What You Need
A straight stick or pole (about 1 metre), a flat surface that receives sunlight, a pendulum (string and a weight — a stone, a bolt, anything heavy enough to swing steadily), any clock (a wall clock, a phone, a watch), and a notebook.
No electronics. No special equipment. No cost.
What You Do
Each day, at approximately the same time around midday, you record three measurements:
1. Solar noon. Plant the stick vertically in the ground or stand it on a flat surface in sunlight. Watch the shadow. The moment it is shortest is solar noon — the Sun is as high as it will get today. Write down the time on your household clock when this happens.
2. Pendulum count. Set the pendulum swinging and count the number of complete swings in exactly 60 seconds, timed by your household clock. Record the count.
3. Clock reading at solar noon. Write down the exact time shown by your household clock at the moment of solar noon.
Repeat this every day for 7 to 14 days. Record every number. Record the weather (clouds affect shadow sharpness). Record anything unusual.
What You Are Looking For
Three clocks: the Sun, the pendulum, and the household clock. They will not agree. Your job is to find out how they disagree and why.
Does solar noon happen at the same clock time every day? If not, is the difference random or does it drift in one direction? (It drifts. This is a real physical effect called the equation of time, caused by the tilt of the Earth’s axis and the shape of its orbit. You are measuring it.)
Does the pendulum give the same count every day? If not, what changed? (Temperature affects the length of the string. Wind affects the swing. Your reaction time affects the count. These are different kinds of error, and learning to tell them apart is one of the most important skills in experimental science.)
Which discrepancies come from the world, which from the instrument, and which from the observer? This is the core question. You cannot answer it on the first day. You need at least a week of data. That is the point.
What This Teaches
A clock is defined by its comparison to another clock, not by its mechanism. Disagreement between clocks is data, not failure. Systematic deviations (the equation of time) are distinguishable from random scatter (reaction time, wind) if you have enough observations. No clock establishes its own correctness in isolation.
These ideas — comparison, systematic error, random error, reproducibility — are the same ideas used in every tier of this programme, from electronic oscillators to atomic clocks to pulsars. You are learning them here with a stick and a string.
Your Notebook
Record everything. A good format for each day:
Solar noon (shadow shortest): clock reads ___:___
Pendulum count in 60 seconds: ___
Weather / conditions: ___
Anything unusual: ___
After 7–14 days, make a table of all your readings. Look for patterns. Draw a graph if you can: day number on the horizontal axis, solar-noon clock time on the vertical axis. What shape do you see?
Going Further
If you have completed two weeks of observations and want to go deeper:
Try adding a fourth clock. The Moon’s phase is a slow clock: it completes one cycle in about 29.5 days. Record the Moon’s appearance each evening. How does this “lunar clock” compare to the solar and mechanical clocks?
Try measuring the pendulum period precisely. Instead of counting swings in 60 seconds, count 100 swings and divide by 100. How does this affect the scatter in your data?
Try building a second pendulum of different length. Compare the two pendulums to each other. Which is more stable? How do you know?
Ready for electronics? Continue to Tier 1: Build.
A printable observation log for this experiment is in development and will be available in the worksheets folder.