An eccentric scheme for the Sun

Demonstration

An Earth-centred model to explain the irregularity of the seasons.

Apparatus and Materials

• Ball, large (7.5 cm diameter approximately)
• Ball, small (2.5 cm diameter approximately)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Hold the large ball - representing the Earth - in one hand. With the other hand, holding the smaller ball to represent the Sun, sweep out a large vertical circle. The Earth should be noticeably off-centre with respect to the Sun's circular orbit. This demonstration needs practice!
2. If an elastic thread is joined between the two balls students can watch the thread and see how the Sun’s apparent speed must change through the seasons if the Earth is off-centre.

Teaching Notes

• The Sun’s motion around the ecliptic is a little faster in the northern winter than in the northern summer so the interval between mid-winter and the spring equinox is a little shorter than between mid-summer and the autumn equinox. The seasons are not quite the same length.
• To imitate this uneven motion, the Sun was imagined to move round a circle at constant rate but the Earth was placed a little off centre. Then the Sun, viewed along a line of sight from the Earth, would seem to move a little faster in December than in June.

This experiment was safety-tested in July 2007.