25.1 Properties of Stars
Characteristics of Stars
 A constellation is an apparent group of
stars originally named for mythical
characters. The sky contains 88
constellations.
 Star Color and Temperature
• Color is a clue to a star’s temperature.
The Constellation Orion
25.1 Properties of Stars
Characteristics of Stars
 Binary Stars and Stellar Mass
• A binary star is one of two stars revolving
around a common center of mass under their
mutual gravitational attraction.
• Binary stars are used to determine the star
property most difficult to calculate—its mass.
Common Center of Mass
25.1 Properties of Stars
Measuring Distances to Stars
 Parallax
• Parallax is the slight shifting of the apparent
position of a star due to the orbital motion of
Earth.
• The nearest stars have the largest parallax
angles, while those of distant stars are too small
to measure.
 Light-Year
• A light-year is the distance light travels in a year,
about 9.5 trillion kilometers.
Parallax
Original Photo
Photo taken 6 months later
25.1 Properties of Stars
Stellar Brightness
 Apparent Magnitude
• Apparent magnitude is the brightness of a star
when viewed from Earth.
• Three factors control the apparent brightness of
a star as seen from Earth: how big it is, how hot
it is, and how far away it is.
 Absolute Magnitude
• Absolute magnitude is the apparent brightness
of a star if it were viewed from a distance of 32.6
light-years.
Distance, Apparent Magnitude, and
Absolute Magnitude of Some Stars
25.1 Properties of Stars
Hertzsprung–Russell Diagram
 A Hertzsprung–Russell diagram shows the
relationship between the absolute
magnitude and temperature of stars.
 A main-sequence star is a star that falls
into the main sequence category on the
H–R diagram. This category contains the
majority of stars and runs diagonally
from the upper left to the lower right on
the H–R diagram.
Hertzsprung–Russell Diagram
25.1 Properties of Stars
Hertzsprung–Russell Diagram
 A red giant is a large, cool star of high
luminosity; it occupies the upper-right
portion of the H–R diagram.
 A supergiant is a very large, very bright
red giant star.
25.1 Properties of Stars
Hertzsprung–Russell Diagram
 Variable Stars
• A Cepheid variable is a star whose brightness
varies periodically because it expands and
contracts; it is a type of pulsating star.
• A nova is a star that explosively increases in
brightness.
Images of a Nova Taken
Two Months Apart
25.1 Properties of Stars
Hertzsprung–Russell Diagram
 Interstellar Matter
• A nebula is a cloud of gas and/or dust in space.
• There are two major types of nebulae:
1. Bright nebula
- Emission nebula
- Reflection nebula
2. Dark nebula
Interstellar Matter
25.2 Stellar Evolution
Star Birth
 Protostar Stage
• A protostar is a collapsing cloud of gas and dust
destined to become a star—a developing star not
yet hot enough to engage in nuclear fusion.
• When the core of a protostar has reached about
10 million K, pressure within is so great that
nuclear fusion of hydrogen begins, and a star is
born.
Nebula, Birthplace of Stars
Balanced Forces
25.2 Stellar Evolution
Star Birth
 Main-Sequence Stage
• Stars age at different rates.
- Massive stars use fuel faster and exist for only
a few million years.
- Small stars use fuel slowly and exist for
perhaps hundreds of billions of years.
• A star spends 90 percent of its life in the
main-sequence stage.
25.2 Stellar Evolution
Star Birth
 Red-Giant Stage
• Hydrogen burning migrates outward. The star’s
outer envelope expands.
• Its surface cools and becomes red.
• The core collapses as helium is converted to
carbon. Eventually all nuclear fuel is used and
gravity squeezes the star.
25.2 Stellar Evolution
Burnout and Death
 All stars, regardless of their size, eventually
run out of fuel and collapse due to gravity.
 Death of Low-Mass Stars
• Stars less than one-half the mass of the sun
never evolve to the red giant stage but remain
in the stable main-sequence stage until they
consume all their hydrogen fuel and collapse
into a white dwarf.
25.2 Stellar Evolution
Burnout and Death
 Death of Medium-Mass Stars
• Stars with masses similar to the sun evolve in
essentially the same way as low-mass stars.
