Eclipses are a common phenomenon in astronomy. In different ways, eclipses account for the phases of the moon, transits of inner planets across our Sun, and provide a means by which astronomers are currently searching for planets outside our solar system.

Do you know?

  • How many Earth moons would fit across the face of the Earth?
  • How many times further our Sun is from the Earth than the Earth is from the moon?
  • How the Earth, sun, and moon are arranged in a solar eclipse?

This procedure will help you to build a mental model of the spatial relationship and relative sizes of the Earth, moon, and Sun. Have plenty of space, you’ll need it!

How Many Steps?

Before you dive into your experiment, you will need to be able to measure long distances by foot and know exactly how far you walked.

Materials per group

Painters’ tape
Long measuring tape (50 feet)


  1. In a space of at least 50 feet, lay down a piece of tape to mark a starting point (0 feet).
  2. Measure a distance of 50 feet and mark that spot with another piece of tape.
  3. With an even and consistent stride, count the number of steps you take, between the pieces of tape and record this number in the table below. Walk like you normally would).
  4. Repeat two more times and calculate (to the nearest foot) the average of your trials. Your average value is the number of steps you will use to measure your model outside.
Trial # Steps for 50 feet


Scaling Earth, Sun, and Moon

In the next part of your investigation, you will create a Sun-Earth-moon system outside and discover your own solar eclipse. Measure the distances between the bodies and check the accuracy of your model. Your eyeball will be the scale model of Earth!

Materials per group

  • Big latex balloon approximately feet in diameter (the Sun)*
  • Air pump (to blow up the balloon)
  • Power extension cord
  • Clamp (to close the balloon)
  • Pearl headpin (the moon)
  • Small marble (about 13 mm or 5 in)
  • Scientific calculator

*You will of course need to take the pump, power cord, and clamp outside to blow up the balloon because you will never get it through a doorway!

Useful data (scaled to a balloon diameter of 50-55”)

The table below lists the actual and scale model diameters of the Earth, sun, and Moon and the actual and scale distances between the Earth and the moon.



Diameters (actual size)

Diameters (your model)

Distance to Earth (actual distance)

Distance to Earth (your model)


1.3 x 104 km

1.3 cm (0.5 in)




1.4 x 106 km

1.5 m (60 in)

1.5 x 108 km

         ?           m


3.5 x 103 km

3.2 mm (0.126 in)

3.8 x 105 km

35 cm (1)

(1)A distance of about 35 cm from eyeball to the head of the pin (the moon) will give a reasonable approximation of the Earth-moon distance.



  1. Find a straight path outside approximately the length of two football fields (500-600 feet) and at one end, mark the spot with some painters’ tape. This will be the location of your balloon sun.
  2. Start walking away from the balloon in a safe and uncrowded Periodically, face the balloon holding the pearl head pin at an approximate arm’s distance of about 35 cm and see if it totally eclipses the balloon. If not, keep walking.
  3. Continue walking and checking until the moon completely eclipses the Mark that spot with another piece of tape.
  4. Identify the tape with a marker and, using the same size steps you previously measured, count the number of steps you have to take to get back to the balloon. Record that number below.
    1. Number of steps back to the balloon _________________ steps
  1. Calculate the distance back to the balloon, using the number of feet per step you calculate previously. Show your calculation below,


  1. Using the dimensions of your model, calculate how many times further the sun is from the Earth, compared to the distance from the Earth to the moon.


  1. Draw and label a diagram that shows the proportional distances between the Earth, sun and moon during a solar eclipse, based on your model.


  1. Using the actual data provided in the table above, show how you would calculate,
    1. How many moons, side by side, would fit across the Earth?
    2. How many Earths would fit across the sun?


  1. You created a total solar eclipse in this lesson by moving your point of view further away from the sun until you reached How does this method differ from what happens in an actual eclipse?


Link to handout


This activity was created for Saturday Morning Astrophysics at Purdue (SMAP). For information about SMAP contact Dr. David Sederberg;