Download Earth in Space

Unit 8: The Earth in Space
I. Motion of Stars and Planets
• Earth and all
other planets
orbit the sun in
elliptical paths.
This kind of
motion is known
as revolution.
The universe according to “Ptolemy” had
the earth at the center
• During the 2nd
Century, Ptolemy’s
followers believed
that all the stars were
on a sphere which
surrounded the earth
and made one trip
around earth each
day.
Copernicus changed this by
postulating that the sun was the
center of our “solar system”
• Earth and all the
planets simply
orbited around the
sun in concentric,
elliptical orbits.
Constellations are figures that
people have observed to mark
the position of the stars.
There are two major motions of
the earth….
• Rotation - the spinning of the earth on its
axis resulting in daily changes such as the
rising and setting of the sun and moon.
• Revolution - the movement of the earth in
its orbit around the sun resulting in
yearly changes such as the changing
seasons.
Evidence for the rotation of the
earth includes star trails…..
The Earth’s rotation on its axis
causes the sun and stars to appear
as if they move through the night
sky.
The apparent movement of the
constellations around “Polaris”...
and the apparent movement of a
Focault pendulum at the north
pole caused by the earth rotating
beneath it…..
The rising and setting of the sun
are caused by the rotation of
Earth
Path of the sun’s apparent motion
in New York State
Note the following….
• At the winter solstice,
the sun rises south of
due east and sets south
of due west.
• At the equinoxes, the
sun rises due east and
sets due west.
• At the summer
solstice, the sun rises
north of due east.
Also note that….
• In New York State,
the sun is NEVER
directly overhead.
• During the winter
the sun’s AOI and
duration are
smallest. Summer,
AOI and duration
are greatest.
Remember that the seasons are
caused by the tilt of the axis
during revolution.
Kepler’s Laws of Orbital
Mechanics
• Law 1 states that the
orbits of planets are
ellipses, with the sun
at one of two foci and
the other focus being
a point out in space.
Kepler’s Second Law…...
• states that the orbital
speed of planets
increases as the sun
and planet grow
closer and decreases
as the distance
between the sun and
planet increase.
Kepler’s Second Law….
• also states that a line
connecting the planet
to the sun sweeps out
an area in such a
manner that equal
areas are swept out
in equal time.
• This area is equal to
• This area
Kepler’s Third Law of Orbital
Mechanics states that…..
• Planets closer to the
sun have a shorter
period of revolution
(a “year”) than
planets farther away
from the sun.
• Earth = 365 days
• Saturn = 29.5 yrs.
Retrograde Motion…..
• When viewed from
earth, Mars moves east
among the stars each
day, then periodically
stops and appears to
move west due to the
fact that Earth has a
faster orbital speed
than Mars and
overtakes it.
All of these elliptical orbits occur because
of the balance between inertia and gravity.
• The planet attempts to
move outward in a
straight line due to
inertia.
• The planet also
attempts to plunge
back into the sun due
to gravity.
• The balance creates an
orbital effect.