• During their collapse from red giants to white
dwarfs, medium-mass stars are thought to cast
off their bloated outer layer, creating an
expanding round cloud of gas called planetary
nebula.
Planetary Nebula
25.2 Stellar Evolution
Burnout and Death
 Death of Massive Stars
• In contrast to sunlike stars, stars that are over
three times the sun’s mass have relatively short
life spans, which end in a supernova event.
• A supernova is an exploding massive star that
increases in brightness many thousands of
times.
• The massive star’s interior condenses and may
produce a hot, dense object that is either a
neutron star or a black hole.
Crab Nebula in the
Constellation Taurus
Stellar Evolution
25.2 Stellar Evolution
Burnout and Death
 H–R Diagrams and Stellar Evolution
• Hertzsprung–Russell diagrams have been
helpful in formulating and testing models of
stellar evolution.
• They are also useful for illustrating the changes
that take place in an individual star during its life
span.
Life Cycle of a Sunlike Star
25.2 Stellar Evolution
Stellar Remnants
 White Dwarfs
• A white dwarf is a star that has exhausted most
or all of its nuclear fuel and has collapsed to a
very small size, believed to be near its final stage
of evolution.
• The sun begins as a nebula, spends much of its
life as a main-sequence star, and then becomes
a red giant, a planetary nebula, a white dwarf,
and, finally, a black dwarf.
Summary of Evolution for
Stars of Various Masses
25.2 Stellar Evolution
Stellar Remnants
 Neutron Stars
• A neutron star is a star of extremely high
density composed entirely of neutrons.
• Neutron stars are thought to be remnants of
supernova events.
 Supernovae
• A pulsar is a source that radiates short bursts or
pulses of radio energy in very regular periods.
• A pulsar found in the Crab Nebula during the
1970s is undoubtedly the remains of the
supernova of 1054.
Veil Nebula in the Constellation Cygnus
25.2 Stellar Evolution
Stellar Remnants
 Black Holes
• A black hole is a massive star that has collapsed
to such a small volume that its gravity prevents
the escape of everything, including light.
• Scientists think that as matter is pulled into a
black hole, it should become very hot and emit a
flood of X-rays before being pulled in.
Black Hole
25.3 The Universe
The Milky Way Galaxy
 A galaxy is a group of stars, dust, and
gases held together by gravity.
 Size of the Milky Way
• The Milky Way is a large spiral galaxy whose
disk is about 100,000 light-years wide and about
10,000 light-years thick at the nucleus.
 Structure of the Milky Way
• Radio telescopes reveal that the Milky Way has
at least three distinct spiral arms, with some
splintering.
Structure of the Milky Way
25.3 The Universe
Types of Galaxies
 Spiral Galaxies
• About 30 percent of all galaxies are spiral
galaxies.
• They have large diameters of 20,000 to 125,000
light-years and contain both young and old stars.
 Elliptical Galaxies
• About 60 percent of galaxies are classified as
elliptical galaxies.
• Elliptical galaxies range in shape from round to
oval.
Spiral Galaxies
Elliptical Galaxy
25.3 The Universe
Types of Galaxies
 Irregular Galaxies
• Only 10 percent of the known galaxies have irregular
shapes and are classified as irregular galaxies.
• In addition to shape and size, one of the major
differences among different types of galaxies is the
age of their stars. Irregular galaxies contain young
stars.
 Galaxy Clusters
• A galaxy cluster is a system of galaxies containing
several to thousands of member galaxies.
Irregular Galaxy
Galaxy Cluster
25.3 The Universe
The Expanding Universe
 Red Shifts
• Red shift, or a Doppler shift toward the red end
of the spectrum, occurs because the light waves
are “stretched,” which shows that Earth and the
source are moving away from each other.
• The red shifts of distant galaxies indicate that
the universe
25.3 The Universe
The Big Bang
 The big bang theory states that at one
time, the entire universe was confined to a
dense, hot, supermassive ball. Then, about
13.7 billion years ago, a violent explosion
occurred, hurling this material in all
directions.
The Big Bang
25.3 The Universe
The Big Bang
 Supporting Evidence
• The red shift of galaxies supports the big bang
and the expanding universe theories.
