• Estimating the size of the Sun.

      Physics is all about understanding the patterns of reality. ‘How do we know?’ is an important question to ask about everything you learn.  How do we know how big the Sun is?  This exercise will enable you to perform a simple experiment outside and come up with your own figure.

      Below we share further information and ask you to complete the calculation. 

      sunPhysics: The Diameter of the Sun – a Practical Investigation

      cartoon graphucIntroduction: Perhaps you have noticed that if you look at the ground under a shady tree on a sunny day there are ‘sunballs’ formed on the ground. (or on a ‘screen’ (piece of paper) held there as in the picture on the right.

      These sunballs are images of the sun formed by little gaps between the leaves acting as ‘pinholes’ like in a pinhole camera.

      The Interesting point is that the ratio of the diameter of the sunball to its distance from the pinhole is the same ratio of the Sun's diameter to its distance from the pinhole. We know the Sun is approximately 150,000,000 km (or 150x109 meters, that’s 150,000,000,000m) from the pinhole, so careful measurements of the ratio of diameter/distance for a sun ball leads us to the diameter of the Sun.


      fraction numerator d i a m e t e r blank o f blank s u n b a l l comma blank d over denominator d i s t a n c e blank t o blank p i n h o l e comma blank h end fraction equals fraction numerator d i a m e t e r blank o f blank s u n blank over denominator blank d i s t a n c e blank t o blank s u n blank end fraction

      sun image

      What to do:

      1. You’ll need a sunny day, a coin of known diameter, d, and instead of a tree of unknown height above the ground you’ll use a piece of card with a small pinhole poked in it with e.g. a pencil.

      2. Put the coin on the ground and hold the card above it until the projected image of the sun is the same size as the coin. Measure the height above the coin of the card, h.

      3. Calculate the diameter of the Sun!


      Diameter of coin, d = ___________m

      Height of card, h = _____________m

      Distance to the Sun = ___________________________ (!) m




      So diameter of the sun =

    • Continuing with the theme of ‘How do we know?’, watch this video which explains how it was possible thousands of years ago to calculate the size of the Earth!  Can you follow this and  confirm the basic maths involved?

    • If you'd like to find out more studying A Level Physics at Huddersfield New College, click here to read the full course guide.