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Neurobics

Astronomical Ping Pong

Lloyd Esler

Some people consider the Earth to be no more significant than a ping-pong ball in the overall scheme of things. If the Earth was the size of a ping-pong ball, what then? For a start, you would be a lot smaller, only about 50 nanometres tall. The Sun would be a 4.14-metre sphere about 450 metres away. The nearest star (Alpha Centauri) at this scale would be120,000 kilometres away (about a third of the way to the Moon).

If the Sun was a ping-pong ball, Earth would be a 0.3mm dot four metres from it. Get someone to hold up a ping-pong ball and move backwards until its size matches that of the Sun in the sky -- you should find yourself about four metres away. Use the same technique to guess the distance from Earth to the Moon if the Moon was a ping-pong ball.

  • Seeing Pluto from Earth is equivalent to seeing a ping-pong ball at a distance of 76 km, or about the distance from Invercargill to Lumsden or Clinton.
  • The solid crust on Earth is about the same relative thickness as the skin of the ping-pong ball -- think of the Earth as ping-pong ball filled with hot golden syrup.
  • We know that the maximum number of colours needed for a map is four, but does it work on a sphere? Imagine your ping-pong ball is a planet. Cover it with countries and work out the maximum number of colours required so that no countries of the same colour are touching.
  • Use a ping-pong ball and a pebble to demonstrate air resistance. Gravity acts with the same force on both, but air resistance means the ping-pong ball falls slower. What would happen if you tried it with a pebble and a ping-pong ball on the Moon?
  • Use a desk lamp and two ping-pong balls to show how the Earth casts a shadow on the Moon to produce a lunar eclipse, and how the Moon casts a shadow on Earth producing a solar eclipse.
  • Use a desk lamp and a ping-pong ball to show how the phases of the Moon are seen from Earth.
  • Use two ping-pong balls to demonstrate the motion of the Earth around the Sun and the Earth turning on its axis.
  • Consider the difference between mass and weight. A ping-pong ball's mass would be the same on the Moon as on Earth, but its weight would be less on the Moon.

Lloyd Esler lives in Invercargill and has an interest in things scientific.

Russell Dear is a Mathematician living in Invercargill