• Scientists discovered a type of energy called
cosmic background radiation. Scientists think
that this radiation was produced during the big
bang.
 Background radiation
• The left over ringing from the original explosion
“Big Bang”
25.3 The Universe
The Big Bang
 The Big Crunch?
• The future of the universe follows two possible
paths:
1. The universe will expand forever.
2. The outward expansion will stop and
gravitational contraction will follow.
• The view currently favored by most scientists is
an expanding universe with no ending point.
• It should be noted, however, that the methods
used to determine the ultimate fate of the
universe have substantial uncertainties.
Chapter
24
Studying the Sun
24.1 The Study of Light
Electromagnetic Radiation
 Electromagnetic radiation includes gamma
rays, X-rays, ultraviolet light, visible light,
infrared radiation, microwaves, and radio
waves.
 The electromagnetic spectrum is the
arrangement of electromagnetic radiation
according to wavelength.
Electromagnetic Spectrum
24.1 The Study of Light
Electromagnetic Radiation
 Nature of Light
• In some instances light behaves like waves, and
in others, like particles. In the wave sense, light
can be thought of as swells in the ocean. This
motion is characterized by a property known as
wavelength, which is the distance from one wave
crest to the next.
 Photons
• A photon is a small packet of light energy.
24.1 The Study of Light
Spectroscopy
 Spectroscopy is the study of the properties
of light that depend on wavelength.
 Continuous Spectrum
• A continuous spectrum is an uninterrupted
band of light emitted by an incandescent solid,
liquid, or gas under pressure.
24.1 The Study of Light
Spectroscopy
 Absorption Spectrum
• An absorption spectrum is a continuous
spectrum produced when white light passes
through a cool gas under low pressure. The gas
absorbs selected wavelengths of light, and the
spectrum looks like it has dark lines
superimposed.
24.1 The Study of Light
Spectroscopy
 Emission Spectrum
• An emission spectrum is a series of bright lines
of particular wavelengths produced by a hot gas
under low pressure.
• When the spectrum of a star is studied, the
spectral lines act as “fingerprints.” These lines
identify the elements present and thus the star’s
chemical composition.
Formation of Spectra
24.1 The Study of Light
The Doppler Effect
 The Doppler effect is the apparent change
in frequency of electromagnetic or sound
waves caused by the relative motions of the
source and the observer.
 In astronomy, the Doppler effect is used to
determine whether a star or other body in
space is moving away from or toward Earth.
The Doppler Effect
24.2 Tools for Studying Space
Refracting Telescopes
 A refracting telescope is a telescope that
uses a lens to bend or refract light.
 Focus
• The most important lens in a refracting
telescope, the objective lens, produces an
image by bending light from a distant object so
that the light converges at an area called the
focus (focus = central point).
Keck Telescope
Simple Refracting Telescope
24.2 Tools for Studying Space
Refracting Telescopes
 Chromatic Aberration
• A chromatic aberration is the property of a lens
whereby light of different colors is focused at
different places.
24.2 Tools for Studying Space
Reflecting Telescopes
 A reflecting telescope is a telescope that
reflects light off a concave mirror, focusing
the image in front of the mirror.
 Advantages of Reflecting Telescopes
• Most large optical telescopes are reflectors.
Light does not pass through a mirror, so the
glass for a reflecting telescope does not have to
be of optical quality.
Viewing Methods with
Reflecting Telescopes
24.2 Tools for Studying Space
Reflecting Telescopes
 Properties of Optical Telescopes
• Both refracting and reflecting telescopes have
three properties that aid astronomers in their
work:
1. Light-gathering power
2. Resolving power
3. Magnifying power
24.2 Tools for Studying Space
Detecting Invisible Radiation
 Radio Telescopes
• A radio telescope is a telescope designed to
make observations in radio wavelengths.
• A radio telescope focuses the incoming radio
waves on an antenna, which, just like a radio
antenna, absorbs and transmits these waves to
an amplifier.
Radio Telescopes
24.2 Tools for Studying Space
Detecting Invisible Radiation
 Advantages of Radio Telescopes
• Radio telescopes are much less affected by
turbulence in the atmosphere, clouds, and the
weather.
• No protective dome is required, which reduces
the cost of construction.
