Over the course of a year you can trace at the path of the midday Sun across the sky to create an analemma.
Throughout the year you may notice that the Sun at midday varies its height above the horizon.
In summer it's higher in the sky than at the same hour in winter. If you measured the angle between the highest summer position of the Sun and its lowest winter position, you would find that the difference is 47 degrees − exactly twice the tilt of the Earth's axis.
However, if you were to chart the exact position of the Sun in the sky at the same time every day you would find something more amazing.
You may like to make this activity a permanent fixture in the school yard to help you calculate the day of the year.
What you need
To do this experiment you will need to gather:
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a flat surface clear of shadows
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a pole, tomato stake or similar
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rocks or tent pegs
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a spirit level or plumb line.
What to do
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Place the pole or tomato stake in the ground, leaving at least 50 centimetres above ground. Using the spirit level or plumb line ensure that the pole is upright and that it will remain immobile for at least a year.
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Every few days, at the same time of day (ignoring daylight saving), use a rock or tent peg to mark where the tip of the shadow falls.
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Keep in mind that these marks have to last for an entire year.
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After a year, join the dots to trace out a figure of eight shape.
What's happening?
Over the year you will discover that the shadow of the Sun doesn't just move up and down along the ground, but also from side to side, tracing out a figure eight.
This shape is called an analemma, and it's been known to astronomers and geographers for centuries.
The closer a satellite is to the object it orbits, the faster it needs to go. The orbit of the Earth around the Sun is not a perfect circle; it's an ellipse. At times it is close to the Sun and must go faster, when it is further away it goes slower.
This causes the Sun to appear 16.4 minutes ahead of our clocks at the end of October, and 14.2 minutes behind them in February.
This fast and slow movement causes the shadow to move in the east-west direction, while the tilt of the Earth's axis, coupled with its orbit around the Sun, causes the shadow to move in a north-south direction.
When these two movements are combined, we see that the Sun traces the figure eight on the ground, provided we plot its position at the same time every day.
See more activities in Physics activities.
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