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Earth Science
Chapter 9
Earth in Space and Beyond
Earth in the Universe
• Because the Universe is so large, the
speed of light (186,000 miles/sec) is used
to measure distances in space.
– A light year is the distance light travels in 1
year (6 trillion miles).
– Polaris is about 300 light years away.
– The farthest known star system is about 12
billion light years away.
Earth in the Universe
• A galaxy is a system of billions of stars.
– Three types of galaxies:
• Spiral galaxies - shaped like pinwheels with
huge spiral arms.
• Elliptical galaxies – look like blobs with no
spirals and can be circular or flattened.
• Irregular galaxies – have no definite shape.
Earth in the Universe
Earth in the Universe
• Milky Way Galaxy
– Spiral galaxy rotating counterclockwise
– 230 million years per revolution around center
– 100,000 light years across
– 15,000 light years thick
– Sun is on one arm 30,000 light years from
nucleus
Earth in the Universe
Earth in the Universe
• Sun
– Mass is about 33,000 times Earth’s
– Diameter is about 109 times Earth’s
– Temp is about 6,000 K at surface and at
surface and 12 million K at center
– Average star (mass, diameter, and density)
– Composed primarily of hydrogen and helium
– About 5 billion years old (5 billion years left)
– Sunspots are regions of cooler gas at surface
The Solar System
• Solar system – includes Sun, 8 planets,
many moons & all that revolves around Sun
– Formed about 5 billion years ago from a giant
cloud of gas and debris
– The type of planet formed depends on conditions
such as temp. and substances (rock fragments,
ice crystals, gases) which exist at such distance.
– Gravity caused planets to be layered according
to density differences in their materials.
The Solar System
• The characteristics of planets are affected
by each planets distance from the Sun.
– Terrestrial planets - planets made primarily of
rock formed near the Sun where its heat
drove off ices & gases.
• Mercury, Mars, Earth, and Venus
• Small dense planets which suggests that
they primarily solid
• Hard rocky surface with craters, canyons,
and volcanoes
The Solar System
– Jovian planets - far from the Sun, where
temps. are low, planets retained volatile
substances resulting in planets made
primarily of gas (gas giants).
• Jupiter, Saturn, Uranus, and Neptune
• Large very low density planets which
suggests they are primarily gas
• Solid cores buried beneath atmospheres
tens of thousands of kilometers thick.
Origin and Fate of the Universe
• Doppler effect – a change of light or sound
wavelengths as a source moves towards or
away from the observer.
– Light waves speeding away through space will
stretch into longer wavelengths (red-shift).
– Light waves speeding toward us will be
squeezed into shorter wavelengths (blue-shift).
– The red-shift is proof that the universe is
expanding as all galaxies display the red-shift
and are therefore moving away from us.
Deep Space Phenomena
• Big Bang – powerful explosion of an
incredibly dense mass that produced the
expanding Universe that exists today.
– Occurred 15 – 20 billion years ago
– All matter in the Universe was concentrated in
a single atom.
– After explosion clouds of hydrogen and
helium began to form, which led to the
beginnings of galaxies and stars.
Deep Space Phenomena
Deep Space Phenomena
• Formation of stars
– Stars begin as nebula (huge masses of dust
and hydrogen gas).
– A nebula is compressed by gravitational
forces and nuclear fusion begins.
– Hydrogen is fused into helium and energy is
released.
– Some energy is released as light and a star is
born.
Deep Space Phenomena
– A pattern was discovered that allows stars to
be compared by brightness and color.
– Each point on an H-R diagram represents a
star whose brightness (absolute magnitude)
and color temperature (spectral type) have
been determined.
– Stars start out in the Main Sequence and as the
core cools, they move into the giant category.
– As the stars continues to collapse they become
very small and very hot white dwarfs.
Celestial Observations
• Celestial sphere – The imaginary sphere
on which all objects in the sky seem to be
located.
– The horizon is where the sphere meets Earth.
– The zenith is the point directly overhead.
– The altitude is the distance above the horizon.
– The azimuth is the distance in degrees
measured clockwise from due north.
Celestial Observations
• All celestial objects appear to move from
E to W across sky due to Earth’s rotation.
• Long-exposure photos of stars show that
the stars form arcs called star trails.
• Circumpolar stars & constellations
– Located between the northern horizon & Polaris
– Never set
– Appear to move in counterclockwise circles
Celestial Observations
• Constellations change slowly as Earth
orbits the Sun.
– Earth takes a year to go once around the Sun,
thus darkened portion of Earth is pointed
toward different directions throughout the year.
– If you follow a specific constellation on
successive evenings, it will rise about
4 minutes earlier every night.
Celestial Observations
• Planetary motions
– Planets also move from E to W across sky
– Over long periods of time the planets seem to
change position relative to the stars around
them.
– Planets don’t shift as much as the stars, thus
the planets actually move eastward relative to
the star fields behind them.
Models to Explain Celestial Motions
• Geocentric model
– Suggested by Ptolemy 2000 years ago
– Earth is stationary (not rotating or revolving)
and all celestial objects move around it at
fixed distances from it.