• Radio telescopes can “see” through interstellar
dust clouds that obscure visible wavelengths.
24.2 Tools for Studying Space
Space Telescopes
 Space telescopes orbit above Earth’s
atmosphere and thus produce clearer
images than Earth-based telescopes.
 Hubble Space Telescope
• The first space telescope, built by NASA, was
the Hubble Space Telescope. Hubble was put
into orbit around Earth in April 1990.
Hubble Space Telescope
24.2 Tools for Studying Space
Space Telescopes
 Other Space Telescopes
• To study X-rays, NASA uses the Chandra X-Ray
Observatory. This space telescope was launched
in 1999.
• Another space telescope, the Compton GammaRay Observatory, was used to study both visible
light and gamma rays.
• In 2011, NASA plans to launch the James Webb
Space Telescope to study infrared radiation.
Images of the Milky Way Galaxy
24.3 The Sun
Structure of the Sun
 Because the sun is made of gas, no sharp
boundaries exist between its various layers.
Keeping this in mind, we can divide the sun
into four parts: the solar interior; the visible
surface, or photosphere; and two
atmospheric layers, the chromosphere and
corona.
24.3 The Sun
Structure of the Sun
 Photosphere
• The photosphere is the region of the sun that radiates
energy to space, or the visible surface of the sun.
• It consists of a layer of incandescent gas less than 500
kilometers thick.
• It exhibits a grainy texture made up of many small,
bright markings, called granules, produced by
convection.
• Most of the elements found on Earth also occur on the
sun.
• Its temperature averages approximately 6000 K
(10,000ºF).
Structure of the Sun
24.3 The Sun
Structure of the Sun
 Chromosphere
• The chromosphere is the first layer of the solar
atmosphere found directly above the
photosphere.
• It is a relatively thin, hot layer of incandescent
gases a few thousand kilometers thick.
• Its top contains numerous spicules, which are
narrow jets of rising material.
Chromosphere
24.3 The Sun
Structure of the Sun
 Corona
• The corona is the outer, weak layer of the solar
atmosphere.
• The temperature at the top of the corona
exceeds 1 million K.
• Solar wind is a stream of protons and electrons
ejected at high speed from the solar corona.
24.3 The Sun
The Active Sun
 Sunspots
• A sunspot is a dark spot on the sun that is cool
in contrast to the surrounding photosphere.
• Sunspots appear dark because of their
temperature, which is about 1500 K less than
that of the surrounding solar surface.
Sunspots
24.3 The Sun
The Active Sun
 Prominences
• Prominences are huge cloudlike structures
consisting of chromospheric gases.
• Prominences are ionized gases trapped by
magnetic fields that extend from regions of
intense solar activity.
Solar Prominence
24.3 The Sun
The Active Sun
 Solar Flares
• Solar flares are brief outbursts that normally
last about an hour and appear as a sudden
brightening of the region above a sunspot
cluster.
• During their existence, solar flares release
enormous amounts of energy, much of it in the
form of ultraviolet, radio, and X-ray radiation.
• Auroras, the result of solar flares, are bright
displays of ever-changing light caused by solar
radiation interacting with the upper atmosphere
in the region of the poles.
Aurora Borealis
24.3 The Sun
The Solar Interior
 Nuclear Fusion
• Nuclear fusion is the way that the sun produces
energy. This reaction converts four hydrogen
nuclei into the nucleus of a helium atom,
releasing a tremendous amount of energy.
• During nuclear fusion, energy is released
because some matter is actually converted to
energy.
• It is thought that a star the size of the sun can
exist in its present stable state for 10 billion
years. As the sun is already 4.5 billion years
old, it is “middle-aged.”
Nuclear Fusion
23.1 The Solar System
The Planets: An Overview
 The terrestrial planets are planets that are
small and rocky—Mercury, Venus, Earth,
and Mars.
 The Jovian planets are the huge gas
giants—Jupiter, Saturn, Uranus, and
Neptune.
 Pluto does not fit into either the Jovian or
the terrestrial category.
Orbits of the Planets
23.1 The Solar System
The Planets: An Overview
 Size is the most obvious difference
between the terrestrial and Jovian planets.
