Chapter Three
The Earth as a Planet
Section One – The Planet Earth
Section Two
– Earth Motions
Section Three -- Tools of Astronomy
The Earth’s Composition
The Earth is a terrestrial planet.
Definition: a planet that is composed mostly of rock
– A planet is a large body that travels around a star.
– The term terrestrial means that it is made mostly of rock
and metal.
The land portion of our planet is known as the lithosphere and
is divided into 4 parts (starting with the innermost):
– INNER CORE/OUTER CORE: a dense combination of
molten iron and nickel
– MANTLE: plastic-like, less dense rock
– CRUST: the outer portion of the Earth, made up of two basic
types of solid rock (granites and basalts)
Granite rocks make up most of the Earth’s continents;
basalts compose most of the ocean floor.
Along with the lithosphere, here are two other parts
that make up our Earth – the atmosphere and the
hydrosphere.
The Earth’s Shape
The shape of the Earth is nearly spherical, which
means it resembles a round ball.
– Before there was hard evidence to support the shape of our
planet, humans theorized about its shape. Some hints that
they had are stated below:
1. The disappearance of ships as they moved over the
horizon.
2. Ancient astronomers observed a curved shadow
move across the moon during a lunar eclipse.
3. The altitude of the North Star changed as they
moved farther south or north.
Currently, scientists have developed the technology
needed to prove the round shape of the Earth.
– However, it is not perfectly round – it is referred to as
an oblate spheroid.
The term oblate means that it is slightly flattened.
The slight flattening of the Earth near the equator is
caused by it’s rapid rotation – that means that the
diameter is slightly larger around the equator than
around the poles.
The equatorial diameter of the Earth is
approximately 7,928 mi, whereas the diameter
between the North and South Poles is 7,900 mi.
The Size of the Earth
The total surface area of the Earth is approximately
197 million square miles.
– More than 70% is covered by ocean.
The Earth’s Rotation
The Earth rotates on it’s axis, connecting the North and
South Poles.
Definition: a straight line around which an object rotates
– Early astronomers questioned whether the heavens
were spinning around the Earth, or the Earth was
spinning on an axis.
The speed at which the Earth is rotating depends on
how far north or south of the equator you are.
– If you are standing near the equator, the rotational
speed is approx. 1,037 mph.
At this speed, the Earth makes one full rotation.
Definition: the circular movement of a body around a
central point called an axis
– A full rotation takes 23 hours, 56 minutes, and 4.1 seconds –
but we round it up to 24 hours, or 1 day.
The rotation of the Earth causes half the world to be
exposed to sunlight while the other half is bathed in
darkness.
– This is why we experience day and night – and helps define
the Earth’s weather and climate regions.
The Earth’s Tilted Axis
The axis of the Earth is tilted approximately 23.5
degrees.
– The tilt of the Earth’s axis causes parts of the Earth to
receive more sunlight than other parts – causing the four
season.
The first day of summer in the Northern Hemisphere
is called the summer solstice and the first day of
winter is called the winter solstice.
– The summer solstice usually falls around June 21st and is
that day that has the longest period of daylight.
– The winter solstice usually falls around December 21st and is
the day that has the shortest period of daylight.
Days in which periods of daylight and night are equal
length are referred to as equinoxes (the sun is directly
overhead at the equator).
– Every year, two equinoxes occur – the vernal equinox and
the autumnal equinox.
– The vernal (spring) equinox usually falls around March
21st, and the day and night are each 12 hours in length.
– The autumnal (fall) equinox usually falls around September
23rd, and the day and night are each 12 hours in length.
The Earth’s Coordinate System:
Latitude and Longitude
A coordinate system is a method of locating an exact
location on a two-dimensional surface.
– The equator divides the planet into two hemispheres.
Definition: the imaginary line, also known as zero degrees
latitude, that divides the Earth in half into the Northern and
Southern Hemisphere
– Each hemisphere is further divided by horizontal lines
that mark the locations north and south of the equator
called lines of latitude.
