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Science project idea

Noon shadow tip marked every week: the analemma figure-eight

If you mark the tip of a stick’s shadow at the same clock time each week, the marks form a long thin figure-eight called the analemma. It combines the Earth’s tilted axis with its elliptical orbit.

Make it yourself8 numbered steps, the materials and the science, all free.Go to the stepsGet a ready-made kitReady-made models, kits and spare parts in the store under Science.Browse ScienceAsk on WhatsAppAsk the head office whether a model or kit for this idea is available.Send a message

What it shows

If you mark the tip of a stick’s shadow at the same clock time each week, the marks form a long thin figure-eight called the analemma. It combines the Earth’s tilted axis with its elliptical orbit.

The science

The Sun’s height at noon changes with the seasons because the Earth’s axis is tilted by about 23.4 degrees, so the shadow is short in summer and long in winter. The Sun also appears to be ahead or behind the clock by up to about 16 minutes in a year, an effect called the equation of time, because the Earth’s orbit is an ellipse and the axis is tilted. Together the two effects shape the figure-eight path of the shadow tip at one clock time. Because India uses one standard time for all places, the moment of true local noon depends on how far east or west you are.

Materials

Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.

  • A vertical stick or nail on a flat board, 20 cm tall
  • A large sheet of white paper fixed under the stick
  • A spirit level or plumb line
  • A phone clock set to an accurate time
  • A pencil and ruler
  • A small compass to align north
  • Tape

Safety

For this project: Do not look at the Sun. Keep the board where nobody trips on it and where it will not be moved.

Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.

All the safety rules

Step by step

8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.

  1. Fix the paper on a flat, level board and fix the stick upright in the centre with the plumb line. Align the board with the compass and tape it down so it never moves.

  2. Choose one clock time near local noon, for example 12:30, and check that the shadow falls on the paper at that time.

  3. At that exact time on the first day, mark the shadow tip with a dot and write the date next to it.

  4. Repeat once a week, always at the same time on a clear day. A common mistake is to change the time by mistake after a time change or to bump the board.

  5. Continue as long as possible. Over about 10 weeks the dots move steadily towards or away from the stick.

  6. Measure the distance of each dot from the base and plot shadow length against the date on a graph.

  7. Measure the sideways position of each dot from the north line and plot it against the date.

  8. If you can follow for a year, join all the dots to see the figure-eight, or compare your pattern with the one published for your latitude.

What you should see

The dots lie on a curve that moves along the paper and sideways over the weeks, with long shadows in winter, short in summer, and a sideways shift of the same size as the time difference between clock noon and true noon.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What causes the seasons?

The tilt of the Earth’s axis changes how directly the Sun’s rays strike a place.

2What is the equation of time?

The difference between solar time and clock time, up to about 16 minutes, from the Earth’s tilt and elliptical orbit.

3Why is the figure-eight tall and thin?

The Sun’s height changes by 47 degrees over the year while its time shift is only a few degrees of angle.

4Why is the board kept still?

Any shift of the board would be mistaken for a change in the Sun’s position.

Free ebook: Viva Questions for Science Projects

Take it further

Use the lengths of the noon shadow on both solstices to calculate the Earth’s axial tilt from the two noon altitudes.

Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.

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