Precession of the equinoxes
Using the model celestial sphere to show the precession of the equinoxes.
Apparatus and materials
Model of the celestial sphere from the experiment Model of the celestial sphere.
Health & Safety and Technical notes
Take care with glassware. Keep the model out of sunlight as flask and water can act as a convex lens and produce localized heating.
Mark a new axis on the sphere, perpendicular to the ecliptic. This can be marked on the flask with small circles of coloured sticky paper indicating where the axis meets the surface. (An ideal arrangement is to fix two small suction caps at these points.)
Hold the model so that it can revolve very, very slowly about that axis, whilst the whole model is imagined to be spinning very rapidly (10 million times faster) round the Pole Star axis.
The precession of the equinoxes, as described by early astronomers (from a geocentric - Earth-centred - point of view), was a very obscure creeping motion of the whole system of stars around a special axis (the axis of the ecliptic). It was as though the whole Zodiac belt slipped very slowly round the celestial sphere, carrying all the stars with it and leaving the celestial equator attached to a fixed Earth. In that model it is difficult to describe, but Copernicus made it much simpler.
The precession of the equinoxes is a slow rotation of the whole pattern of stars around the ecliptic axis, one revolution taking 26 000 years. This motion was discussed by Hipparchus (~190 BC to ~120 BC).
This experiment was safety-checked in April 2007