Definition: parallel lines the run east and west across Earth’s
surface, measuring locations north and south of the equator
– These lines are marked in degrees – the equator being
represented by 0o, and increasing in both directions as
you move away from the equator line.
Lines of latitude are useful only for determining an
exact location either north or south of the equator –
they become more helpful if used in combination with
the lines of longitude.
Definition: coordinate lines used on the earth’s surface
that run north and south from pole to pole and measure
a location east and west of the prime meridian
– Degrees of longitude are also marked by degrees, only
there is no natural halfway point that divided the Earth
vertically – so astronomers have designated a place on
our planet (called the prime meridian) to represent 0o
degrees.
Topographical Maps
Maps the we use to represent a three-dimensional
surface of the Earth are called topographical maps.
They represent changes in elevation on the Earth’s
surface by using contour lines  they are drawn on a
map to represent a specific elevation of the land surface
above sea level.
Definition: isolines that mark areas of equal elevation on a
topographical map
– By adding contour lines to a map, it is possible to see
the true shape of the land.
Also found on a topographical map, along with contour
lines, are contour intervals.
Definition: the specific change in elevation associated
with each contour line on a topographical map
– Each contour interval is represented as the blank space
between two lines – on a map of this type, each contour
line represents a predetermined increase in height and the
space between represents the steepness of the land.
Chapter Three
The Earth as a Planet
Section One
– The Planet Earth
Section Two – Earth Motions
Section Three -- Tools of Astronomy
Apparent Motion of Celestial Objects
Because our Earth is spinning, objects in the
nighttime sky appear to move in regular motions.
– These objects are called celestial objects, which
include planets, moons, stars, comets, asteroids, and
any other object located outside of the Earth’s
atmosphere.
The movement of celestial objects is called apparent
motion – this apparent motion travels from east to west
across the sky.
– The speed at which apparent motion travels is measured in
degrees, with the sky representing 180 degrees from
horizon to horizon – this is broken down so that the speed
of the celestial objects travel is approximately 15 degrees
of sky per hour.
Early Models of the Universe
As a result of the apparent motion of celestial objects,
for thousands of years humans believed that the
planets and stars revolved around the Earth.
– Claudius Ptolemy, the great Romanian mathematician,
geographer, and astronomer, who lived almost 2,000 years
ago, was the first scientist to formulate this idea.
In the Ptolemaic system, or geocentric view of the universe,
Ptolemy described the planets and stars are revolving around
the Earth in perfect circular orbits.
Definition: an early model of the universe which puts the
Earth at the center of the solar system and universe
The Geocentric model was accepted for over one thousand
years until Copernicus changed the way that humans
looked at the heavens.
Nicolaus Copernicus observed the motions of stars
and planets for decades and eventually published his
theory called the Copernicus heliocentric system.
Definition: a model of the solar system that puts the Sun at its
center with the Earth and other planets revolving around it
– Copernicus’ theory created much controversy, and lacked
sufficient proof to verify it.
– Galileo Galilei took up Copernicus’ idea when he used
improved telescopes of his own design to help support
the heliocentric model.
Galileo first observed the Earth’s moon with his new
telescopes revealing that its surface was much like the
Earth, consisting of mountains, valleys, and craters.
– He then started to observe the moons of other planets and
he noticed that each of those moons were revolving around
their planets.
Galileo also observed the surface of the sun with his
telescopes.
– Although it made him nearly blind, his observations
discovered its unique sunspots.
– After carefully plotting the location of these dark patches on
the Sun, he discovered that it was also most likely rotating
on its axis, like the Earth.
While Galileo was trying to prove his theories, the
work of German astronomer, Johannes Kepler,
caused another breakthrough for modern astronomy.
– Kepler used the work of another astronomer to reveal the
true nature of the orbits of the planets
– It was previously thought that the orbits were circular, but he
recognized that the only way to accurately explain the
positions of the planets was to describe their orbits as being
elliptical.
