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Lecture Slides
CHAPTER 2: Patterns in the Sky—Motions of Earth and
the Moon
Understanding Our Universe
SECOND EDITION
Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal
Prepared by Lisa M. Will,
San Diego City College
Copyright © 2015, W. W. Norton & Company
Patterns in the Sky
What do we observe on
the sky due to the:
 Rotation of Earth?
 Revolution of Earth?
 Axial tilt of Earth?
 Orbit of the Moon?
Rotation of the Earth
 One day is 24 hours.
• One rotation with respect to
stars = 23 hr 56 min =
sidereal day
• One rotation with respect to
the Sun = 24 hours = solar
day
 Earth rotates
counterclockwise and also
orbits the Sun in the
counterclockwise direction.
Class Question
The Sun rises in the east and sets in the west.
Which way is Earth rotating?
A.
B.
C.
D.
Toward the west
Toward the north
Toward the east
Toward the south
Celestial Sphere
 The celestial
sphere
is a projection
of Earth’s axes
and equator
into space.
 The celestial
sphere
rotates around
the north and
south celestial
poles each
day.
Celestial Sphere: Celestial Equator and Ecliptic
 Celestial equator:
midway between the
two poles.
 Ecliptic: apparent path
of the Sun, inclined
23.5 degrees to
equator.
Celestial Sphere: Earth’s North Pole
 At Earth’s North Pole, we would see half of the
celestial sphere.
 The north celestial pole would be at the zenith, the
point directly overhead.
Celestial Sphere: Circumpolar
 No star rises or sets =>circumpolar.
 Observers never see the south celestial pole.
Celestial Sphere: Circumpolar (Cont.)
Celestial Sphere: Circumpolar (Cont.)
Celestial Sphere: Circumpolar (Cont.)
Class Question
From what location on Earth would all of the stars
on the celestial sphere be visible?
A.
B.
C.
D.
Equator
North Pole
Prime Meridian
South Pole
Celestial Sphere: Hemispheres
 At Earth’s equator, all stars rise and set.
 The celestial poles are on the northern and southern
horizons.
Class Question
What would the sky look like from a latitude
of 30° N?
A.
B.
C.
D.
All stars would be circumpolar.
All stars would be visible.
No stars would be visible.
Some stars would be circumpolar; some
would never be visible.
Celestial Sphere: Stars Visibility
 Some stars are circumpolar.
 Some stars are never visible.
Revolution of the Earth
 The average distance to the Sun is called the
astronomical unit, or AU.
 1 AU = 150 million km = 8.3 light-min
Revolution of the Earth: Zodiac
 As Earth revolves, the Sun is seen against different
constellations—the zodiac.
 The constellations are along the ecliptic.
Axial Tilt of the Earth
 Earth’s axis is tilted at an angle of 23.5°with respect
to the ecliptic plane.
Axial Tilt of the Earth: In Summer
The tilt causes the seasons. In summer:
 The angle of sunlight is more direct.
 Energy is more concentrated and days are longer.
Axial Tilt of the Earth: In Summer (Cont.)
Axial Tilt of the Earth: In Summer (Cont.)
Axial Tilt of the Earth: Summer and Autumnal Seasons
 Summer solstice:
• Sun is at its highest point
above the ecliptic (June 21).
• Longest day.
 Autumnal equinox:
• Sun on the celestial equator
(Sept. 22).
• Equal hours of day
and night.
Axial Tilt of the Earth: Winter and Vernal Seasons
 Winter solstice:
• Sun at its lowest point below
the ecliptic (December 21).
• Shortest day.
 Vernal equinox:
• Sun on the celestial equator
(March 20).
• Equal hours of day
and night.
Axial Tilt of the Earth: Winter and Vernal Seasons (Cont.)
Axial Tilt of the Earth: Winter and Vernal Seasons (Cont.)
Class Question
If the ecliptic were aligned with the celestial
equator, what would happen to the seasons?
A.
B.
C.
D.
Each season would last longer.
Nothing. They would be the same as now.
We would not have seasons at all.
Cannot tell from the information given.
Axial Tilt of the Earth: Changes Over Years
 Earth’s axial tilt changes
orientation over a period of
26,000 years due to
precession.
 The location of the poles
slowly shifts.
 Currently the north celestial
pole is near the bright star
Polaris.
Orbit of the Moon
 Synchronous
rotation.
 The Moon completes
one full rotation in
one full revolution
around Earth.
Orbit of the Moon: Moon From the Earth
 The Moon shines
because of reflected
sunlight.
 Half of the Moon
is always illuminated
by the Sun.
Phases of the Moon
The phase of the Moon is
determined by how much
of the bright side we see.
Phases of the Moon: New and Crescent Moon
 New Moon: Moon
between Earth and
the Sun.
 Crescent: Less than
half of the Moon is lit
up from the Earth’s
point of view.
Phases of the Moon: Quarter, Gibbous, and Full Moon
 Quarter: Moon at right
angles with Earth and
the Sun.
 Gibbous: More than
half of the Moon is lit
up from Earth’s point
of view.
 Full Moon: Moon
on opposite side
of Earth from the Sun.
Class Question
At what phase of the Moon is a solar eclipse
possible?
A.
B.
C.
D.
First Quarter
Full
New
Third Quarter
Eclipses: Solar Eclipse
 Solar eclipses happen at new Moon.
 Only a small portion of Earth can witness each one.
Eclipses: Solar Eclipse (Cont.)
Eclipses: Solar Eclipse (Cont.)
Eclipses: Types of Solar Eclipse
 Three types of solar
eclipses:
• Total: the Moon
completely blocks the Sun.
• Partial: only part of Sun is
blocked.
• Annular: the Sun appears
as a bright ring
surrounding the Moon.
Class Question
At what phase of the Moon is a lunar eclipse
possible?
A.
B.
C.
D.
First Quarter
Full
New
Third Quarter
Eclipses: Lunar Eclipse
 Lunar eclipses occur at full Moon.
 Visible over a wider area of Earth.
 Longer duration than solar eclipses.
Eclipses: Lunar Eclipse (Cont.)
Eclipses: Lunar Eclipse (Cont.)
Eclipses: Lunar Eclipse (Cont.)
Eclipses: Lunar Eclipse (Cont.)
Class Question
Do we observe eclipses every month?
A. Yes
B. No
Orbit of the Moon
 Eclipses do not occur
every month because
the Moon’s orbit is tilted
about 5 degrees with
respect to Earth’s orbit
around the Sun (the
ecliptic).
Class Question
If Earth rotated in 12 hours but its orbit were the
same, which of the following would NOT be true?
A. Each season would happen twice per year.
B. We would still see only one hemisphere of the
Moon.
C. The cycle of lunar phases would last as long
as it does now.
D. Observers at the North Pole would only see one
half of the celestial sphere.
Chapter Summary
 The daily patterns of rising and setting => Earth’s
rotation.
 The annual patterns of the stars and constellations in
the sky => Earth’s revolution.
 The changes in the length of day and intensity of
sunlight that create the seasons => Axial tilt of the
Earth.
 The phases of the Moon => Moon’s revolution around
the Earth.