# Model to illustrate precession of the Earth

##### Demonstration

This demonstration imitates the Earth’s precession, with a spinning flywheel ‘rocked’ by the pull of a taut rubber band.

#### Apparatus and materials

Large gyroscope

Mount for large gyroscope

#### Health & Safety and Technical notes

A suitable gyroscope and mount are available from ASCOL.

A small massive flywheel, which can be set spinning in a frame, is held by another frame hung by a long thread.

Nylon is suitable for the long thread used to support the apparatus because it does not make things spin when used for suspending an object like ordinary strings do. Any torque from the suspension will be unwelcome.

#### Procedure

a Hang the frame carrying the rotating flywheel on a long thread, preferably from the ceiling.

b Spin the flywheel rapidly: this can be done by hand by bringing it against a rapidly rotating wheel attached to a fractional horse-power motor. Be careful that clothing does not become entangled in the rotating motor.

c Set the wheel so that its axis points in a definite direction, say 45 % from the vertical and across the pupils’ line of sight.

d Slip an elastic band over the hooks on the frame and wheel axle. Precession will start. The moment the band is slipped off, precession stops immediately.

#### Teaching notes

1 Precession is the slow, conical motion of the Earth’s spin-axis round the axis of the ecliptic; that is, around a line through the Sun, perpendicular to the Earth’s orbit; an axis at 23.5o from the Earth’s polar axis. Newton showed that this motion is a necessary consequence of gravitation and the Earth’s spin.

A spherical Earth, whether spinning or not, would keep its axis pointing in a constant direction among the stars as it followed its orbit around the Sun. An Earth with an equatorial bulge will suffer extra gravitational pulls exerted by the Sun (and Moon) on the bulge. These extra forces applied to a spinning body show that rocking forces applied to a spinning body do not succeed in rocking the body, but produce precession instead.

2 This is a modification of ordinary gyroscope demonstrations, specially arranged for teaching the way in which precession of the equinoxes is caused.

Initially the wheel spins with its axis pointing in an unchanging direction. When a rubber band is installed, between the outer frame and a hook on the inner frame, precession starts, the spin axis moving slowly round a cone. When the rubber band is unhooked (without interrupting the spin) precession stops.

The pull of the rubber band represents the Sun’s net gravitational pull on the Earth’s equatorial bulge.