 Density, chemical makeup, and rate of
rotation are other ways in which the two
groups of planets differ.
Planetary Data
23.1 The Solar System
The Planets: An Overview
 The Interiors of the Planets
• The substances that make up the planets are
divided into three groups: gases, rocks, and ices.
 The Atmosphere of the Planets
• The Jovian planets have very thick atmospheres
of hydrogen, helium, methane, and ammonia.
• By contrast, the terrestrial planets, including
Earth, have meager atmospheres at best.
Scale of the Planets
23.1 The Solar System
Formation of the Solar System
 Nebular Theory
• A nebula is a cloud of gas and/or dust in space.
• According to the nebular theory, the sun and
planets formed from a rotating disk of dust and
gases.
23.1 The Solar System
Formation of the Solar System
 Planetesimals
• Planetesimals are small, irregularly shaped
bodies formed by colliding matter.
Formation of the Universe
Planetary Composition, Distance
from the Sun, and Melting Point
23.2 The Terrestrial Planets
Mercury: The Innermost Planet
 Mercury is the innermost and second
smallest planet; it is hardly larger than
Earth’s moon.
 Surface Features
• Mercury has cratered highlands, much like the
moon, and vast smooth terrains that resemble
maria.
 Surface Temperatures
• Mercury has the greatest temperature extremes
of any planet.
Mercury’s Surface
23.2 The Terrestrial Planets
Venus: The Veiled Planet
 Surface Temperatures
• The surface temperature of Venus reaches
475oC, and its atmosphere is 97 percent
carbon dioxide.
23.2 The Terrestrial Planets
Venus: The Veiled Planet
 Venus is similar to Earth in size, density,
mass, and location in the solar system.
Thus, it has been referred to as “Earth’s
twin.”
 Surface Features
• Venus is covered in thick clouds that visible light
cannot penetrate.
• About 80 percent of Venus’s surface consists of
plains covered by volcanic flow.
Venus
23.2 The Terrestrial Planets
Mars: The Red Planet
 The Martian Atmosphere
• The Martian atmosphere has only 1 percent of
the density of Earth’s.
• Although the atmosphere of Mars is very thin,
extensive dust storms occur and may cause the
color changes observed from Earth.
 Surface Features
• Most Martian surface features are old by Earth
standards. The highly cratered southern
hemisphere is probably 3.5 billion to 4.5 billion
years old.
Mars
23.2 The Terrestrial Planets
Mars: The Red Planet
 Water on Mars
• Some areas of Mars exhibit drainage patterns
similar to those created by streams on Earth.
• Images from the Mars Global Surveyor indicate
that groundwater has recently migrated to the
surface.
Water on Mars
23.3 The Outer Planets
Jupiter: Giant Among Planets
 Jupiter has a mass that is 2 1/2 times
greater than the mass of all the other
planets and moons combined.
 Structure of Jupiter
• Jupiter’s hydrogen-helium atmosphere also
contains small amounts of methane, ammonia,
water, and sulfur compounds.
Jupiter and the Great Red Spot
23.3 The Outer Planets
Jupiter: Giant Among Planets
 Jupiter’s Moons
• Jupiter’s satellite system, including the 28 moons
discovered so far, resembles a miniature solar
system.
 Jupiter’s Rings
• Jupiter’s ring system was one of the most
unexpected discoveries made by Voyager 1.
Jupiter’s Largest Moons
23.3 The Outer Planets
Saturn: The Elegant Planet
 The most prominent feature of Saturn is its
system of rings.
 Features of Saturn
• Saturn’s atmosphere is very active, with winds
roaring at up to 1500 kilometers per hour.
• Large cyclonic “storms” similar to Jupiter’s Great
Red Spot, although smaller, occur in Saturn’s
atmosphere.
Cassini Approaching Saturn
23.3 The Outer Planets
Saturn: The Elegant Planet
 Saturn’s Rings
• Until the discovery that Jupiter, Uranus, and
Neptune have ring systems, this phenomenon
was thought to be unique to Saturn.
• Most rings fall into one of two categories based
on particle density.
 Saturn’s Moons
• Saturn’s satellite system consists of 31 moons.
• Titan is the largest moon, and it is bigger than
Mercury.
