Seasons Motions of the Sun

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1 ASTR1010 Lecture 6 31 Jan 13 Today How the Sky Works: Phases, Eclipses, Precession, Planets Announcements: Exam 1 Review sheets available up front Old-Fashioned Homework 1 ed, due ON PAPER Tues SkyGazer not needed today; laptops only for notes (in front) Moon Balls session now & after class outside do it! What does retrograde motion refer to? A. The acceleration and deceleration of a planet on an elliptical orbit B. The apparent backwards motion of a planet seen in the sky C. The actual backwards motion of some planets orbiting the Sun D. The moon s motion relative to the stars, not the Sun E. A secret play the 49ers will use in the Superbowl Here is a neat moonrise video. archive/2013/01/full-moonsilhouettes/272673/ -Jason Reading Reminders What is retrograde motion? What are the heliocentric and geocentric models of the solar system? What is stellar parallax? What were Galileo s main discoveries? Your Questions via JiTT2 1) How does angular size tell us distance? (several people) 2) How does precession of the earth's axis work, or what does it mean? (several people) 3) Can longitude/latitude of the sky be covered a bit more? (several people) 4) More about equinoxes/solstices, celestial equator (perhaps with demo)? (several) 5) More about the moon phases, and what must be known for the class regarding phases? (several) 6) What is the celestial sphere, and how does it affect our view of the night sky? 7) Can retrograde motion be described a bit more? Outline for Next Three Lectures Stars recap Seasons Lunar Phases Eclipses Precession Planetary motion The main point is linking observations to mental models don t just memorize We ve seen the value of a good mental model what if there are TWO???? Seasons Motions of the Sun What causes the seasons? How is the Earth oriented relative to the Sun for different seasons? How high does the sun get at different times of the year? Where does the sun rise & set at different times of the year?

2 Excellent Student Question I had a question relating to our discussion of the seasons. I was wondering why, if the axis is roughly at the same angle, and both hemispheres are exposed to the same amount of direct sunlight, Fall is a colder season, in general, than Spring is in Colorado? Around the fall equinox, where is the Sun directly overhead at noon? A: all latitudes B: 23.5 degrees north C: only on the equator D: 23.5 degrees south E: not at any latitude Around the fall equinox, where is the Sun directly overhead at noon? A: all latitudes B: 23.5 degrees north C: only on the equator D: 23.5 degrees south E: not at any latitude Where will the Sun be directly overhead on 21 June? Where will the Sun be directly overhead on 21 June? Where will the Sun be directly overhead on 21 March?

3 Where will the Sun be directly overhead on 21 March? Here s the Sun as we see in December at noon. How will it be different at noon in June? a. Larger because we re closer b. Higher in the sky c. Larger and higher in the sky d. No significant change Here s the Sun as we see in December at noon. How will it be different at noon in June? a. Larger because we re closer b. Higher in the sky c. Larger and higher in the sky d. No significant change Rules for rising and setting Only on the equinoxes (March 21 or Sept. 21) does the sun sit right on the celestial equator. In December, the Sun is south of the celestial equator, north in June Rising and setting directions depend only on and objects position north, south or of the celestial equator Motions of the Sun wrap-up The date (season) determines whether the sun rises NE, E or SE The location determines the path of rising/setting, which affects how high an object will appear What if Imagine Earth was shaped like a soup can, surrounded by a tube of stars far overhead. Will stars change as you travel? How can you prove Earth isn t like this? At equator! At 23.5 deg N!

4 Imagine Irth was shaped like a cylinder, surrounded by a tube of stars far overhead. How can you prove Earth isn t like this? The failing answer: Earth is a sphere, not a cylinder The C answer: Stars above Irth would move differently from ours The B answer: Circumpolar stars would be different on Irth The A answer: Irth would have no circumpolar stars at any location Expect an exam question where a different model leads to different observations (or vice-versa). Full-credit answers depend on specific observations As in this case, there can be multiple correct approaches Lunar Phases What causes phases of the Moon? How are the Moon, Earth and Sun positioned for different phases? How long does the cycle of phases take? When do the different phases of the moon rise, set and appear near overhead? How well do you understand lunar phases? A. Pretty well B. OK, I guess C. Pretty shaky D. Hardly at all Why does the Moon undergo phases over a month? A: Earth s orbital motion around the Sun B: Earth s rotation C: The moon s orbital motion around the Earth D: More than one of the above is very important E: Trick question: moon goes through its cycle of phases each day Hint: Which motion(s) would you need to turn off to stop the cycle of phases? Why does the Moon undergo phases every month? A: Earth s orbital motion around the Sun B: Earth s rotation C: The moon s orbital motion around the Earth D: More than one of the above is very important E: Trick question: moon goes through its cycle of phases each day What time does the full moon set? A: Noon B: sunset C: midnight D: dawn E: any of the above - depends on time of year

5 What time does the full moon set? A: Noon B: sunset C: midnight D: dawn E: any of the above - depends on time of year Lunar Phases The Essential Diagram Draw the Earth, Sun & Moon as seen from above the north pole Add orbital & rotational motions Shade in night side Place moon in position for first quarter, shade night side What time of day does this phase rise? Draw person watching moonrise Lunar Phases The Essential Diagram Great Practice: Choose two of three and find the third! Phase: full, new, first/last quarter, crescent Time of Day: noon, midnight, sunrise, sunset Observation: rise, set, overhead, not visible Putting it All Together I need some volunteers! We live on a rotating sphere which orbits the Sun with its axis slightly tilted (at Polaris) The stars lie at great distance The moon orbits the Earth keeping the same face to the Earth Eclipses Why do they happen? What phases of the moon make eclipses possible Do eclipses happen every month? If not, why not? How would eclipses be different if the objects were different sizes or distances? Do solar eclipses occur every month?" A. Yes, but some are partial and we don t even notice them any more B. Yes, but at different locations on Earth so they are pretty rare here C. No, the new moon doesn t always line up with the Sun D. No, sometimes the moon is so far away it doesn t block the Sun

6 Do solar eclipses occur every month?" A. Yes, but some are partial and we don t even notice them any more B. Yes, but at different locations on Earth so they are pretty rare here C. No, the new moon doesn t always line up with the Sun D. No, sometimes the moon is so far away it doesn t block the Sun About how far apart should the globe and ball be placed to represent the Earth-Moon system? A) 10 centimeters (or less) B) 1 meter C) 10 meters D) 100 meters E) 1 km (or more) About how far apart should the globe and ball be placed to represent the Earth-Moon system? Good logic? Or not? A) 10 centimeters (or less) B) 1 meter C) 10 meters D) 100 meters E) 1 km (or more) Precession: Earth s Tilt - with a twist How does it affect Earth s motions? How does it affect the sky, seasons, etc.? Gyroscope Demo Spinning things obey rules not seen in everyday life Earth represented by spinning gyroscope What happens as I move & rotate? Moving the weight represents the extra gravitational tugs by the Sun & Moon on our equatorial bulge What happens as these tugs become important?

7 Precession Precession changes the direction of Earth s tilt over 26,000 years Precession does NOT change the amount of the tilt - always 23.5 degrees Discussion questions: So what? How does precession affect our sky? How does precession affect the seasons???? What if precession took 1 year, keeping pole towards Sun??? How does precession affect Earth over its 26,000 year cycle? A: it dramatically changes the intensity of seasons B: it changes which star (if any) is the North Star C: it changes which way the sun rises on warm summer days (southeast vs. northeast) D: more than one of the above E: none of the above