– Widely accepted because it explained all of
the daily motions of the Sun, planets, & stars.
– The retrograde motion (backwards) of some
planets was explained by showing they move
in small circles called epicycles.
Models to Explain Celestial Motions
• Heliocentric model
– Suggested by Copernicus in the early 16th
century.
– Model used at present time
– Earth and planets revolve around the Sun and
Earth rotates on its axis.
– The apparent retrograde motion of some
planets is caused by the fact that each planet
revolves around the Sun at a different speed.
Orbital Forces
•
Kepler’s Laws of Planetary Motion
1. The orbits of the planets around the Sun are
ellipses, with the Sun at one foci.
• Planets travel in closed curves called
ellipses.
• The center of the ellipse consists of two
fixed points, called foci.
Orbital Forces
• Eccentricity is the out of roundness of an
ellipse (e = d/L).
eccentricity = distance between foci
length of major axis
Orbital Forces
2. An imaginary line joining a planet to the Sun
will sweep over equal areas in equal periods
of time.
– A planet travels fastest and farthest when
it is nearest the Sun (perihelion).
– A planet travels slowest and covers the
least distance when it is farthest from the
Sun (aphelion).
Orbital Forces
3. The square of any planet’s orbital period is
proportional to the mean radius of its orbit
cubed.
– The period of a planet equals its year.
– The farther a planet is from the Sun the
larger its orbit and the longer its period.
Orbital Forces
• Newton’s Universal Law of Gravitation
– The gravitational force between any two objects
is in the Universe is directly proportional to the
product of their masses and inversely
proportional to the square of the distance
between their centers.
• As the masses increase, the gravitational
force increases.
• As distance between the objects increases,
gravitational force between them decreases.
Earth’s Rotation
• Evidence of Earth’s rotation
– Foucault Pendulum
• A swinging pendulum changes its direction
of motion in a predictable manner as Earth
rotates under it.
• The rate of change in the direction of a
pendulum on Earth depends on its latitude.
– At the equator it is 0 deg/hour.
– At the poles it is 15 deg/hour.
– At our latitude it is 10.5 deg/hour.
Earth’s Rotation
• Motions of the sun
– Due to Earth’s rotation, the Sun appears to
move E to W at a rate of 15 degrees per hour.
– Because Earth’s axis is tilted at 23.5 degrees,
the latitude at which direct rays strike Earth’s
surface changes in a cyclic pattern.
– Seasonal changes are caused by:
• Tilt of Earth’s axis
• Parallelism of Earth’s axis
• Revolution around the sun
Moon and Its Effects
• The Moon, like all celestial objects,
appears to move from E to W across the
sky.
• As the Moon makes one revolution around
Earth, it also makes one rotation on its
axis.
– The same side of the Moon is always facing
Earth.
Moon and Its Effects
• Because Earth orbits the Sun as the Moon
revolves around Earth, the Moon must
actually travel farther than one revolution
to complete a cycle of phases.
– 27 1/3 days to complete one revolution
around Earth (sidereal month)
– 29 1/2 days to complete a cycle of phases
(synodic month)
Moon and Its Effects
• The shape of the Moon’s orbit is elliptical
– Its closest distance from Earth (perigee)
is about 356,000 km
– Its farthest distance from Earth (apogee)
is about 407,000 km
• The position of the Moon with respect to
the Sun and Earth is responsible for Earth
tides, Moon phases, and eclipses.
Tides
• The cyclic changes in Earth-Moon Sun
alignments are responsible for cyclic
fluctuations in the high and low tides of
oceans and other large bodies of water.
– Tides fluctuate throughout the course of the
day as well as the month.
• Every day large bodies of water experience
two cycles of high tides and two cycles of
low tides.
Tides
– When Earth, Moon, and Sun are positioned in
a straight line spring tides occur.
• Spring tides feature the largest range in
ocean levels between high and low tides.
– When Earth, Moon, and Sun are at right
angles to one another neap tides occur.
• Neap tides show the smallest range of
ocean levels between high and low tides.
Moon Phases
• At any given time half of the Moon’s
surface is illuminated by the Sun.
• As the moon revolves around the Earth,
the amount of the illuminated portion of the
Moon that faces Earth varies in a cyclic
fashion called phases.
Moon Phases
Eclipses
• During a solar eclipse, the Moon passes
directly between Earth and the Sun, and
people standing in the path of the Moon’s
shadow see an eclipse of the Sun.
– Because the Sun is a sphere there are 2 parts
of the Moon’s shadow:
• Umbra = area of total darkness (total eclipse)
• Penumbra = area of partial darkness (partial
eclipse)
Eclipses
• During a total solar eclipse when the moon
is at apogee, the Moon’s umbra does not
reach all the way down to Earth and we
will see the outer edge of the Sun.
• This is called an annular eclipse.
Eclipses
• During a lunar eclipse, the Moon moves
through Earth’s shadow and people on the
nighttime side of the Earth see an eclipse
of the Moon.
– Occurs only at full moon phase
– Why don’t we have eclipses every month?
• Angle between Earth orbit and Moon orbit
is about 5 degrees so the Moon is usually
above or below the plane of the Earth.