Seasons
Dr. Pamela Gore
Georgia Perimeter College
Objectives
- Describe the tilt of the Earth's axis.
- Explain the terms vernal equinox, summer solstice, autumnal equinox, and winter solstice.
- Sketch a diagram showing the Earth orbiting the sun with tilted axis, and how this relates to the seasons in the northern and southern hemispheres.
- Explain when and how it can be light at one of the poles 24 hours a day.
- Explain when and how it can be dark at one of the poles 24 hours a day.
- Explain why it is winter in one hemisphere while it is summer in the other hemisphere.
- Explain how the seasons relate to the length of the day; explain why the day is longer in summer and shorter in winter.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
- SKE1. Students will describe time
patterns (such as day to night and night to day) and objects (such as sun, moon,
stars) in the day and night sky.
- SKE1c. Recognize that the Sun supplies heat and light to Earth.
- S1E1. Students will observe, measure, and communicate weather data to
see patterns in weather and climate.
- S1E1c. Correlate weather data (temperature, precipitation, sky conditions, and
weather events) to seasonal changes.
- S2E2. Students will investigate the position of sun and moon to show patterns throughout the year.
- S2E2c. Relate the length of the day and night to the change in seasons (for example: Days are longer than the night in the summer.)
- S4E2. Students will model the position and motion of the earth in the solar system and will explain the role of relative position
and motion in determining sequence of the phases of the moon.
- S4E2c. Demonstrate the
revolution of the earth around the sun and the earth’s tilt to explain the
seasonal changes.
- S6E2c. Relate the tilt of the Earth to the distribution of sunlight throughout the year and its effect on climate.
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This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
- Because the Earth turns daily on an axis that is tilted relative to the plane of the earth's yearly orbit around the sun, sunlight falls more intensely on different parts of the earth during the year. The difference in heating of the earth's surface produces the planet's seasons and weather patterns.
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This section addresses, in whole or in part, the following National Science Education Standards:
- The sun is the major source of energy for phenomena on the earth's surface, such as growth of plants, winds, ocean currents, and the water cycle. Seasons result from variations in the amount of the sun's energy hitting the surface, due to the tilt of the earth's rotation on its axis and the length of the day.
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Seasonal Changes
Note that the Earth's axis is tilted at 23 and a half degrees to the orbital plane around the Sun.
The Earth orbits the sun in 365.24220 days.
This is the TROPICAL YEAR.
At this point, the Sun returns to its starting position on the celestial sphere.
(Note that the .24220 days is equal to 5 hours, 48 minutes and 46 seconds.
This is a little less than 6 hours. This is the reason that we have a LEAP YEAR every 4 years.)
Diagram showing the Earth with tilted axis revolving around the Sun, and how this relates to the seasons in the northern and southern hemispheres.
Note that the axis is always tilted in the same direction.
When the axis and pole point TOWARD the Sun, it is SUMMER in that hemisphere.
When the axis and pole point AWAY FROM the Sun, it is WINTER in that hemisphere.
Basic terms relating to the seasons:
- Summer solstice = The sun is as high or as far north as it gets all year.
First day of Summer (June 21 or 22)
The Sun is directly above the Tropic of Cancer (N-hemisphere)
- Winter solstice = The Sun is at its lowest, and the farthest south it gets all year.
First day of Winter (Dec 21 or 22)
The Sun is directly above the Tropic of Capricorn (S-hemisphere).
- Equinox = The Sun is directly above the equator. Rises due E and sets due W.
- Vernal equinox = Spring equinox.
First day of spring (March 21 or 22).
- Autumnal equinox = Fall equinox
First day of autumn (Sept 22 or 23).
Unusual things having to do with the seasons:
The Arctic Circle is at 66.5o N latitude.
The latitude may seem odd, but note that 90o - 23.5o (which is the angle of tilt of the earth's axis) = 66.5o
In the northern hemisphere summer, north of the Arctic Circle there are 24 hours of daylight. (Land of the Midnight Sun.)
The sun does not rise or set during the day. It stays up.
The sun rises to a maximum altitude of 23.5o above the horizon at the Summer Solstice.
The sun moves around in a circle above the horizon during the day.
In winter, there are 24 hours of darkness north of the Arctic Circle. The sun stays below horizon.
Sun rises above horizon at North Pole on the Vernal Equinox, and sets on Autumnal Equinox.
At the South Pole (south of the Antarctic Circle), similar patterns of darkness and light occur, but opposite to those of the Arctic Circle.
When the northern hemisphere is having its summer, the southern hemisphere is having its winter.
When it is light 24 hours a day at the North Pole, it is dark 24 hours a day at the South Pole.
Also note that the length of the day is longer in summer and shorter in winter.
This is related to the tilt of the Earth's axis as it goes around the sun.
You can easily see that it is light in summer 24 hours a day at the North Pole.
The length of day (daylight) is longer in summer (and shorter in winter) where you live, for the same reason.
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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA
Page created March 29, 2005
Updated to facstaff, email updates August 23, 2008