Chapter 25: The Earth’s Motion
Earth’s Rotation
The Earth is tilted 23.5 perpendicular to the plane of the Earth’s Orbit.
The Earth turns on this axis, the axis of rotation; this turning motion is
called rotation. The tip of this axis is always pointed at the northern
star, Polaris. This behaviour is constant as the Earth rotates around the
sun. The behaviour has been termed the “parallelism of the axis”. The
parallelism of the axis is the first reason for the seasons.
The rotation of the Earth is defined as the time it takes for the
Earth to make one full turn on its axis. Thus, the rotation of the Earth
results in day time periods and night time periods. When looking down
at the Earth from the North Pole the direction of rotation can be
determined. The Earth is rotating from west to east or counter-clock
wise. This is why the sun rises in the east and sets in the west.
The tilt of the Earth results in uneven sunlight to the northern and
southern hemisphere.
Evidence that shows the earth rotates about its axis:
1.) Foucault pendulum: pendulum swings back
and forth while the Earth rotates under it.
Every hour there is an 11 degree shift in a
clockwise direction.
2.)
As the Earth rotates under the moving air, the winds appear
to be deflected. High to low
pressure.
3.)
The sun rises in the East and
sets in the West.
Earth’s Revolution
The movement of the Earth in orbit around the sun is called
revolution. The stars provide evidence that the Earth is moving around
the sun. The apparent shift of close stars in comparison to distant stars is
called parallax.
The Earth is revolving in a counter-clock wise motion around the
sun, as well as in an elliptical orbit. The Earth is revolving about 1 a
day around the sun.
Seasons on Earth
3 reasons for the seasons:
1. the revolution of the
Earth around the sun
2. the tilt of Earth’s axis
3. the parallelism of
Earth’s axis
Summer Solstice
The first day of summer in the Northern Hemisphere occurs on
June 21 each year, thus making it the longest day of the year and
therefore the shortest day of the year in the Southern Hemisphere. On
this day the Tropic of Cancer 23.5 N latitude, is experiencing vertical
rays.
Winter Solstice
The first day of winter in the Northern Hemisphere occurs on
December 21, thus making it the shortest day of the year and therefore
the longest day of the year in the Southern Hemisphere. On this day the
sun is straight overhead at the Tropic of Capricorn 23.5 S latitude.
The Equinoxes
The equinoxes occur two days out of the year, during this time the
days are equal in length in both the Northern and Southern Hemispheres.
The spring equinox occurs on March 21 and the autumn equinox
occurs on September 23. During these days the sun is directly overhead
the equator.
66.5º
Chilliwack 49º
23.5º
N
0º
23.5º
66.5º
PLACE
North Pole
Arctic Circle
Chilliwack
Tropic of Cancer
Equator
Tropic of Cap
Antarctic Circle
South Pole
SPRING
EQUINOX
12 hrs
12 hrs
12 hrs
12 hrs
12 hrs
12 hrs
12 hrs
12 hrs
SUMMER
SOLSTICE
24 hrs
24 hrs
18 hrs
12 hrs
11.8 hrs
11.5 hrs
0 hrs
0 hrs
FALL
EQUINOX
12 hrs
12 hrs
12 hrs
12 hrs
12 hrs
12 hrs
12 hrs
12 hrs
WINTER
SOLSTICE
0 hrs
0 hrs
8 hrs
11.5 hrs
11.8 hrs
12 hrs
24 hrs
24 hrs