An eccentric scheme for the Sun
Practical Activity for 14-16
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
- 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!
- 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.
- 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.