Saturn’s Rings
23.3 The Outer Planets
Uranus: The Sideways Planet
 Instead of being generally perpendicular to
the plane of its orbit like the other planets,
Uranus’s axis of rotation lies nearly parallel
with the plane of its orbit.
Uranus
23.3 The Outer Planets
Neptune: The Windy Planet
 Winds exceeding 1000 kilometers per hour
encircle Neptune, making it one of the
windiest places in the solar system.
Neptune
23.3 The Outer Planets
Pluto: Planet X
 Pluto’s orbit is highly eccentric, causing it
to occasionally travel inside the orbit of
Neptune, where it resided from 1979
through February 1999.
23.4 Minor Members of the Solar System
Asteroids: Microplanets
 An asteroid is a small, rocky body whose
diameter can range from a few hundred
kilometers to less than a kilometer.
 Most asteroids lie between the orbits of
Mars and Jupiter. They have orbital periods
of three to six years.
Irregular Orbits of Asteroids
23.4 Minor Members of the Solar System
Comets
 Comets are small bodies made of rocky
and metallic pieces held together by frozen
gases. Comets generally revolve about the
sun in elongated orbits.
23.4 Minor Members of the Solar System
Comets
 Coma
• A coma is the fuzzy, gaseous component of a
comet’s head.
• A small glowing nucleus with a diameter of only
a few kilometers can sometimes be detected
within a coma. As comets approach the sun,
some, but not all, develop a tail that extends for
millions of kilometers.
Comet’s Tail Points Away from the Sun
23.4 Minor Members of the Solar System
Comets
 Kuiper Belt
• Like the asteroids in the inner solar system, most
Kuiper belt comets move in nearly circular orbits
that lie roughly in the same plane as the planets.
 Oort Cloud
• Comets with long orbital periods appear to be
distributed in all directions from the sun, forming
a spherical shell around the solar system called
the Oort cloud.
23.4 Minor Members of the Solar System
Comets
 Halley’s Comet
• The most famous short-period comet is Halley’s
comet. Its orbital period is 76 years.
23.4 Minor Members of the Solar System
Meteoroids
 A meteoroid is a small, solid particle that
travels through space.
 A meteor is the luminous phenomenon
observed when a meteoroid enters Earth’s
atmosphere and burns up, popularly called
a shooting star.
 A meteorite is any portion of a meteoroid
that reaches Earth’s surface.
23.4 Minor Members of the Solar System
Meteoroids
 Most meteoroids originate from any one of
the following three sources: (1)
interplanetary debris that was not
gravitationally swept up by the planets
during the formation of the solar system, (2)
material from the asteroid belt, or (3) the
solid remains of comets that once traveled
near Earth’s orbit.
Major Meteor Showers
22.1 Early Astronomy
The Birth of Modern Astronomy
 Nicolaus Copernicus
• Copernicus concluded that Earth is a planet. He
proposed a model of the solar system with the
sun at the center.
22.1 Early Astronomy
The Birth of Modern Astronomy
 Johannes Kepler
• Kepler discovered three laws of planetary motion:
1. Orbits of the planets are elliptical.
2. Planets revolve around the sun at varying
speed.
3. There is a proportional relationship between
a planet’s orbital period and its distance to
the sun.
22.1 Early Astronomy
The Birth of Modern Astronomy
 Johannes Kepler
• An ellipse is an oval-shaped path.
• An astronomical unit (AU) is the average
distance between Earth and the sun; it is about
150 million kilometers.
Planet Revolution
22.1 Early Astronomy
The Birth of Modern Astronomy
 Galileo Galilei
• Galileo’s most important contributions were his
descriptions of the behavior of moving objects.
• He developed his own telescope and made
important discoveries:
1. Four satellites, or moons, orbit Jupiter.
2. Planets are circular disks, not just points of light.
3. Venus has phases just like the moon.
4. The moon’s surface is not smooth.
5. The sun has sunspots, or dark regions.
Gravity’s Influence on Orbits
22.1 Early Astronomy
The Birth of Modern Astronomy
 Sir Isaac Newton
• Although others had theorized the existence of
gravitational force, Newton was the first to
formulate and test the law of universal
gravitation.
 Universal Gravitation
• Gravitational force decreases with distance.
