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The Earth revolves around the Sun on its own orbit together with the other celestial bodies in our Solar System. One revolution around the Sun takes one year. The axis of the Earth is inclined at about at 23 degrees compared to the orbit plane. This fact and the revolution cause the seasons. The drawing below shows the orbit position of the Earth during various seasons (VH, Finland).

Aim: To explore the phenomena caused by the regular revolution of the Earth around the Sun and the inclination of the Earth’s axis.


Explorations: Explore by means of the models the revolution of the Earth around the Sun and the seasonal circumstances at various parts of the Earth’s orbit. On the northern hemisphere the Polar Regions are towards the Sun in the summer. That is why the summer nights are warm and light through the night. During the winter, on the other hand, the northern Polar Regions do not see much of the Sun. Therefore, during the polar night (i.e. kaamos) there is twilight even in the middle of the day. The Sun does not rise above the horizon for many weeks. However, at the same time on the southern hemisphere, it is summer. In contrast, during the autumn time on the northern hemisphere it is spring in the south. At the vernal and autumnal equinox especially, the Sun is in the zenith at the equator. That is when the day and the night are equally long everywhere on the Earth.

An ordinary bulb may indicate the Sun. It sheds light simultaneously to all parts of the imaginary orbit. The lamp need not be moved during the exploration which creates a stronger impression of sunlight spreading simultaneously everywhere. A styrox ball indicates the Earth. Explore various seasonal circumstances in a darkened classroom. Ponder the following: What is special in each case? What is exceptional or common to all cases?

1)  First prepare the Earth from a styrox ball with a diametre of about 10 cm. Push a stick into it (along the diametre) to indicate the axis. Use a marker pen to draw the equator, the tropics, polar circles, maybe Finland and some other interesting object on the ball. Craft colours may be used to draw green and brown continents and blue seas on the ball. Poles left white indicate everlasting snow and ice.

2)  In the light of the sun lamp, in a darkened classroom, discuss first on a general level the motions of the Earth on the orbit around the Sun. Keep the inclination and the axis of the Earth constantly in the same posture through the whole revolution. Ask the students to observe the sunlit regions on the Earth and their change. Each student may, in turn, carry the Earth around the Sun. The teacher ensures that the posture of the Earth is correct.

3)  Place the Earth, in turn, on the middle point of every season, i.e. at the winter and summer solstice as well as at the vernal and autumnal equinox. Find out the circumstances on both hemispheres at these points. Explore the angle of incident rays, the illuminated areas, the lengths of a day and night, the border of the midnight sun in the south as well as the border of the polar night (i.e. kaamos) in the north. In addition to these, explore at the equinoxes the equal spreading of the sunlight on both hemispheres.

4) *In order to explore shadows, fix small sticks into various parts of the Earth-ball. Implement this exploration at various times of a day and a year. Discuss special cases as to the northern and southern area. In Finland, the Sun rises in the east, travels its path via the south and sets in the west. As a matter of fact, the Sun only seems to travel due to the direction of the Earth’s rotation around its axis from the west to the east. In Southern Australia the Sun’s path goes from the east via the north to the west. With the help of the shadows produced by the sticks explore the case when the Sun is in the zenith, i.e. vertically above the head of the observer, and there is no shadow. Likewise, explore the cases on the tropics and the area between them. On the tropic of Capricorn the Sun is in the zenith at the winter solstice and on the tropic of Cancer at the summer solstice. On the area between the tropics the Sun is in the zenith in two days of the year. It looks like the Sun travels back and forth on that area. Depending on where between the tropics the observer is, the amount of the times when the Sun goes via the north and the south varies. For instance, if the Sun goes via the north on 20 days of the year and via the south on 340 days, the observer must be very near the tropic of Cancer on the southern side of it.  (See ARCI The daily path of the Sun)

5) *Explore more closely the four special positions on the orbit, i.e. the solstices and the equinoxes. At each point discuss the distribution of thermal energy and amount of light to various parts of the Earth. The students are encouraged to make explorations independently and present their findings for further discussion.

6) * Using a model, explore the horizontal point of the sunrise and sunset during the seasons. The Sun rises in the summer nearly in the northeast and in the winter in the southeast. In summer the Sun sets in the northwest, and in winter the path of the Sun is short; the horizontal point of the sunset is approximately in the southwest. Try to illustrate these cases in a darkened classroom by using a model.

Methods: Using a scale model, explore the revolution of the Earth around the Sun. Illustrate all four seasons in a darkened classroom lit by a lamp by positioning the Earth on the imaginary orbit according to the seasons. Compare the circumstances on the northern and southern hemisphere during various seasons.

Materials: Styrox ball, long sticks, marker pens, a bright lamp, literature.


Pondering: It is worthwhile to discuss with the model at hand the following questions: How illustrative is the model? Is the scale of the model valid? What are the possible biases of the model if there are any?


Evaluation of results: The teacher must ensure that the students are able to interpret the cases produced by the model correctly and that possible misunderstandings have been corrected. It is easy to make a quick test with the model. Change the preconditions and ask the students to create the case with the model according to the given information. Or position the parts of the model to a case and ask the students to interpret it and share the detailed interpretation with the others. The test works in both ways.


Hints: It is absolutely necessary to test how the students understand various cases. For instance, the students can be asked to position the globe according to the season or to present a 24-hour case by using a stick to produce a shadow. They can also be asked to position the stick to illustrate a certain observation result. The students can also test each other by setting certain preconditions to a case and asking other students to find a case that fulfills those preconditions.


Keywords: Seasons, Earth’s orbit, solstices, equinoxes, polar night (kaamos), midnight sun, polar circle, tropic, zenith.




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