His research is now known as Kepler’s laws of
planetary motion:
– The first law states that the planets all revolve around the Sun
in elliptical orbits.
– The second law explains that as the planets revolve around
the Sun, their velocity changes in relationship to their distance
to the Sun.
– The third law explains that the period of time it takes for a
planet to orbit the Sun is related to the size of its elliptical path.
Finally in 1687, the English astronomer and
mathematician Isaac Newton published his three laws
of gravitation- these helped to explain how Kepler’s laws
worked.
Getting to this point shows how the advancement of
human knowledge progresses through time as
scientists build upon the theories of others.
Orbital Motion
The motions of all celestial objects are based on the
concept of an ellipse.
Definition: the oval-like path of the orbit of a celestial
object around two points known as foci, one of which is
the Sun
– The ellipse can be generally described as the oval-like
path of a celestial object.
The path of the ellipse, known as the orbit, is defined
by two points, individually known as a focus, and
together called foci.
The oval nature of elliptical paths can be
mathematically described as their eccentricity.
Definition: the mathematical expression of how far as ellipse is
from a perfect circle, which can be determined by dividing the
distance between the foci by the length of the major axis
An ellipse with an eccentricity of 0 represents a perfect circle
and an ellipse with an eccentricity of 1 is regarded as a flat line.
– The closer the eccentricity is to 1, the more eccentric or oval
shaped the orbital path is.
– Pluto’s orbit is the most eccentric, or furthest from being a
perfect circle.
– Venus has the least eccentric orbit, which is closest to being
a perfect circle.
The time it takes for a planet to make one complete orbit
around the Sun is called one revolution.
Definition: the movement of an object in an orbit around
another object
For the Earth, the perihelion occurs around the first of
January, when the Earth is approx. 91,349,000 mi from
the Sun.
Definition: the point in a planet’s orbit around the Sun
when it is closest to the Sun
For the Earth, the aphelion happens during the first
week in July, when the Earth is approximately
94,454,000 miles from the Sun.
Definition: the point in the orbit of a planet when it is
farthest from the Sun
– Although the distance from the Earth to the Sun
changes as a result of its elliptical orbit, this does not
affect the Earth’s climate- the Earth’s tilted axis has a
much greater effect on seasonal change and temperature
than the aphelion or perihelion.
Kepler’s second law describes how the velocity (speed)
of a planet changes in its orbital path relative to its
distance from the Sun.
– When the Earth is at its closest position to the Sun, the
increased gravitational attraction causes an increase in
the velocity.
– When the Earth is at its farthest position to the Sun, the
decreased gravitational attraction causes a decrease in
the velocity.
Knowledge of the elliptical orbits of planets and how
they are governed by gravity has become a useful tool
for exploring outer space.
– Astronomers have put this knowledge to practical use with
what is called gravity assist.
Definition: a technique also known as the “slingshot”
effect which is used to control the direction and velocity of
many interplanetary spacecraft by applying an
understanding of elliptical orbits and gravitational
acceleration
Chapter Three
The Earth as a Planet
Section One
– The Planet Earth
Section Two
– Earth Motions
Section Three -- Tools of Astronomy
Locating Celestial Objects
Astronomers have used various coordinating systems,
known as celestial coordinates, to help them map the
locations of celestial objects, and also track their
movements.
– One of these systems is called the horizontal coordinate
system – this uses coordinates called azimuth and
altitude to mark precise locations in the sky.
AZIMUTH: the location of an object around the horizon,
which is divided into the 360 degrees of a circle.
ALTITUDE: the angle of a celestial object above the horizon,
which represents an angle between 0 and 90 degrees.
– The horizon represents 0 degrees of altitude with the zenith
representing 90 degrees above the horizon.
Definition: the point in the sky that is directly above the
observer, or 90 degrees above the horizon
Another system more widely used by astronomers is
called the equatorial coordinate system.