• The greater the mass of an object, the greater is
its gravitational force.
22.2 The Earth–Moon–Sun System
Motions of Earth
 The two main motions of Earth are rotation
and revolution. Precession is a third and
very slow motion of Earth’s axis.
Stonehenge, an Ancient Observatory
22.2 The Earth–Moon–Sun System
Motions of Earth
 Rotation
• Rotation is the turning, or spinning, of a body on
its axis.
• Two measurements for rotation:
1. Mean solar day is the time interval from one
noon to the next, about 24 hours.
2. Sidereal day is 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.
Sidereal Day
22.2 The Earth–Moon–Sun System
Motions of Earth
 Revolution
• Revolution is the motion of a body, such as a
planet or moon, along a path around some point
in space.
• Perihelion is the time in January when Earth is
closest to the sun.
• Aphelion is the time in July when Earth is
farthest from the sun.
22.2 The Earth–Moon–Sun System
Motions of Earth
 Earth’s Axis and Seasons
• The plane of the ecliptic is an imaginary plane
that connects Earth’s orbit with the celestial
sphere.
• Because of the inclination of Earth’s axis to the
plane of the ecliptic, Earth has its yearly cycle of
seasons.
22.2 The Earth–Moon–Sun System
Motions of the Earth–Moon System
 Perigee is the point at which the moon is
closest to Earth.
 Apogee is the point at which the moon is
farthest from Earth.
22.2 The Earth–Moon–Sun System
Motions of the Earth–Moon System
 Phases of the Moon
• The phases of the moon are the progression of
changes in the moon’s appearance during the
month.
• Lunar phases are a result of the motion of the
moon and the sunlight that is reflected from its
surface.
Phases of the Moon
22.2 The Earth–Moon–Sun System
Motions of the Earth–Moon System
 Lunar Motions
• The synodic month is based on the cycle of the
moon’s phases. It lasts 29 1/2 days.
• The sidereal month is the true period of the
moon’s revolution around Earth. It lasts 27 1/3
days.
22.2 The Earth–Moon–Sun System
Motions of the Earth–Moon System
 Lunar Motions
• 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.
• The moon’s period of rotation about its axis and
its revolution around Earth are the same, 27 1/3
days. It causes the same lunar hemisphere to
always face Earth.
Lunar Motions
22.2 The Earth–Moon–Sun System
Eclipses
 Solar eclipses occur when the moon
moves in a line directly between Earth and
the sun, casting a shadow on Earth.
 Lunar eclipses occur when the moon
passes through Earth’s shadow.
 During a new-moon or full-moon phase,
the moon’s orbit must cross the plane of
the ecliptic for an eclipse to take place.
Solar Eclipse
Lunar Eclipse
22.3 Earth’s Moon
The Lunar Surface
 Craters
• A crater is the depression at the summit of a
volcano or a depression produced by a
meteorite impact.
• Most craters were produced by the impact of
rapidly moving debris.
• Rays are any of a system of bright, elongated
streaks, sometimes associated with a crater on
the moon.
The Moon’s Surface
Mare Imbrium
(Sea of Rains)
Kepler
Crater
Copernicus
Crater
Mare Tranquillitatus
(Sea of Tranquility)
Formation of a Crater
22.3 Earth’s Moon
The Lunar Surface
 Highlands
• Most of the lunar surface is made up of densely
pitted, light-colored areas known as highlands.
 Maria
• Maria, ancient beds of basaltic lava, originated
when asteroids punctured the lunar surface,
letting magma bleed out.
• A rille is a long channel associated with lunar
maria. A rille looks similar to a valley or a trench.
22.3 Earth’s Moon
The Lunar Surface
 Regolith
• The lunar regolith is a thin, gray layer on the
surface of the moon, consisting of loosely
compacted, fragmented material believed to
have been formed by repeated impacts of
meteorites.
Major Topographic Features of the Moon
22.3 Earth’s Moon
Lunar History
 The most widely accepted model for the
origin of the moon is that when the solar
system was forming, a body the size of
Mars impacted Earth. The resulting debris
was ejected into space, began orbiting
around Earth, and eventually united to form
the moon.
Formation of Earth’s Moon