By comparing this with the regular beat of his pulse, he found something remarkable: as long as the lamp swung through small angles - not always the same angle - the time for one complete swing remained constant.
Children can construct pendulums using a 100g mass, suspended at the end of long piece of string or wire so that it swings freely. Ask them to investigate Galileo's idea by pulling the weight to one side and timing 10 or 20 complete swings. Use a stopwatch if possible.
They should then try using different angles of swing, by pulling the weight further sideways before release. See how far the swing can be increased before the time varies too much. Now try varying the length of the string to see if this affects the time of swing.
Another idea is to tie a string horizontally and then suspend several pendulums from it. Set just one of the pendulums swinging and see the effect on the stationary ones. It is possible to make patterns using a pendulum and some fine dry sand.
Suspend a one-metre rule horizontally across the backs of two chairs.
Attach a loop of string, to each end of the ruler so that it reaches halfway down to the floor. At the lowest point tie a single piece of string holding a plastic bag filled with the sand.
The bag needs a small hole that can be resealed, for example using a clothes peg. Place newspaper on the floor and get the children to try two experiments.
First, remove the peg and push the pendulum to swing back and forth. Watch the sand pattern on the floor.
Next, try pushing the loop in one direction and the bag of sand in the opposite direction as it is released. The sand makes patterns on the ground, often in a figure of eight.
Ray Oliver teaches science at St Albans Girls' School, Hertfordshire