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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