– This system is based on Earth’s latitude/longitude
coordinate system, which is extended out into what is called
the celestial sphere.
Telescopes
Probably the most widely used tool of astronomers is
the telescope.
Definition: a scientific instrument used to observe objects
that are far away
– Galileo was the first person we know of to use this
device to observe celestial objects.
There are three basic types of optical telescopes.
Definition: a telescope that uses glass lenses and mirrors
to magnify the light given off by an object
BASIC TYPES:
– REFRACTING TELESCOPES  uses a combination of
concave and convex lenses to magnify an image; they
provide clear, high-resolution images of very small sections
of the sky.
– REFLECTING TELESCOPES  uses a large concave
mirror to focus light to magnify an image; they can have very
large apertures providing a wide view of the sky with
excellent clarity and brightness.
– COMPOUND TELESCOPES  uses a combination of both
lenses and mirrors to provide an excellent high-resolution
magnification of the sky.
One of the limits of ground based optical telescopes is
the distortion and interference caused by the Earth’s
atmosphere.
– To alleviate this problem, NASA has designed and launched
space based telescopes like the Hubble Space Telescope.
– This large reflecting telescope orbits the Earth at about 375
miles above the surface of the planet – it provides some of
the highest resolution images of space ever seen by
astronomers.
Photography and computer technology also have
added to the capabilities of optical telescopes.
– Many optical telescopes are fitted with spectroscopes.
Definition: a scientific instrument used to analyze the
visible light portion of the electromagnetic spectrum
– These spectroscopes work like a prism, which separates
light into individual colors – this allows astronomers to
view the unique spectrum of celestial objects.
A continuous spectrum shows all of the colors of
visible light, however the interaction of visible light
with matter causes specific wavelengths of light to be
absorbed.
The wavelengths of light that are absorbed leave gaps in
the spectrum known as absorption, or dark line spectrum
– every element and compound has its own unique dark line
signature, which can be used like a fingerprint to identify
specific substances.
Definition: a spectrum of light where specific wavelengths are
absorbed, leaving gaps in the form of dark lines within the
spectrum, also known as the absorption spectrum
A similar process also occurs
when objects give off radiation –
this is called an emission, or
bright light spectrum.
Definition: a type of spectrum
where bright lines of specific
wavelengths of visible light
appear within a spectrum that represent wavelengths of light
emitted from a substance
Radio telescopes are another type of telescope used
by astronomers.
Definition: a type of telescope that senses longer wave
electromagnetic radiation in the form of microwaves and
radio waves
– The main component is the antennae, which is usually
disk-shaped – they sense radiation from far out into
the universe and sends it to computers to be analyzed.
– The radio telescope has made many significant contributions
to astronomy, including the detection of the first planets
discovered outside of the solar system.
Often a network of radio telescopes is used to create
interferometry.
Definition: the combination of radio signals received by a
network of radio telescopes used to create one large
telescope
Space Exploration
Space exploration is the physical investigation of
celestial objects outside of the Earth’s atmosphere – it
is undertaken in two fundamental ways.
– The first employs the use of robotic spacecraft or
satellites to visit objects in the solar system.
– The second is human space-flight, which utilizes
spacecraft to transport humans into space for the purpose of
exploration.
One of the most difficult aspects of both methods of
space exploration is the problem of getting off the
Earth itself.
– So far the best way to do this is to use rocketry – rockets
use solid and liquid fuels, which are burned rapidly to create
great amounts of thrust that rapidly accelerates the vehicle.
The development of new technologies to propel
spacecraft are currently under way, and will certainly
play an important role in the future of space
exploration.
– Cordless power tools, smoke detectors, light emitting
diodes (LEDs), protective helmets, robotics, global
positioning systems (GPSs), wireless communication,
night vision cameras, heart monitors, and scratchresistant lenses are just a few of the technologies that
are a result of space exploration – this is known as
technology transfer.
Definition: the transfer and application of technology to
improve quality of life that was developed as a result of
scientific investigation