Transcript 1-4 The Moon`s Phases 1. The rotation and

Courtesy of Hubble Space Telescope Comet Team and NASA
Chapter 1
Sections 1-4 thru 1-9
Courtesy of STScI/NASA
The Quest
Ahead
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1-4 The Moon’s Phases
1. The rotation and revolution period of the Moon are
equal and can be explained by the law of universal
gravitation.
2. Rotation is the spinning of an object about an axis that
passes through it.
3. Revolution is the orbiting of one object around
another.
4. Phases of the Moon—the changing appearance of the
Moon during its cycle—are caused by the relative
positions of the Earth, Moon, and Sun.
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Figure 1.24a: The Moon in various phases seen from above the
Earth’s North Pole.
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Figure 1.24b: Photos of the Moon in phases that correspond to the
positions shown in Fig. 1-24a. (A) The waxing crescent Moon.
(a-g) Photos courtesy of UCO/Lick Observatory
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5. The phases follow the sequence of waxing crescent,
first quarter, waxing gibbous, full Moon, waning
gibbous, third (or last) quarter, waning crescent, new
Moon.
6. Elongation is the angle of the Moon (or planet) from
the Sun in the sky.
7. A sidereal revolution of the Moon takes about 27 1/3
days.
8. A sidereal period is the amount of time required for
one revolution (or rotation) of a celestial object with
respect to the distant stars.
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Figure 1.25: Sidereal vs synodic period
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9. A synodic period is the time interval between
successive similar alignments of a celestial object
with respect to the Sun.
10. A synodic revolution of the Moon takes about 29 1/2
days.
11. Lunar month is the Moon’s synodic period, or the
time between successive phases: 29d12h44m2s.9.
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Question 1
What, ultimately, is the reason for the differences
between the Lunar sidereal and synodic periods?
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1-5 Lunar Eclipses
1. Lunar eclipse is an eclipse in which the Moon passes
into the shadow of the Earth.
2. Umbra is the portion of a shadow that receives no
direct light from the light source.
3. Penumbra is the portion of a shadow that receives
direct light from only part of the light source.
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4. Eclipse season is a time of the year during which a
solar or lunar eclipse is possible.
5. A lunar eclipse does not occur at each full Moon
because the Moon’s plane of revolution is tilted 5°
compared to the Earth’s plane of revolution around
the Sun. Only during the two eclipse seasons that
occur each year are the Earth and Moon positioned so
that the Moon will enter the Earth’s shadow during a
full Moon.
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Types of Lunar Eclipses
1. Penumbral lunar eclipse is an eclipse of the Moon in which
the Moon passes through the Earth’s penumbra but not
through its umbra.
2. Total lunar eclipse is an eclipse of the Moon in which the
Moon is completely in the umbra of the Earth’s shadow.
3. Partial lunar eclipse is an eclipse of the Moon in which only
part of the Moon passes through the umbra of the Earth’s
shadow.
4. A total eclipse of the Moon is never totally dark because some
light is refracted toward the Moon by the Earth’s atmosphere.
Most of this refracted light reaching the Moon is red; the blue
portion has been scattered out.
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Figure 1.26 abc: Total vs partial lunar eclipse
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Figure 1.31: Shadows of Moon on Earth
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Figure 1.29: Moon during eclipse
Photo by Jim Rouse
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1-6 Solar Eclipses
1. Solar eclipse is an eclipse of the Sun in which light
from the Sun is blocked by the Moon.
2. Total solar eclipse is an eclipse in which light from the
normally visible portion of the Sun (the photosphere)
is completely blocked by the Moon.
3. The corona—the outer atmosphere of the Sun—is
visible during a total solar eclipse.
Courtesy of Alex York
Figure 1.32: Sun at total eclipse
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The Partial Solar Eclipse
1. In a partial solar eclipse only part of the Sun’s disk is covered
by the Moon.
The Annular Eclipse
1. An annular eclipse is an eclipse in which the Moon is too far
from the Earth for its disk to cover that of the Sun completely,
so the outer edge of the Sun is seen as a ring or annulus.
Figure 1.34: When the Moon is far away during a solar eclipse, the eclipse will be annular.
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Figure 1.35a: During an annular eclipse, we can see the entire ring—
annulus—of the Sun around the Moon.
Courtesy of AURA/NOAO/NSF
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Question 2
Why do eclipses only occur so often, why not once a
month?
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1-7 Observations of Planetary Motion
1. Five planets are visible to the naked eye: Mercury,
Venus, Mars, Jupiter, Saturn.
2. The planets lack the simple, uniform motion of the Sun
and Moon. They sometimes stop their eastward
motion among the stars and move westward for a
while. This is called retrograde motion.
3. The planets always stay near the ecliptic. In addition,
Mercury and Venus never appear very far from the
position of the Sun in the sky. Thus their elongation
(the angle in the sky from an object to the Sun) is
small.
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Figure 1.36a: Mars' retrograde motion in 2003
Courtesy of NASA/JPL-Caltech
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Question 3
In your own words describe and explain retrograde
motion.
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1-8 Rotations
1. Solar day is the amount of time that elapses between
successive passages of the Sun across the meridian.
2. Meridian is an imaginary line that runs from north to
south, passing through the observer’s zenith.
3. Sidereal day is the amount of time that passes
between successive passages of a given star across
the meridian.
4. The Earth’s solar day and sidereal day differ by about
4 minutes.
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Figure 1.37: Solar vs sidereal day
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1-9 Units of Distance in Astronomy
1. To measure distances in a planetary system such as
ours we use the astronomical unit (AU), which is the
average distance between Earth and Sun.
2. For greater distances we use the light-year, the
distance light travels in one year.
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