Friday, May 3, 2013

The Seasons, Day and Night

Ever since, many years ago, I observed a student teacher erroneously  demonstrating to a 4th grade class how the sun orbits around the Earth I have made it a point to include a class session on the reasons for the seasons and why days and nights vary in length.

Apart from this obvious error there are all kinds of misinformation students can develop in their own "private universes" that we must guard against. On the left is a classic picture found in many textbooks that attempts to show the tilt of the Earth in relation to the sun. Unfortunately, the picture also shows the earth significantly larger than the Sun as well as showing how the Earth appears to get closer or further away from the sun as it makes its annual orbit. These misconceptions are caused by the scale and perspective of the drawing. Both of these pieces of misinformation can lead to the development of major misconceptions that can live in the private universe for a lifetime.

I usually try to demonstrate the changing seasons by having the students sit in a circle with me in the center holding a flash light. They then pass the globe around the circle making sure to keep the Earth tilted toward the front wall of the classroom at all times. The other piece that goes along with the tilt of the Earth is the idea of insolation. This is basically the different intensity of the suns rays on the Earth's surface caused by the angle at which they meet the Earth's surface. This can be modeled by holding the flashlight virtically above the desk and then at a shallow angle when the pool of light (and heat in the case of the sun) becomes much less intense and more spread out.

As the globe moves around the circle I try to shine the flashlight on different parts of the Earth's surface making sure that it is always horizontal. As the globe goes around the circle you can then see how the overhead sun moves from the extremes of Tropic of Cancer (our summer in the north) to the Tropic of Capricorn in the south (our winter). It also demonstrates how the north pole can rotate through 360 degrees and never be in the dark in summer or always in the dark in the middle of winter.  It helps to have the students imagine they are standing on the Earth's surface and observing the sun at these different times of year.

The same strategy also works for exploring day and night. Imagine you are watching the sun behind the Adirondack Mountains in the west across Lake Champlain. Now reconceptualize the mountains "coming up" to cover the sun rather than the sun going down and you can develop a much better sense of the rotation of the earth on its axis and the reason for day and night.

Finally, there's this remarkable website that gives you a great sense of global issues such as the days as well as the different lengths of day and night between the summer and winter. The shaded part on the map is night time. Look how much shorter the nights are in summer in the northern hemisphere than they are in the southern hemisphere. Isn't that way cool? You can also see where it is tomorrow. 

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