Welcome to post number 2 in this series on guesstimation. If this is your entry point into the world of guesstimation then please go back to the first post in this series to get some more context as to what we are trying to do in this series.

Today is all about doing your first guesstimate. We’re going to estimate the distance to the centre of the Earth, using nothing but a bit of elementary maths and our everyday experiences. Intriguing, right?

We’re going to find that we can be quite cavalier with our numbers so long as they are not more than 10 times too big or too small - it turns out that (most of the time) small errors tend to cancel out which is what makes guesstimation so powerful.

So, our goal today is to calculate the distance to the centre of the earth to within a factor of 10. How are we going to do this?

We’re going to need to think about:

- the connection between the circumference of the Earth and its radius 🥧
- what personal experiences we can use to extract some useful numbers, e.g. distances, speeds and times 🛩
- what cavalier approximations can we use to make our calculations as simple as possible 🔵 ➡ 🟦

I’ll say no more and let the video guide you through this mini adventure.

Good luck!

P.S. Let me know what answer you get by posting on this little web-app I made (you can also see other peoples answers too)