Planetary paths

Demonstration

The apparent motion of the Sun and planets around the Earth, including epicyclic motion.

Apparatus and Materials

• Links to animations, planetariums and orreries that could be used in this experiment are given in the guidance note:

  Teaching aids: star charts and model planetariums

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Draw a large sketch of the path of a planet as seen from the Earth. Planets that are currently observable in the night sky can be found in The Yearbook of Astronomy (Macmillan).

2. Show students an oblique view of an epicycloid. This can be drawn freely with a felt-tip pen by moving the pen round in a small circle (diameter about 10 cm) whilst sweeping the hand round a larger circle of diameter about 50–100 cm.
3. Tear out a patch of paper from the resulting pattern and hold it obliquely so that students can see the pattern almost edge on.

Teaching Notes

• A few stars show entirely different behaviour from the others. They were the ones singled out by some of the earliest observers to be watched with great care and awe. We call them planets, using the Greek name which means wanderer.
• Like the Sun, the planets sweep round the star pattern in daily motion. Freezing out that daily motion, we find that each planet slips slowly backwards from west to east through the star pattern in the course of years, along a path in the Zodiac belt. Unlike the steady motion of the Sun round the ecliptic, the outer planets have an irregular motion through the star pattern. They slide backward for a time, come to a stop and then move forward again. The backward motion from west to east predominates, carrying the planet, Jupiter, for example, all the way round the Zodiac in 12 years. The short forward motion in which the planet makes a loop (seen almost sideways on) occurs almost once a year.

• The path that we see the planet taking through the star pattern seems to be an epicycloid; that is, a compound of motions round a small circle and a big one.
• Optional: display the Information about the planets (see below).

This experiment was safety-tested in March 2007.