lecture outlines powerpoint chapter 21 tarbuck/lutgens€¦ · motions of the earth-moon system...
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Lecture Outlines
PowerPoint
Chapter 21
Earth Science 11e
Tarbuck/Lutgens
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Earth Science, 11e
Origin of Modern
Astronomy
Chapter 21
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Early history of astronomy
Ancient Greeks
• Used philosophical arguments to explain
natural phenomena
• Also used some observational data
• Most ancient Greeks held a geocentric (Earth-
centered) view of the universe
• "Earth-centered" view
• Earth was a motionless sphere at the center of
the universe
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Early history of astronomy
Ancient Greeks
• Most ancient Greeks held a geocentric (Earth-
centered) view of the universe
• "Earth-centered" view
• Stars were on the celestial sphere
• Transparent, hollow sphere
• Celestial sphere turns daily around Earth
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Early history of astronomy
Ancient Greeks
• Most ancient Greeks held a geocentric (Earth-
centered) view of the universe
• Seven heavenly bodies (planetai)
• Changed position in sky
• The seven wanderers included the
• Sun
• Moon
• Mercury through Saturn (excluding Earth)
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Early history of astronomy
Ancient Greeks
• Aristarchus (312-230 B.C.) was the first Greek
to profess a Sun-centered, or heliocentric,
universe
• Planets exhibit an apparent westward drift
• Called retrograde motion
• Occurs as Earth, with its faster orbital speed,
overtakes another planet
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Early history of astronomy
Ancient Greeks
• Ptolemaic system
• A.D. 141
• Geocentric model
• To explain retrograde motion, Ptolemy used two
motions for the planets
• Large orbital circles, called deferents, and
• Small circles, called epicycles
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The universe according to
Ptolemy, second century A.D.
Figure 21.4 A
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Retrograde motion as
explained by Ptolemy
Figure 21.4 B
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Early history of astronomy
Birth of modern astronomy
• 1500s and 1600s
• Five noted scientists
• Nicolaus Copernicus (1473-1543)
• Concluded Earth was a planet
• Constructed a model of the solar system that put
the Sun at the center, but he used circular orbits
for the planets
• Ushered out old astronomy
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Early history of astronomy
Birth of modern astronomy
• Five noted scientists
• Tycho Brahe (1546-1601)
• Precise observer
• Tried to find stellar parallax – the apparent shift
in a star's position due to the revolution of Earth
• Did not believe in the Copernican system
because he was unable to observe stellar parallax
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Early history of astronomy
Birth of modern astronomy
• Five noted scientists
• Johannes Kepler (1571-1630)
• Ushered in new astronomy
• Planets revolve around the Sun
• Three laws of planetary motion
• Orbits of the planets are elliptical
• Planets revolve around the Sun at varying
speed
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Kepler’s law of equal areas
Figure 21.10
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Early history of astronomy
Birth of modern astronomy
• Five noted scientists
• Johannes Kepler (1571-1630)
• Three laws of planetary motion
• There is a proportional relation between a
planet's orbital period and its distance to the
Sun (measured in astronomical units (AU’s)
– one AU averages about 150 million
kilometers, or 93 million miles)
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Early history of astronomy
Birth of modern astronomy
• Five noted scientists
• Galileo Galilei (1564-1642)
• Supported Copernican theory
• Used experimental data
• Constructed an astronomical telescope in 1609
• Four large moons of Jupiter
• Planets appeared as disks
• Phases of Venus
• Features on the Moon
• Sunspots
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Early history of astronomy
Birth of modern astronomy
• Five noted scientists
• Sir Isaac Newton (1643-1727)
• Law of universal gravitation
• Proved that the force of gravity, combined with
the tendency of a planet to remain in straight-
line motion, results in the elliptical orbits
discovered by Kepler
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Constellations
Configuration of stars named in honor of mythological characters or great heroes
Today 88 constellations are recognized
Constellations divide the sky into units, like state boundaries in the United States
The brightest stars in a constellation are identified in order of their brightness by the letters of the Greek alphabet – alpha, beta, and so on
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Positions in the sky
Stars appear to be fixed on a spherical shell (the celestial sphere) that surrounds Earth
Equatorial system of location
• A coordinate system that divides the celestial sphere
• Similar to the latitude-longitude system that is used on Earth's surface
• Two locational components
• Declination – the angular distance north or south of the celestial equator
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Positions in the sky
Equatorial system of location
• Two locational components
• Right ascension – the angular distance measured
eastward along the celestial equator from the
position of the vernal equinox
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Astronomical coordinate system
on the celestial sphere
Figure 21.16
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Earth motions
Two primary motions
• Rotation
• Turning, or spinning, of a body on its axis
• Two measurements for rotation
• Mean solar day – the time interval from one
noon to the next, about 24 hours
• Sidereal day – the time it takes for Earth to make
one complete rotation (360º) with respect to a
star other than the Sun – 23 hours, 56 minutes, 4
seconds
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The difference between a solar
day and a sidereal day
Figure 21.19
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Earth motions
Two primary motions
• Revolution
• The motion of a body, such as a planet or moon,
along a path around some point in space
• Earth's orbit is elliptical
• Earth is closest to the Sun (perihelion) in
January
• Earth is farthest from the Sun (aphelion) in July
• The plane of the ecliptic is an imaginary plane that
connects Earth's orbit with the celestial sphere
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Earth motions
Other Earth motions
• Precession
• Very slow Earth movement
• Direction in which Earth's axis points continually
changes
• Movement with the solar system in the
direction of the star Vega
• Revolution with the Sun around the galaxy
• Movement with the galaxy within the universe
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Precession of Earth
Figure 21.22
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Motions of the Earth-Moon
system Phases of the Moon
• When viewed from above the North Pole, the
Moon orbits Earth in a counterclockwise
(eastward) direction
• The relative positions of the Sun, Earth, and
Moon constantly change
• Lunar phases are a consequence of the motion
of the Moon and the sunlight that is reflected
from its surface
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Phases of the Moon
Figure 21.23
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Motions of the Earth-Moon
system Lunar motions
• Earth-Moon
• Synodic month
• Cycle of the phases
• Takes 29 1/2 days
• Sidereal month
• True period of the Moon's revolution around
Earth
• Takes 27 1/3 days
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The difference between the sidereal
month and the synodic month
Figure 21.24
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Motions of the Earth-Moon
system Lunar motions
• Earth-Moon
• The difference of two days between the synodic and
sidereal cycles is due to the Earth-Moon system also
moving in an orbit around the Sun
• Moon's period of rotation about its axis and its
revolution around Earth are the same, 27 1/3
days
• Causes the same lunar hemisphere to always face
Earth
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Motions of the Earth-Moon
system Eclipses
• Simply shadow effects that were first
understood by the early Greeks
• Two types of eclipses
• Solar eclipse
• Moon moves in a line directly between Earth
and the Sun
• Can only occur during the new-Moon phase
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Solar eclipse
Figure 21.25
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Motions of the Earth-Moon
system Eclipses
• Two types of eclipses
• Lunar eclipse
• Moon moves within the shadow of Earth
• Only occurs during the full-Moon phase
• For any eclipse to take place, the Moon must be
in the plane of the ecliptic at the time of new- or
full-Moon
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Motions of the Earth-Moon
system Eclipses
• Two types of eclipses
• Lunar eclipse
• Because the Moon's orbit is inclined about 5
degrees to the plane of the ecliptic, during most
of the times of new- and full-Moon the Moon is
above or below the plane, and no eclipse can
occur
• The usual number of eclipses is four per year
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Lunar eclipse
Figure 21.26
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End of Chapter 21