Transcript The Earth`s Motion - Fort Thomas Independent Schools

The Earth’s
Movement
Earth’s Motion is
Relative
Earth rotates…
Earth revolves…
Earth precesses…
Earth nutates…
Earth moves with the solar system…
Solar System moves with spiral arm of Milky Way
galaxy…
Milky Way rotates with other galaxies in the Local
Group…
Local Group moves within the universe…
All this while space is expanding!
TOPICS
Earth Cycles
Sun’s path and the ecliptic plane
Seasons
Cyclic motions of the Sun and stars in our sky
are due to motions of the Earth
1. ROTATION=the spin of the Earth on its axis. It takes one
day for the Earth to complete one rotation.
2. REVOLUTION=the movement of the Earth in orbit around
the sun. It takes one year for the Earth to complete one
revolution.
3. PRECESSION=the slow conical (top-like) motion of the
Earth’s axis of rotation. It takes 26,000 years for the
Earth to complete one cycle of precession.
4. NUTATION=a small back and forth wobble in the Earth’s
axis caused by varying effects of the gravitational forces
from the Moon and the Sun.
THE FOUR MOTIONS
OF THE EARTH
R = rotation
P = precession
N = nutation
r = revolution
r
ROTATION
Not all stars in our sky rise and set, and different latitudes view the
movements of the stars differently.
The stars near the poles of the celestial
sphere (shown here) move in trails that
circle the pole and never set. They are
called circumpolar.
The view of the stars movement
depends on our latitude. (Note:
these views represent east or west
orientations)
NORTH
POLE
EQUATOR
MIDDLE
LATITUDES
DESCRIPTION OF
EARTH MOVEMENT
Cyclic motions of the Sun and stars in our sky are
due to motions of the Earth
ROTATION=the spin of the Earth on its axis. It takes one
day for the Earth to complete one rotation. The daily
motion of the Earth is called diurnal motion.
REVOLUTION=the movement of the Earth in orbit
around the sun. It takes one year for the Earth to
complete one revolution.
PRECESSION=the slow conical (top-like) motion of the
Earth’s axis of rotation. It takes about 26,000 years for
the Earth to complete one cycle of precession.
NUTATION=an irregular nodding motion caused by tidal
forces that occurs as the Earth precesses.
WHICH WAY TO THE
STARS APPEAR TO
MOVE?
The apparent westward motion of the Sun, Moon, and stars
across our sky each day is caused by Earth’s rotation.
We generalize this motion to make statements
such as, “The Sun rises in the east and sets in the
west.”
The same is true for the Moon, planets and the
stars.
At middle latitudes, we see the Sun, Moon, and
many of the stars first come into view moving
upward, rising at some point along the eastern
horizon. Then, they appear to arc across the sky,
gaining greatest altitude at the local meridian.
Finally, they disappear somewhere along the
western horizon.
http://www.opencourse.info/astronomy/introduction/02.motion_stars_sun/
The apparent westward motion of the Sun,
Moon, and stars across our sky each day is
caused by Earth’s rotation.
At middle latitudes, we see the Sun, Moon, and many of the stars
first come into view moving upward, rising at some point along the
eastern horizon. Then, they appear to arc across the sky. Finally,
they disappear somewhere along the western horizon.
We generalize this motion to make statements such as, “The Sun
rises in the east and sets in the west.”
The Sun’s Path
Each day we see the Sun’s path across our sky, it is a
snapshot of a series of 365 separate paths that occur
throughout the year.
Each daily “snapshot” of the Sun is just one position of the
Sun along the ecliptic plane (as seen from the Earth).
The seasons we experience are linked to the motion of the Sun along the
celestial sphere.
The point of the Sun’s path
farthest north on the celestial
sphere is called the summer
solstice (JUN 21), while the point
of the ecliptic farthest south is
called the winter solstice (DEC
21).
The two points on the ecliptic
where the Sun crosses the
celestial equator are called
equinoxes. During the vernal
equinox (MAR 21), the Sun is
moving north, while during the
autumnal equinox (SEPT 21), the
Sun is moving south.
Remember that the seasonal
names of the equinoxes and the
solstices refer to seasons in the
NORTHERN hemisphere. The
seasons occurring in the
SOUTHERN hemisphere are
exactly opposite.
The Sun rises at different points along the horizon at different times of the year
and also peaks at different heights.
Different parts of the world experience different times of day as the Earth
rotates.
TIME ZONES which can be used to calculate the time of day in any given
part of the world.
EARTH REVOLVES
As the Earth revolves, it changes position relative to Sun.
This causes two effects:
• Zodiac constellations appear to shift along the ecliptic
slightly less than 1 degree per day. Eventually,
constellations visible at night become invisible during the
day.
• The altitude of the Sun in the sky at local meridian
changes daily. This alters the Sun’s path across the sky.
Variations in the
Earth’s rotation
Earth averages one rotation every 24 hours,
which means that some days are slightly
longer or shorter than the average.
