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Introduction

How can we measure the stars if we can’t reach out and touch them?

The Earth and Moon. Image Credit: NASA/ESA

In the formative years of astronomy there was a burning desire to measure the size and distance to objects in our solar system. This tutorial will explore the following 5 questions using nothing but naked eye astronomy, geometry and trigonometry:

  • How big is the earth?
  • How big is the moon?
  • How far away is the moon?
  • How big is the sun?
  • How far away is the sun?

​Let's warm up with a simple yet powerful question: how can you prove the earth is not flat using shadows?

Is the earth flat?

A flat earth was once a commonly held belief, it persisted in some areas until the middle ages. Definitive proof of a spherical earth did not come until  Ferdinand Magellan successfully traveled around the earth (1519-1521). However this belief was challenged much earlier thanks to major advancements in ancient Greek astronomy. 

The image depicts a man crawling under the edge of the sky, as if it were a solid hemisphere. This image first appeared in Camille Flammarion's L'atmosphère: météorologie populaire (1888) Image Credit: Heikenwaelder Hug

Plato (427–347 BC) was convinced that the earth was spherical: "My conviction is that the earth is a round body in the centre of the heavens...Also I believe the Earth is very vast". Significantly, he could not prove this was the case. It was his prized student Aristotle who began offering evidence for a spherical earth. One of his more convincing pieces of evidence was the shape of the earth’s shadow visible during a Lunar Eclipse. Notice the subtle curvature of the shadow cast on the moon:

Another way to prove that the earth is spherical came from Eratosthenes (276–194 BC). His method involves looking at the shadows cast by the sun on the surface of the earth. This is the basis for a sundial:

A sundial. Image Credit: John CarmichaelBut what can shadows tell us about the earth? Eratosthenes did an interesting experiment using two vertical poles located in two distant cities in Egypt. You could do this using telephone poles separated by several hundred kilometers or more. (phone a friend!)

Image Credit: Brit CruiseFirst wait until the sun is directly overhead point A, which is the moment when the pole casts no shadow. At the same time have a friend observe an identical pole at point B. You will find out that the pole at point B will cast a shadow. Since we can assume the sun's rays reach the earth in parallel lines this is visual proof that the surface must be curved!

Try out the interactive illustration below. You can click and drag the pole to see how it affects the shadow it casts.

Pause now and think about how could these shadows could allow you to determine the radius of the earth. We have a similation next which will help. You will need to understand how to calculate arc length so let's review that too. Have fun!