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Day and night periods

The Earth rotates about its axis when revolving around the Sun. The change from day to night is caused by the sunlight hitting the ground on various places in turn. In reality, Earth’s revolution is not absolutely steady but is slightly decelerating. That is why there are very tiny periods of time – like a blink of an eye – added between our 24-hour periods every now and then, very rarely though.  The basic unit of time, a second, is nowadays defined by means of an atomic clock. That is why the lengthening of a day and night has no effect on the unit. The time needed for the Earth’s rotation and revolution deviates in the long run a little from that of the atomic clocks. That is why the Coordinated Universal Time (UTC) must be adjusted a little every now and then in order to keep the times approximately the same. Our everyday life is deeply dependable on the rotational and revolutionary periods.  Therefore it is quite unthinkable to start living only under the precise guidance of some mysterious atomic clock. To ensure the similar pace the Universal Time must be adjusted by adding extra seconds. The next picture shows the border of the dark and light regions moving to the left when the Earth rotates to the right, towards the east (picture IH, Fell Lapland Visitor Centre, Enontekiö, Finland).


Aim: To illustrate what happens when the sunlight hits the ground on various parts of the globe at various times.


Explorations: A styrox ball with a diametre of about 10 cm represents the Earth. Fix a long stick through the ball to indicate the axis. The oceans and continents can be painted on the ball, if wanted.

1)  Prepare for exploration an Earth-ball with an axis and a darkened classroom with a lamp indicating the Sun. The best way of illustrating the sunlight is to use a bulb that sheds light simultaneously to all directions. Now it is simple and easy to illustrate all possible cases during a year. In the picture there is a model of the Earth, prepared for the explorations (design VH, Finland).





2) Rotate the Earth slowly in the sunlight and at the same time discuss with the students their observations. It is important to remember to keep the axis constantly inclined at 23 degrees in every phase of the revolution. Revolve the Earth around the Sun, simultaneously rotating it about its axis. It is difficult at first to handle with two motions at the same time. After rehearsing a little it will manage. Let the students do this exercise without hurrying up.  The teacher keeps track of the posture of the axis and corrects it if needed.

3)  Explore a 24-hour case by keeping the Earth at one point on the orbit. Ask the students to describe the distribution of the sunlight on the Earth. Discuss the circumstances on both hemispheres. Try to find common facts and cases as well as differences.

4) * Recognize special cases on the light region caused by the direction of the sunrays and recall their names, e.g. the equinoxes and solstices. Depending on the location of the observer on the Earth, the sunlight can at noon hit the ground vertically from above, from the south or north, low from the horizon or just by reflecting from the particles in the atmosphere.

5) * Explore the border of the light and dark region at various times. Compare the size of the light and dark regions with each other. Try to illustrate the twilight at various latitudes.

6) * Explore with the model the sunrises and sunsets in summer and winter as well as at the equinoxes. Compare the lengths of a day with each other in these cases.

Methods: Explore Earth’s rotation with the help of a scale model. Illustrate changes between a day and a night during various seasons in the light of a lamp indicating the Sun. Discuss the diversity of cases.

Materials: Styrox ball, long sticks, bulb, marker pens, colours, literature.


Pondering: Is the posture of the Earth’s axis important in these explorations? What kind of a case would give a biased result? What kind of a case would prevent life on the Earth?


Evaluation of results: It is important to find out how the students understand the substance in order to avoid misunderstandings. Common discussions during the exploration as well as individual tests are an easy way to ensure the correct understanding of the concepts and cases. The students can also be asked to draw a picture of a certain case. The teacher then corrects the misinterpretations if there are any.


Hints: The explorations made in a darkened classroom with the scale model at hand are the best way in increasing the understanding of the phenomena. The changing and repetitive cases help in recalling and testing the basic knowledge.  


Keywords: 24-hour period, day, night, twilight, polar night (kaamos), midnight sun, zenith.




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Seuraava sivu: Seasons