Earth’s rotational speed throughout
geologic time has slowed due to it’s
gravitational (tidal) interaction with the
Moon.
Variations in The Earth’s
Revolution
The Earth’s orbital speed varies because
its orbital path is not a perfect circle; it is
elliptical.
The Earth orbits fastest when it is furthest
away from the Sun (at aphelion) and
slowest when it is closest (at perihelion).
http://www.windows.ucar.edu/tour/link=/physical_science/physics/mechanics/orbit/perihelion_aphelion.html
http://www.mhhe.com/earthsci/geology/mcconnell/earths_climate/intro.htm
Reason for the Seasons
2
Changing Angle of Sunlight
• The hemisphere tilted toward the Sun receives sunlight at
higher angles than the hemisphere tilted away from the Sun.
• Higher insolation angle means greater intensity of radiant
energy (the sun shines more directly on a smaller area).
WHY DOES THE EQUATOR
RECEIVE MORE ENERGY FROM
THE SUN THROUGHOUT THE
YEAR THAN FORT THOMAS?
Since the angle of the
Sun at the equator is
higher above the horizon
than at Fort Thomas,
more solar radiation is
received at the Equator
due to the higher
insolation angle.
The angle of the Sun
above the horizon at
Fort Thomas varies
from 74 to 27 degrees
during the year.
The angle of the Sun above the horizon at the
equator is never lower than 66.5 degrees during
the year.
Identify the Seasons and the Equinox or Solstice, and comment about the number
of daylight vs. night hours
Season _________
_________
Equinox or Solstice
Season _________
Season _________
Equinox or Solstice
Equinox or Solstice
Season ________
Equinox or Solstice
Seasons are caused because the Earth’s axis is tilted and as the Earth
revolves around the Sun, different parts of the Earth receive more direct
sunlight (summer), while other parts of the Earth receive sunlight which is
more spread out (winter).
Time and Seasons
2
Why do seasons change?
• Factors that affect season change are tilt of axis, revolution
and rotation.
• The reason that the seasons change on Earth is because the
tilt of the Earth at 23.5° as it revolves around the Sun.
• This causes two effects:
• 1) sunlight strikes the Earth’s surface at different angles
during different times of the revolution year.
• 2) the number of hours of daylight each rotational day
varies because the Sun is up for different numbers of
hours per day.
THE ECLIPTIC
It is the flat plane of the orbit
of the Earth around the Sun.
(Earth orbits at an angle)
The apparent path of the Sun
across the sky. (Sun takes a
slightly different path across
the sky each day)
The angle between the ecliptic plane and
the celestial equator is 23.5o.
The Earth also revolves around the Sun, which changes our view of the stars.
From our perspective, the Sun
appears to move through the
stars along a special path
called the ecliptic.
From an outside view, we see the
Earth revolve around the Sun. We
define the plane of the Earth’s orbit
as the ecliptic plane.
Seasons
Tilted axis and rotation of Earth combined with revolution around Sun causes the
seasons.
1) Tilted axis (23.5 degrees), rotation and revolution cause the northern
hemisphere to receive more direct sunlight for more hours than the southern
hemisphere during the summer and less direct sunlight for less hours during the
winter.
2) So, as one hemisphere is warmer, the other is colder. This condition reverses
every 6 months. In other words, this condition is seasonal.
3) The fall and spring represent 3-month transitions from summer to winter and
from winter to summer.
PRECESSION
PRECESSION OF THE
EQUINOXES
Sunrise during the vernal equinox will occur in a different
zodiac constellation every 2000-3000 years or so. This is due
to the precession of the equinoxes.
This precession causes the position of the North Celestial Pole to slowly
change over time. Today, the North Celestial Pole is near the star
Polaris, which we call the “North Star.” However, in 3000 BC, Thuban
was close to the North Celestial Pole and in 14,000AD, Vega will be in
this location.
Gravitational forces of the Sun and the Moon pulling on
the Earth as it rotates causes the Earth to undergo a
top-like motion called precession. Over a period of
26,000 years, the Earth’s rotation axis slowly moves in
a circular motion.
North Star isn’t
always Polaris!
LATITUDE VS.
LONGITUDE
latitude
+
longitude
=
Earth’s Orbit Around
the Sun
The Earth revolves around the Sun, which changes our view of the
stars.
Seasons are caused because the Earth’s axis is tilted and as the
Earth revolves around the Sun.
As the Earth revolves, different parts of the Earth receive sunlight at
different angles.
The more direct the sunlight, the more energy delivered.
The angle of the Sun’s path across the sky determines the daylight
hours.
Daylight hours + the angle of the sun affect the amount of light
energy received by the Earth.
One common misconception is
that the Moon is only visible at
night. However, the time of
day in which the Moon is in our
sky varies depending on its
phase. This picture clearly
displays the Moon, visible
during the day.