Motions of the Earth
Rotation
• The Earth rotates on its axis.
•‘Rotation’ is the spinning of the Earth,
from west to east, on its axis.
•It makes a complete 3600 turn in 24 hours
at a speed of 1600 km/h at the equator.
•Most places on earth experience a period
of daylight and one of darkness during this
24 hour period.
• The most important effects of rotation are
related to changes in light, heat and humidity
that occur between night and day. By imposing
a daily rhythm of day and night on many
things, this motion has had a profound effect
on life and development of all forms of plants
and animals.
• The earth’s rotation also has an important
influence on the movement of air and water on
the earth’s surface.
Revolution
• Earth ‘revolves’ around the sun once in 365.242
days. (Leap Year: add one (1) day to February every
four (4) years to make up the four 1/4 days). It travels
an average distance of 150,000,000km. The average
velocity of the earth is 106,300 km/h.
• The path or orbit made by the earth as it revolves
around the sun is elliptical in shape rather than a
circle. The distance from the sun at aphelion
(apogee) is 152 million km (July 4) and at perihelion
(perigee) the distance is 147 million km (Jan. 3). This
variation has little effect on the temperature of the
earth.
• The path followed by the earth around the
sun can be thought of as forming a flat surface
or plane. This is known as the plane of orbit
or the plane of the ecliptic.
• The earth’s axis is tilted 23.5 degrees from
the perpendicular to the plane of orbit, in other
words the angle between the axis and the
plane of orbit is 66.5 (90 - 23.5).
• The axis always maintains this degree of
tilt, remaining parallel to all its previous
positions. This is known as the ‘parallelism
of the axis’.
• The revolution of the earth about the sun
on an inclined axis has two very important
effects: seasons (seasonal temperature
differences) and variations in the length of
day and night. Neither would happen if the
axis was not inclined.
• Four important dates should be noted and are
related to five important lines of latitude. Time to
review Latitude and Longitude!
1. June 21st: The sun’s direct rays are on the Tropic
of Cancer. This day is called Summer Solstice in the
Northern Hemisphere. The days are longer because
on this day the Northern Hemisphere is inclined
towards the sun. During one complete rotation, all
places in the Northern Hemisphere spend a greater
proportion of the 24 hours in daylight than they do in
darkness. As you travel further north the day becomes
longer. Any point north of the Arctic Circle receives
24 hours of daylight and any point south of the
Antarctic Circle receives 24 hours of night.
This is an important slide:
2. Sept. 21st: The sun’s direct rays are over the
Equator. This day is called Fall (autumnal)
Equinox. The days are very close to being 12
hours day and 12 hours night.
3. Dec. 21st: The sun’s direct rays are on the
Tropic of Capricorn. This time is called Winter
Solstice in the Northern Hemisphere. Days are
shorter. Nights are longer.
4. March 21st: The sun’s direct rays are over the
Equator again. This day is called Spring (vernal)
Equinox. The days are very close to being 12
hours day and 12 hours night.
Equinox
Solstice
• There are two principal causes for the seasonal
differences in temperature created by the tilt of the earth
and the curvature of the earth. INTENSITY and
DURATION.
Duration
• Duration refers to the amount of daylight any place
receives. The longer the length of daylight a place
experiences (ie. the greater the duration), the greater the
amount of heat that place will receive.
• If you study the diagram on the next slide you will notice
how the position of the sun overhead and its movement in
the sky during the day, determines the length of sunlight.
The length of day or the arch that the sun travels in the sky
depends on the solstices and equinoxes.
Follow the Arches!
Intensity
• Variations in intensity are caused by the
difference in the angle at which the sun’s rays
strike the earth’s surface.
• If you study the diagram on the next slide, you
will notice that in Fig. 2 the curvature of the earth
forces the sun’s rays to cover a larger area than
in Fig. 1. It is colder in Fig. 2 than in Fig. 1. During
winter solstice when the northern hemisphere is
tilted away from the sun the intensity of the sun is
even less therefore it is even colder in Fig. 2.
This is another way of looking at the concept of Intensity.
Time Zones
• There is one last effect caused by the rotation
of the earth. Not all points on the earth begin
their day at the same time. All places lying on
the same line of longitude have the same time.
In theory, this means that the place you live in
actually has a different solar time of day from
any other place located east or west.
Depending on the distance the difference could
be seconds, minutes or hours.
• Before high speed travel and worldwide
communications differences in time were not that
important.
• Today we need a system of time zones. Sir Sanford
Fleming, a Canadian, developed a standardized time
zone system for the earth in 1886.
• The earth is divided into 24 time zones. Since the
earth is a 360 degree circle. The sun movers through
15 degrees of longitude every hour. (360/24=15)
• Some modifications to the time zone boundaries
were made to ensure that political regions
remained in the same time zone.
• Since the Prime Meridian (0 degrees longitude)
is located at Greenwich, England, it was decided
that the time zones would start there. The 180
degree line (on the opposite side of the earth)
became the International Date Line. (If you cross it
going from west to east you would go back one
day on your calendar, in a sense you are gaining a
day).
• Especially
in urban areas, many human
activities begin well after sunrise and continue
after sunset. Daylight saving time is obtained
by setting ahead all clocks by one hour the
first Sunday in April and is discontinued on the
last Sunday in October.
• Think of the economics behind daylight
saving time.
The End