The movement of the Earth and the seasons

What the Activity is for

This is a simple but effective demonstration of the daily and annual movement of the Earth.

The first activity is essential to grasp the point that the axis of rotation of the Earth is tilted at 23.5 ° to the plane in which it orbits the Sun. This means that the angle of incidence of the Sun's rays will vary between winter and summer.

The major point of the second pair of activities is to show that if you change the angle of incidence of the Sun, it makes a major difference to the amount of energy received locally at the Earth's surface and explains the seasonal variation in temperature. The first involves a model which leads to some predictions and the second is an experiment which tests those predictions. This combination of activities therefore illustrates aspects of the way in which scientists work.

What to Prepare

• an Earth globe – preferably a large inflatable plastic one (the bigger the better)
• a bright lamp (A household lamp is best, but you can make do with a slide projector or OHP – but then you'll have to explain that the Sun does not really send light out in a beam.)
• 1 metre rule
• a dark room

What Happens During this Activity

Tell the pupils that we believe that the Earth orbits the Sun once a year. Secondly, we know that the Earth spins on an axis like a top. However the axis is not at right angles to the plane in which the Earth travels around the Sun, and this causes the seasons. The following model demonstrates the effect:

• Place the lamp (Sun) in the middle of the room, on a trolley if possible.
• Clear the tables or a space so that it is possible to walk around the lamp in a circle.
• Show the Earth (globe) and, holding it by the North Pole, ask how it moves in one day (spins once every 24 hours).
• Now show that once every year the Earth goes right around the Sun spinning every day. Walk around showing this or better still ask a pupil to model the movement for the class (it is a good idea to follow the Earth around with the lamp, if this produces a beam, so that the Earth remains in the Sun's light).
• Tell the class that there is one further detail to be aware of: the axis on which the Earth spins is at 23.5 ° to the plane in which it goes around the Sun.

• Now show the position of the Earth in winter (for Northern Hemisphere). Hold a ruler to show how the rays of the Sun are incident. Point to the shallow angle.
• Now walk the Earth around to its position in summer. Using the ruler show the angle at which the rays come in. Point to the steeper angle.
• Conclude by emphasising that the tilt leads to a large change in the angle of the incident rays between summer and winter. Ask what effect this is likely to have.