Universe 8/e Chapter 2

1
CHAPTER 2 Knowing the Heavens
2
(No Transcript)
3
What you will learn

• The importance of astronomy in ancient
  civilizations around the world
• That regions of the sky are divided around groups
  of stars called constellations
• How the sky changes from night to night

• How astronomers locate objects in the sky
• What causes the seasons
• The effect of changes in the direction of Earths
  axis of rotation
• The role of astronomy in measuring time
• How the modern calendar developed

4
(No Transcript)
5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
(No Transcript)
19
(No Transcript)
20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
(No Transcript)
26
(No Transcript)
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
38
(No Transcript)
39
(No Transcript)
40
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).

41
Key Ideas

• Seasons and the Tilt of the Earths Axis The
  Earths axis of rotation is tilted at an angle of
  about 23.5 from the perpendicular to the plane
  of the Earths orbit.
• The seasons are caused by the tilt of the Earths
  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 23.5.
• The ecliptic crosses the celestial equator at two
  points in the sky, the vernal (March 21) and
  autumnal (September 21) equinoxes.
• The northernmost point that the Sun reaches on
  the celestial sphere is the summer solstice (June
  21), and the southernmost point is the winter
  solstice (December 21).

42
Key Ideas

• Precession The orientation of the Earths axis
  of rotation changes slowly, a phenomenon called
  precession.
• Precession is caused by the gravitational pull of
  the Sun and Moon on the Earths equatorial bulge.
• Precession of the Earths axis causes the
  positions of the equinoxes and celestial poles to
  shift slowly as seen against the background of
  more distant stars.
• Because the system of right ascension and
  declination is tied to the position of the vernal
  equinox, the date of observation must be
  specified when giving the position of an object
  in the sky.

43
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.