Solstick: Measuring the Earth

Follow in the footsteps of ancient astronomers to recreate the methods they used to measure the size of the Earth in this UK national science experiment, run and hosted by the Schools' Observatory' and the Royal Astronomical Society.

Back in the days before we had satellites and specialist instruments to help us calculate the size of Earth, early scientists found clever ways to calculate the approximate size of the planet. More than 2000 years ago, Eratosthenes realised that he could use the length of shadows to work out an estimated size - and it turns out that by taking two simple measurements, at midday on the summer solstice he was able to calculate a surprisingly accurate estimate of the size of Earth.

We've created a tool that uses this same maths, and your measurements - so you can have a go at working out the size of Earth.

All you have to do is take a few simple measurements around the summer solstice on the 21st June 2023, enter them into our calculator and you will be rewarded with your very own measurement of the Earth!

Your measurements will be added into our database and after the solstice you can check back to see how accurate your result was compared to our national average.

If we get enough measurements we will also then be able to use all the measurements to calculate the size in a better way, so remember to get all your friends to take part.

You will need:

A long, straight stick. Your stick can be a pencil, a dowel, a skewer or anything thin and straight. The longer the better!
A ruler or tape measure long enough to measure your stick.

Then all you need to do is:

On Monday 19th June, Tuesday 20th June, Wednesday 21st June, Thursday 22nd June or Friday 23rd June place your stick vertically on the ground in the sunshine (outside or in a sunny spot by a window).
You can prop your stick up, or place it in a pot of soil, or dig it slightly into the ground. You could also use blutac to stick it to the ground and keep it upright. Or even just ask someone to hold it steady for you!
You must be somewhere where the ground is flat and sunny.
Measure how tall your stick is.
At 1PM measure the length of the shadow created by your stick. (Don't worry if you are a few minutes early or late).
Come back to this page to give us your measurements and get your results!

If it isn’t sunny on the first day you try, try again! You can enter your results as many times as you want.