Reasons for Seasons. Question: TRUE OR FALSE. Question: TRUE OR FALSE? What causes the seasons? What causes the seasons?

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1 Reasons for Seasons Question: TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. Question: TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter. Question: 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 U.S., it is winter in Australia. Seasons are opposite in the N and S hemispheres, so orbital distance cannot be the reason. Tilt of the Earth s rotation axis. Earth s rotation axis always points to Celestial North Pole, tilted 23.5 degs from the axis of the Earth s orbital axis. (Drawing not to scale!) Tilt of the Earth s rotation axis. Earth s rotation axis always points to Celestial North Pole, tilted 23.5 degs from the axis of the Earth s orbital axis. (Drawing not to scale!) March 21 March 21: Spring equinox June 21 Dec 21 June 21: Summer solstice Dec 21: Winter solstice Sept 21 (Labels only for N. Hemisphere) Sept 21: Fall equinox 1

2 Summer solstice Winter Solstice Summary: The Reason for Seasons Earth s axis points in the same direction all year round (to Celestial North Pole), so its orientation relative to the Sun changes as Earth orbits the Sun. Summer occurs in your hemisphere when sunlight hits it more directly: more energy absorbed per area of land. Winter occurs when the sunlight is less direct: less energy absorbed per area of land. ROTATION AXIS TILT is the key to the seasons. Without it, we would not have seasons on Earth. 2

3 Why doesn t distance matter? Progression of the seasons summer solstice: maximum tilt towards Sun for N. Hemisphere. June 21 ( first day of summer ) The Earth s orbit is nearly an exact circle. Only small (3%) variation of the Earth-Sun distance over the orbit. (In fact, Earth is closer to the Sun in the Southern Hemisphere winter.) The small variation in the Earth s distance is overwhelmed by effects of axis tilt. winter solstice: maximum tilt away from Sun for N. Hemisphere. Dec 21 ( first day of winter ) spring equinox, fall equinox: Sun shines equally on both hemispheres Note: Some planets have greater distance variation in their orbit that DOES affect their seasons (Mars, Pluto). Summer solstice: highest path (longest day) Summer solstice: Sun follows the highest path in the sky (longest day) Rise and set at most extreme north of due east. rise and set at most extreme north of due east. Winter solstice: lowest path (shortest day) rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west. Day is precisely 12 hours long. Winter solstice: Equinoxes: Sun follows the lowest path in the sky (shortest day) Rise and set at most extreme south of due east. Sun rises precisely due east and sets precisely due west. Person at the Equator will see Sun passing directly overhead. 3

4 Seasonal changes are more extreme at high latitudes Path of the (circumpolar) Sun on the summer solstice at the Arctic Circle (66 degrees N latitude) Does the orientation of Earth s axis change with time? Does the orientation of Earth s axis change with time? YES. It actually precesses over about 26,000 yrs. The Moon: Phases and Eclipses YES. It actually precesses over about 26,000 yrs. The rotation axis tilt remains 23.5 degs (so the seasonal pattern is not affected), but the axis orientation changes very slowly over time. Polaris wonʼt always be the North Star! Constellations in which the Sun is located shift 360 degrees around the Zodiac over this 26,000 year period yrs ago the spring equinoxes occurred with the Sun in Aries. Now they occur in Pisces. In another 2000 years, they will be in Aquarius ( Age of Aquarius ). Full moon viewed from the Space Shuttle: which direction is the Sun? Phases of the Moon: 29.5-day cycle It is safe to try this at home! Half the Moon is illuminated by Sun and half is dark. We see some combination of the bright and dark faces. 4

5 Earth and Moon viewed from the Galileo spacecraft on its way to Jupiter A quarter Earth seen from the Moon (Apollo 8, December 1968) What causes eclipses? The Earth and Moon cast shadows. When either passes through the other s shadow, we can see an eclipse on the Earth. Eclipses When can lunar eclipses occur? Total lunar eclipse Why is the Moon red, not black? Lunar eclipses can occur only at full moon (when the Earth is between the Moon and the Sun.) Lunar eclipses can be penumbral, partial, or total. 5

6 When can solar eclipses occur? Africa: June 2001 Solar eclipses can occur only at new moon (when the Moon is between the Sun & the Earth). Solar eclipses can be partial, total, or annular. Solar eclipses only occur over part of the Earth. La Paz, Mexico: July 1990 Total solar eclipse One of the most spectacular natural wonders to witness 1991: Baja, Mexico (also seen in Hawaii) Why do not we have an eclipse at every new and full moon? The Moon s orbit is tilted 5 to ecliptic plane So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon. 6

7 Two conditions must be met to have an eclipse: Predicting Eclipses Eclipses recur with the 18 yr, 11 1/3-day saros cycle. 1. It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes). 7