rising/setting points, and noon-time height return to the same points. Constellations return to the same pattern But planets “wander” in front of the zodiac at different rates, and even dance! Sunrise Planting Calendars – Southwestern US Peoples Walpi (Hopi) horizon calendar. McCloskey, after Stephen. Observations from Bear Clan House. Sunrise Planting Calendars – Southwestern US Peoples From Zeilik (1985) Hopi planting calendar. Village of Shungopavi. How does the sky change over… A year? The Sun’s height changes slowly Photo of the sun, taken @ same time every day, over a year How does the sky change over a year? How does the sky change over a year? Sun’s “speed” across the sky varies Upper loop = Summer, Earth moves slower in its orbit lower loop = Winter, Earth moves faster in its orbit The analemma Bergamo, Italy Photos by John Nystuen, University of Michigan How does the sky change over… A year? Summer Sun higher in the sky, shining more DIRECTLY Sun up longer, more hours in the day Winter Sun lower in the sky, less direct light Sun not up as long TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. (Hint: When it is summer in the United States, it is winter in Australia.) TRUE OR FALSE! Earth is closer to the Sun in sun summer and farther from the Sun in winter. • Seasons are opposite in the N and S hemispheres, so distance cannot be the reason. • The real reason for seasons involves Earth’s axis tilt. What causes the seasons? Seasons depend on how Earth’s axis affects the directness of sunlight. Direct light causes more heating. Directness of Light Axis tilt changes directness of sunlight during the year. Why Does the Flux of Sunlight Vary Sun’s altitude changes w/ seasons Sun’s position at noon in summer: higher altitude means more direct sunlight. Sun’s position at noon in winter: lower altitude means less direct sunlight. The Real Reason for Seasons Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun. Summer occurs in your hemisphere when sunlight hits it more directly; winter occurs when the sunlight is less direct. AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth. Why doesn’t distance matter? • Variation of Earth–Sun distance small ~ 3% • Small variation overwhelmed by effects of axis tilt. How do we mark the progression of the seasons? • We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinox Recognize solstices and equinoxes by the Sun’s path across the sky. Summer solstice: Highest path, rise and set at most extreme north of due east Winter solstice: Lowest path, rise and set at most extreme south of due east Equinoxes: Sun rises precisely due east and sets precisely due west. Seasonal changes are more extreme at high latitudes. Path of the Sun on the summer solstice at the Arctic Circle How does the sky change over… A year? Constellations return to the same pattern But planets “wander” in front of the zodiac at different rates, and even dance! Tunc Tezel, apod031216 How does the sky change over… A lifetime? Annual patterns repeat: Moon Phases, Seasons, constellations Longer patterns apparent: Planetary motions, solar eclipses How does the sky change over… A millenium? Slow change in location of “North” star How does the sky change over… A millenium? Slow change in location of “North” star Slower, even smaller change in direct positions of stars themselves
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