Download The Earth

SCIENCE
PROGECT
• The Earth
• Earth, which is our base from which we look
into space, is constantly moving. Understanding
this movement is one of the most useful and
important things in astronomy.
The earth orbits the sun in an elliptical orbit and the
moon orbits the earth with the same kind of orbit.
Looking down from the north pole, the earth spins in a
counterclockwise direction on an imaginary line called its
axis once every day. This accounts for the fact that the
sun rises in the east and sets in the west. The earth’s
axis is tilted with respect to the plane of its orbit at an
angle of about 23.4 degrees. If we position ourselves
high above the north pole, we would see that the earth
orbits the sun in a counterclockwise motion, coming to
the same position among the stars every 365.26 earth
days. We would also see that the moon also orbits the
earth in a counterclockwise motion. This is illustrated in
the following example.
• The average distance from the earth to the sun,
the semimajor axis of its orbit, is 149,597,890
km. This distance was not known until recently
and it is called the astronomical unit or AU. The
distances of the other planets to the sun are
usually measured in astronomical units.
Because of the tilt of the earth, not every place
on earth gets light every day. Also, some places
have extremely short days.
• As the earth revolves around the sun, the place where
light shines the brightest changes. This motion gives
us the different seasons. For instance, the poles
receive less light than does the equator because of
the angle that the land around the poles receive the
sun’s light. When the north pole is tilted toward the
sun, the northern hemisphere is presented to the sun
at a greater angle than the southern hemisphere and
the northern hemisphere gets warmer. When this
happens, the northern hemisphere gets summer while
the southern hemisphere gets winter. When the south
pole is tilted toward the sun, the two seasons reverse
hemispheres. This is illustrated in the following image.
• The earth’s orbit is called the ecliptic. The plane
which contains the ecliptic is the reference
plane for the positions of most solar system
bodies. Viewed from earth, the ecliptic is the
apparent motion of the sun among the stars.
• The earth’s equator is a circle going around the
earth which is on a plane that is perpendicular
to the earth’s axis. The equator and the plane
on which it lies are illustrated in the following
image.
• This equatorial plane is one of the most important in astronomy
because it intersects the plane of the ecliptic and gives us a
reference point in space by which we can measure the
positions of stars. This plane also divides the earth into halves,
the northern half being the northern hemisphere, the other half
being the southern hemisphere. The intersection of these
planes is a line, which for convenience we will call the line of
equinoxes. The real definition of equinox is the point on the
celestial sphere which intersects this line, but since the celestial
sphere is an imaginary sphere with any size, the equinoxes are
really lines. Also, for some purposes and illustrations, it is more
convenient to think of the equinoxes as a line extending into
space. For other purposes, it is convinient to think of the
equinoxes as directions. The two planes are illustrated below.
• One half of this line is called the vernal equinox; the
other half is called the autumnal equinox. At two points
in the earth’s orbit this line intersects the sun. These
two places mark the start of two of the four seasons,
autumn or spring. The autumnal equinox starts
autumn around September 23. From earth, this marks
the time when the sun looks as if it is crossing the
plane of the equator on its way south. The vernal
equinox starts spring around March 21. This marks the
time when the sun looks as if it is crossing the plane of
the equator on its way north. The earth carries the
plane of the equator along with it. When the sun looks
as if it is on its way north or south, the earth is actually
carrying the equatorial plane along so that it crosses
the sun.
• Perpendicular to this line of equinoxes is a line which contains
the solstices. The solstices are points on the ecliptic which start
the other two seasons, summer and winter, when they cross the
sun. The summer solstice is one half of this line, the winter
solstice is the other half of this line. The half of this line that is
north of the celestial equator is the summer solstice, the half
that is south of the celestial equator is the winter solstice.
Currently, the winter solstice starts winter for the northern
hemisphere at about the time the earth is closest to the sun.
This line is illustrated in the following example.
•
• Because of centrifugal force involved when an
object spins, the earth is not a perfect sphere,
but is somewhat flattened at the poles and
bulges out at the equator. The distance from
any point on the equator to the center of the
earth is longer than the distance from either
pole to the center of the earth. This is illustrated
in the following image which is exaggerated for
clarity. The form caused by this equatorial bulge
is called a geoid.
