Earth’s Moon
and Solar System
Phases of the Moon
The apparent shape of the moon depends
upon the changing relative positions of Earth
 As the moon completes one revolution
around Earth each month, the growing and
shrinking lighted area makes the moon
appear to change in shape
 The moon’s periods of rotation and
revolution are equal


Because of this, the same side of the moon
always faces Earth
Phases of the Moon
One complete orbit of the moon around
Earth takes about 27 days
 A complete cycle of the moon’s phases
takes 29 ½ days

This 2 ½ day difference occurs because as the
moon orbits Earth, Earth orbits the sun
 When the moon gets back to its original
position, it must move through an extra angle of
about 30° to compensate for Earth’s orbital
motion around the sun

Eclipse of the Moon
An eclipse of the moon occurs when the full
moon moves into Earth’s shadow
 During a lunar eclipse, the moon turns a
coppery red


You can still see the
moon because sunlight
is bent by Earth’s
atmosphere, which
causes a weak
illumination of the moon
Eclipse of the Sun
An eclipse of the sun occurs when the new
moon briefly moves in front of the sun
 At this time the
moon casts its
shadow on Earth

Angular Diameter
Angular diameter is the angle formed
between the sides of an object and your eye
 The angular diameter of any object depends
upon the actual
size of the
object and
how far away
it is from
the observer

Angular Diameter of the Sun
Observations of the sun’s angular diameter
tell us that Earth is closest to the sun in
January and farthest from the sun in July
 Seasonal variations result from the tilt of the
Earth’s axis and Earth’s shape not from
Earth-sun distance
 The sun’s angular diameter is larger in
winter and smaller in summer

Angular Size and Shape of Orbit
Because the moon seems to change size
more than the sun, we can infer that changes
in the relative distance between the moon
and Earth are greater than changes in the
relative distance between the sun and Earth
 Since both changes are small compared to
the magnitude of the average distance, we
can infer that the orbit of the moon around
Earth and the orbit of Earth around the sun
are nearly circular

Earth-Moon Orbit
The Tides
Every point along the ocean experiences
two low tides and two high tides per day
 The difference between high tide and low
tide is usually less than a meter (3 feet)
 The cause of tides is gravitational attraction
off the moon and the sun


The sun and moon pull on the water in the
oceans and on the solid part of Earth
The Tides
The water of the oceans is pulled toward
the moon, which causes high tide
 Another high tide occurs on the opposite
side of Earth, where the solid part of Earth
is pulled away from the oceans
 The highest high tides and the lowest low
tides occur about twice a month near the
full and new moon phases

The Tides
The Geometry of Orbits

Planets revolve in an ellipse around the sun
An ellipse has two fixed points called foci that
are on either side of the center of the axis
 The sun lies at one focus and is not the center
of Earth’s orbit

The Geometry of Orbits

If the two foci are located
near the ends of the axis, an
ellipse is long and narrow
 Many
comets have
this type of path

If the foci move closer
together, the shape of the
ellipse becomes circular
Calculating Eccentricity
(elongation) of an ellipse
Eccentricity = distance between the foci
length of the major axis
e = d/L
The Force of Gravity

Gravity is a force of attraction between
objects that is dependent on the masses
of the objects and the distance between
them
Gravity and the Planets
Gravity is the force that holds the planets
and other objects in the solar system in
their orbits
 Any object that orbits another object in
space is known as a satellite

Earth is a satellite of the sun
 The moon is Earth’s satellite

Gravity and the Planets

The elliptical path
of any satellite is
a result of inertia
and gravity

Inertia is the
tendency of an
object to remain
at rest, or, if it is
moving, to move
with the same
speed in the
same direction
Gravity and the Planets

If a satellite has a circular orbit, inertia and
the force of gravity are constant

There is no change in speed, but there is a
constant change in direction, producing a
circular path
Gravity and the Planets

If a satellite has
an elliptical orbit,
gravity causes the
speed to change

The satellite will
move faster when
it’s near its
primary and
slower when it’s
farther away
Gravity and the Planets

The closer a planet is to the sun, the faster
it moves in its orbit

Mercury, the planet closest to the sun, travels
about 1.6 times as fast as Earth and 10 times
the speed of Pluto
Planets of our Solar System

The planets can be divided into two groups
 Rocky
(terrestrial) planets
 Mercury
 Venus
 Earth
 Gas
(density = 5.5 g/cm3)
giants
 Jupiter
 Saturn
 Uranus
 Neptune
Planets of our Solar System
Similarities Between
Mercury and Earth’s Moon

