Orbits, Seasons, and Ice Ages
Introduction:
At the same time the Earth rotates, or spins, on its axis, it is moving in orbit
around the sun. This motion around the sun is called revolution. The Earth’s orbit is an
eclipse with the sun a tone focus. This elliptical, or not perfectly round, orbit results in
certain astronomical consequences that were explained by the work of the astronomer
Johannes Kepler.
To understand how seasons are caused by the revolution of the Earth, we must
consider the orientation of the Earth’s axis with the plane of the Earth’s orbit. This plane,
or flat surface that contains the Earth’s orbit, is known as the plane of the ecliptic.
Diagram A below shows the Earth axis tilted, or inclined, both from the perpendicular
(straight up and down) and from the plane of he Earth’s orbit.
The Earth completes its orbit around the sun in 365 ¼ days. This period is known
as the tropical year. Every 4 years an extra day is added to February to accommodate the
extra quarter days and to correct the calendar. When viewed from above the Earth-sun
system, the Earth moves in a counterclockwise direction around the sun.
The seasons are a result of the generally constant tilt or parallelism of the Earth’s
axis in relation to the plane of the ecliptic. See Diagram 1 on your Report Sheet. Notice
the North Pole of the Earth is always pointed in the same direction. The North Pole is
always pointed toward Polaris, the North Star, regardless of the position of the Earth in
its orbit. Interestingly, the Earth is farthest from the sun, or at aphelion, on June 21.
Over long periods of time, however, this has not always been the case.
Notice Diagram 1 on your Report Sheet that the most direct or vertical rays of the
sun strike Earth at varying locations during the year. Diagram B below shows how the
sun illuminates the Earth on different dates as the Earth orbits the sun.
Because of the constant tilt of the Earth’s orbit, the Northern Hemisphere leans
toward the sun during part of the year, away from the sun for part of the year, and at
certain points, neither toward nor away from the sun.
On June 21 or 22, the Northern Hemisphere reaches its maximum tilt toward the
sun. The date of this position is called the summer solstice. Six months later, on
December 21 or 22, the Southern Hemisphere reaches its maximum inclination toward
the sun. This opposite date and position is known as the Winter Solstice. Notice that
halfway in orbit, and on the calendar, the equinoxes take place. The Vernal, or Spring,
Equinox is on March 20 or 21. The Autumnal, or Fall, Equinox is on September 22 or
23.
The shape of the Earth’s orbit and the angle of tilt have changed over the 4.6
billion year history of the Earth. The information up to this point in the Introduction is a
description of the characteristics and results of the Earth’s revolution that occur during
the period of one year and have occurred for the last several thousand years.
Consider the following information. The inclination, or tilt, of the Earth’s axis to
the plane of its orbit varies as much as 30 over 41,000 years. The shape, or eccentricity,
of the Earth’s orbit varies during a cycle of approximately 100,000 years. In addition, the
orbital position of the Summer Solstice varies during a cycle of 26,000 years in length.
The interactions and combined effects of the three cycles have controlled the amount of
solar energy received during any given season for any particular location. Diagram C
shows how these changes have proceeded for the past 11,500 years.
At present, summer in the Northern Hemisphere occurs near aphelion when the
Earth is farthest from the sun. As of 11,500 years ago, however, summer in the Northern
Hemisphere occurred at perihelion, the point in its orbit when Earth is closest to the sun.
Objective: After you have completed this investigation, you should be able to:
1) Explain how the Earth’s orbit and motion are related to seasons and climate
changes.
Materials:
none
Procedure:
1) Label (on the lines provided) the date for each of the Earth positions in Figure 1
on the Report Sheet.
2) Label the location of aphelion (Earth’s farthest distance from the sun) on the
diagram in Figure 1 on your Report Sheet. There is no line provided for this
location.
3) Label the location of perihelion (Earth’s closest distance from the sun) on the
diagram in Figure 1 on your Report Sheet. There is no line provided for this
location.
4) Identify the dates for aphelion and perihelion in the spaces provided in question 2
on your Report Sheet.
5) In question 3, record the latitudes for the location of the vertical ray of the sun on
the four dates shown in Figure 2 on your Report Sheet.
6) Figure 2 on your Report Sheet shows carrying angles of insolation, or incoming
solar radiation, which would be experienced during winter months in the Northern
Hemisphere. Note that areas A, B, and C, which are equal in size, receive varying
amounts of insolation depending on the angle of insolation. Answer questions 4
and 5 on your Report Sheet.
7) Complete the remaining questions on your Report Sheet.