• The moon is the earth’s only natural satellite. Its
average distance from the earth is 384,403 km. Its
revolution period around the earth is the same length
and direction as its rotation period, which results in the
moon always keeping one side turned toward the
earth and the other side turned away from the earth.
This type of motion is called synchronous rotation. The
side turned away from the earth is called the moon’s
dark side, even though it is lit half of the time. The
moon’s sidereal period of revolution is about 27.32
days long. This means that a line drawn through the
center of the earth and the moon would point to the
same star every 27.32 days. Due to slight variations in
the orbital velocity of the moon, over a 30 year period,
59% of the moon’s surface is made visible. This is
known as libration.
• The moon’s orbit is not in the plane of the ecliptic and
because of the elliptical nature of the moon’s orbit, it is
not always the same distance from the earth. At the
two intersections of the moon’s orbit and the plane of
the ecliptic are two nodes. These nodes regress along
the plane of the ecliptic, making one complete rotation
every 18.61 years. See Orbits.
• The Effect of the Moon
• The moon has a noticeable effect on the earth in the
form of tides, but it also affects the motion and orbit of
the earth. The moon does not orbit the center of the
earth, rather, they both revolve around the center of
their masses called the barycenter. This is illustrated
in the following animation.
• The sun attracts the moon in such a way that it
perturbs its orbit every 31.807 days, this phenomenon
is called evection. The moon also changes the
position of the earth’s equinoxes. The sun and moon
each attract the earth’s equatorial bulge, trying to bring
it into alignment with themselves. This torque is
counteracted by the rotation of the earth. The
combination of these two forces is a slow rotation of
the earth’s axis, which in turn results in a slow
westward rotation of the equinoxes. Looking down
from the north pole, the equinoxes would appear to be
rotating in a clockwise motion. The equinoxes and
poles complete a rotation every 25,800 years. The
equinoxes move at a rate of about 50.27 arc seconds
per year. This phenomenon in known as the
precession of the equinoxes and is illustrated in the
following image.
• The north pole is currently pointing to a spot near the star
Polaris. Because the vernal equinox is the starting point for
most star charts, the charts must be made for a certain period.
The star charts must be updated periodically to account for
this movement of the reference point.
• Because of the seasonal changes in the ice, snow,
atmospheric distribution, and perhaps because of movements
in the material within the earth, the geographic poles
constantly change position in relation to the earth’s surface.
This phenomenon is known as the Chandler wobble.
Scientists have resolved the change into two almost circular
components, the first with a radius of about 6 meters and a
period of 12 months, the second with a radius of 3-15 meters
and a period of about 14 months.
• The sun and moon, because of their varying distances and
directions in relation to the earth, constantly vary their
gravitational attractions on the earth. This makes the poles
wander irregularly by about + or - 9 arc seconds from its
average, or mean, position. This phenomenon is known as
nutation and has a period of about 18.6 years. The primary
component of this is from the moon and is known as lunar
nutation.
• The sun and moon also constantly change the
earth’s rate of spin.
• Star charts use the mean equinox instead of the
true equinox for their zero points. The mean
equinox is the position of the equinox corrected
for the slight but noticeable changes caused by
nutation and the Chandler wobble. The mean
equinox is still affected by precession, however,
and does change position, but does it at a
constant, predictable rate. Scientists requiring
up-to-date precision information about the
position of the earth can use the International
Earth Rotation Service or IERS. This
information can be found at the IERS web site
at http://maia.usno.navy.mil/
• The Sun
• Because of the elliptical nature of the earth’s orbit and
constant changes in the earth’s rate of spin because
of the previously mentioned phenomena, the sun, as
seen from earth, is moving at a non-uniform rate. This
makes it difficult to use the real position of the sun as
a reference for time keeping. For these purposes, a
point which moves at a constant rate around the earth
is used instead of the real position of the sun. This
point is called the mean sun and is the basis for mean
solar time.
• For more information, go to:
• Nasa: JPL: Solar System Exploration: Planets: Earth
• US Naval Observatory: Seasons and the Earth's Orbit:
Milankovitch Cycles