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
Both the moon and Mercury are smaller than Earth
Both have a dark surface covered with craters from
meteorites
Neither have a significant atmosphere


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They are not protected from meteor impacts and their
craters do not erode quickly
Soil samples would show no chemical weathering
They both have extreme temperatures
Both have very slow rotations making days and
nights longer than on Earth
Planet Surface Temperatures

Are a result of the distance from the Sun



Venus is a little hotter due to a very dense
atmosphere of carbon dioxide producing a
“greenhouse effect”
Earth and Mars are cooler because of the greater
distance from the sun


Hottest Planets are Mercury and Venus
Mars has a very thin atmosphere of mostly carbon
dioxide with minimal “greenhouse effect”
The gas giant planets do not have “surface
temperatures” as they are composed of gases
that increase in density with depth and pressure
Earth is Unique
Earth is the only planet that has abundant
liquid water
 The presence of liquid water on Earth may
be the reason why living organisms have not
been detected elsewhere in the solar system
 The Earth’s atmosphere is the only planet
that has an atmosphere with abundant free
oxygen that is released when plants extract
carbon from carbon dioxide by
photosynthesis

Asteroids
Located mostly between Mars and Jupiter
in a belt of thousands of rocky objects
 They range from the size of pebbles to
600 miles in diameter
 A few have orbits that
can cross Earth’s orbit

Meteors
Small solid particles from
space can be caught by
Earth’s gravity and dragged
down through the atmosphere
 As the objects fall, they are heated by
friction with the Earth atmosphere and
burn up, producing streaks of light
(“shooting stars”) visible at night
 Meteors that survive their fall and hit the
ground are called meteorites

Comets
Icy objects which usually originate in a region
outside of the planets
 Some of them come close to the sun in very
elliptical orbits



Heating by the sun causes them to partially
vaporize producing a tail
Comets are visible for
weeks and do not
streak across the
night sky
The Sun

The nearest star to Earth

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A star is a large, self-luminous body in space that
creates its own energy
The sun gets its energy from nuclear fusion
Dark spots on the sun’s surface are known as
sunspots


Sunspots are temporary
storms visible on the
surface of the sun
Sunspots come
and go in cycles
of about 11 years
Classifying Stars
The Hertzprung-Russell diagram is used
to classify stars by temperature and size
 Our sun is a fairly typical star
 Although the sun Is brighter than most of
the nearest stars, it is small compared with
most of the stars we see at night

Galaxies
A galaxy is a huge body of stars and other
matter in space
 Our own galaxy is
called the Milky Way
named for its
faint white color
 The sun is one
of about 100 billion
stars in the
Milky Way

The Milky Way

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The Milky Way is a spiral galaxy
Our solar system is
located in a spiral arm
well away from the
galactic center
The Earth and sun and
other nearby stars orbit
around the center of the
Milky Way galaxy

It takes about 220 million
years to complete this
revolution
Spectroscopes
The light given off by stars is marked by
dark lines in certain colors
 A spectroscope is an instrument that
separates light into its component colors
 Since stars are primarily hydrogen and
helium, the lines
we usually see
are in the orange,
yellow, green
and blue areas

Edwin Hubble


In the early part of the 20th century, Edwin Hubble
discovered that light that reached Earth from
distant galaxies shows special lines that are
shifted toward the red end of the spectrum
He suggested that the red-shifted lines are
evidence that distant galaxies are moving away
from us

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Observations of distant
galaxies in all directions
showed the red shift
The more distant the
galaxy, the greater the
red shift
Evolution of the Universe

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The red-shift and other observations led scientists
to the conclusion that the universe is expanding
Computer models that reverse the expansion, lead
to the idea that at one time the universe was a
concentrated object of incredible mass and density
that exploded.
This theory of the origin of the universe is known as
the “big bang”


Scientists can detect radiation
remaining from the big bang
Scientists currently believe the universe
is about 10 to 15 billion years old
The Size of the Universe

The distance light can travel in one year is called
a light-year, which is about 10 trillion kilometers
Light could circle Earth seven times in one second
 Light takes about one and a half seconds to get to
the moon
 Light from the sun takes about eight minutes to
reach earth
 Light from the nearest star (not our sun) takes
about four years to reach us


The universe is thought to be about 25 billion
light-years in diameter
The Future of the Universe
Some astronomers think that the expansion
of the universe will continue forever
 Some astronomers believe the force of
gravity will eventually reverse the expansion
and the universe will fall back together in the
“big crunch”
 Some astronomers think it is possible that
the universe will pulsate between explosions
and contractions
