Copyright © 2009 Pearson Education, Inc. Chapter 2 Discovering the Universe for YourselfCopyright © 2009 Pearson Education, Inc. Summary: 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.Copyright © 2009 Pearson Education, Inc. Why doesn’t distance matter? • Variation of Earth–Sun distance is small—about 3%; this small variation is overwhelmed by the effects of axis tilt.Copyright © 2009 Pearson Education, Inc. How do we mark the progression of the seasons? • We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinoxCopyright © 2009 Pearson Education, Inc. We can 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.Copyright © 2009 Pearson Education, Inc. Why doesn’t distance matter? • Variation of Earth–Sun distance is small—about 3%; this small variation is overwhelmed by the effects of axis tilt.Copyright © 2009 Pearson Education, Inc. How do we mark the progression of the seasons? • We define four special points: summer solstice winter solstice spring (vernal) equinox fall (autumnal) equinoxCopyright © 2009 Pearson Education, Inc. We can 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.Copyright © 2009 Pearson Education, Inc. Seasonal changes are more extreme at high latitudes. Path of the Sun on the summer solstice at the Arctic Circle Insert ECP 5e Figure 2.16Copyright © 2009 Pearson Education, Inc. How does the orientation of Earth’s axis change with time? • Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years. — Polaris won’t always be the North Star. — Positions of equinoxes shift around orbit; for example, the spring equinox, once in Aries, is now in Pisces! Earth’s axis precesses like the axis of a spinning top.Copyright © 2009 Pearson Education, Inc. What have we learned? • What causes the seasons? — The tilt of the Earth’s axis causes sunlight to hit different parts of the Earth more directly during the summer and less directly during the winter. — We can specify the position of an object in the local sky by its altitude above the horizon and its direction along the horizon.Copyright © 2009 Pearson Education, Inc. What have we learned? • How do we mark the progression of the seasons? — The summer and winter solstices are when the Northern Hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight. • How does the orientation of Earth’s axis change with time? — The tilt remains about 23.5° (so the season pattern is not affected), but Earth has a 26,000-year precession cycle that slowly and subtly changes the orientation of the Earth’s axis.Copyright © 2009 Pearson Education, Inc. 2.3 The Moon, Our Constant Companion • Why do we see phases of the Moon? • What causes eclipses? Our goals for learning:Copyright © 2009 Pearson Education, Inc. Why do we see phases of the Moon? • Lunar phases are a consequence of the Moon’s 27.3-day orbit around Earth.Copyright © 2009 Pearson Education, Inc. Phases of the Moon • Half of the Moon is illuminated by the Sun and half is dark. • We see a changing combination of the bright and dark faces as the Moon orbits Earth.Copyright © 2009 Pearson Education, Inc. Phases of the Moon Phases of the MoonCopyright © 2009 Pearson Education, Inc. Moon Rise/Set by Phase Time the Moon Rises and Sets for Different PhasesCopyright © 2009 Pearson Education, Inc. Phases of the Moon: 29.5-day cycle new crescent first quarter gibbous full gibbous last quarter crescent waxing • Moon visible in afternoon/evening • Gets “fuller” and rises later each day waning • Moon visible in late night/morning • Gets “less” and sets later each day }}!Copyright © 2009 Pearson Education, Inc. Thought Question A. first quarter B. waxing gibbous C. third quarter D. half moon It’s 9 A.M. You look up in the sky and see a moon with half its face bright and half dark. What phase is it?Copyright © 2009 Pearson Education, Inc. A. first quarter B. waxing gibbous C. third quarter D. half moon It’s 9 A.M. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? Thought QuestionCopyright © 2009 Pearson Education, Inc. We see only one side of the Moon. Synchronous rotation: The Moon rotates exactly once with each orbit. This is why only one side is visible from Earth.Copyright © 2009 Pearson Education, Inc. What causes eclipses? • The Earth and Moon cast shadows. • When either passes through the other’s shadow, we have an eclipse.Copyright © 2009 Pearson Education, Inc. Lunar Eclipse Lunar EclipseCopyright © 2009 Pearson Education, Inc. When can eclipses occur? • Lunar eclipses can occur only at full moon. • Lunar eclipses can be penumbral, partial, or total.Copyright © 2009 Pearson Education, Inc. Solar Eclipse Evolution of a Total Solar EclipseCopyright © 2009 Pearson Education, Inc. When can eclipses occur? • Solar eclipses can occur only at new moon. • Solar eclipses can be partial, total, or annular.Copyright © 2009 Pearson Education, Inc. Why don’t we have an eclipse at every new and full moon? — The Moon’s orbit is tilted 5° to the ecliptic plane. — So we have about two eclipse seasons each year, with a lunar eclipse at new moon and solar eclipse at full moon. Insert ECP 5e Figure 2.20Copyright © 2009 Pearson Education, Inc. Summary: Two conditions must be met to have an eclipse
View Full Document
Unlocking...