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Universe Eighth Edition
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  1. Roger A. Freedman • William J. Kaufmann III Universe Eighth Edition CHAPTER 2 Knowing the Heavens

  2. Homework 2 Read Chapter 3 (Eclipses and the Motion of the Moon) Online quiz from Chapter 2 DUE Friday 9/10 @ 5 pm REMINDER: No class on Monday 9/6 (Labor Day)

  3. 2-1 The importance of astronomy in ancient civilizations around the world 2-2 That regions of the sky are divided around groups of stars called constellations 2-3 How the sky changes from night to night 2-4 How astronomers locate objects in the sky 2-5 What causes the seasons 2-6 The effect of changes in the direction of Earth’s axis of rotation 2-7 The role of astronomy in measuring time 2-8 How the modern calendar developed By reading this chapter, you will learn

  4. Most evidence suggests ancient astronomers were inspired to look at the sky because • they wanted to create scientific theories of the world in which they lived. • observation of star positions allowed calendars to be created. • ancient civilizations associated star patterns with gods and mystical figures. • Both a and c. • Both b and c. Q2.2

  5. Most evidence suggests ancient astronomers were inspired to look at the sky because • they wanted to create scientific theories of the world in which they lived. • observation of star positions allowed calendars to be created. • ancient civilizations associated star patterns with gods and mystical figures. • Both a and c. • Both b and c. A2.2

  6. In the southern hemisphere • stars rise in the east and set in the west. • stars rise in the west and set in the east. • all stars are circumpolar. • no stars are circumpolar. • a or b, depending on the time of day. Q2.4

  7. In the southern hemisphere • stars rise in the east and set in the west. • stars rise in the west and set in the east. • all stars are circumpolar. • no stars are circumpolar. • a or b, depending on the time of day. A2.4

  8. Summer occurs in the northern hemisphere of the Earth in June, July, and August because • the northern hemisphere of Earth is closer to the Sun than the southern hemisphere. • days are longer in the northern hemisphere than in the southern hemisphere. • the sunlight strikes the northern hemisphere of the Earth at an angle closer to the vertical. • the Earth is closer to the Sun. • Both b and c. Q2.5

  9. Summer occurs in the northern hemisphere of the Earth in June, July, and August because • the northern hemisphere of Earth is closer to the Sun than the southern hemisphere. • days are longer in the northern hemisphere than in the southern hemisphere. • the sunlight strikes the northern hemisphere of the Earth at an angle closer to the vertical. • the Earth is closer to the Sun. • Both b and c. A2.5

  10. If the Earth’s axis were not tilted, • a day and night would last 365 Earth days. • the effect of seasons would be exaggerated. • there would be no seasons. • the Earth would always keep the same side facing toward the Sun. • The Earth would be completely covered with ice. Q2.6

  11. If the Earth’s axis were not tilted, • a day and night would last 365 Earth days. • the effect of seasons would be exaggerated. • there would be no seasons. • the Earth would always keep the same side facing toward the Sun. • The Earth would be completely covered with ice. A2.6

  12. The point on the ecliptic (see diagram) where the Sun crosses from the southern to the northern hemisphere is the • summer solstice. • winter solstice. • autumnal equinox. • vernal equinox. • celestial equator. Q2.10

  13. The point on the ecliptic (see diagram) where the Sun crosses from the southern to the northern hemisphere is the • summer solstice. • winter solstice. • autumnal equinox. • vernal equinox. • celestial equator. A2.10

  14. Key Ideas • Constellations and the Celestial Sphere: It is convenient to imagine the stars fixed to the celestial sphere with the Earth at its center. • The surface of the celestial sphere is divided into 88 regions called constellations. • Diurnal (Daily) Motion of the Celestial Sphere: The celestial sphere appears to rotate around the Earth once in each 24-hour period. In fact, it is actually the Earth that is rotating. • The poles and equator of the celestial sphere are determined by extending the axis of rotation and the equatorial plane of the Earth out to the celestial sphere. • The positions of objects on the celestial sphere are described by specifying their right ascension (in time units) and declination (in angular measure).

  15. Key Ideas • Seasons and the Tilt of the Earth’s Axis: The Earth’s axis of rotation is tilted at an angle of about 231⁄2° from the perpendicular to the plane of the Earth’s orbit. • The seasons are caused by the tilt of the Earth’s axis. • Over the course of a year, the Sun appears to move around the celestial sphere along a path called the ecliptic. The ecliptic is inclined to the celestial equator by about 231⁄2°. • The ecliptic crosses the celestial equator at two points in the sky, the vernal and autumnal equinoxes. • The northernmost point that the Sun reaches on the celestial sphere is the summer solstice, and the southernmost point is the winter solstice.

  16. Key Ideas • Because the system of right ascension and declination is tied to the position of the vernal equinox, the date (or epoch) of observation must be specified when giving the position of an object in the sky.

  17. Key Ideas • Timekeeping: Astronomers use several different means of keeping time. • Apparent solar time is based on the apparent motion of the Sun across the celestial sphere, which varies over the course of the year. • Mean solar time is based on the motion of an imaginary mean sun along the celestial equator, which produces a uniform mean solar day of 24 hours. Ordinary watches and clocks measure mean solar time. • Sidereal time is based on the apparent motion of the celestial sphere. • The Calendar: The tropical year is the period between two passages of the Sun across the vernal equinox. Leap year corrections are needed because the tropical year is not exactly 365 days. The sidereal year is the actual orbital period of the Earth.