Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Model of the celestial sphere

Demonstration

Making a model that shows the apparent motion of the Sun and stars around the Earth.

Apparatus and materials

Flask, large (e.g. 2 litre wide neck)

Bung to fit flask

Knitting needle, long

Polystyrene ball (2.5 cm or suitable diameter to fit through neck)

Retort stands, bosses and clamps, 2

Sellotape, coloured

Tripod with round top

Health & Safety and Technical notes


Take normal care with glassware. Keep the model out of sunlight since flask and water can act as a convex lens and produce localized heating.

diagram showing model of celestial sphereTo construct the sphere, mount the polystyrene ball on the point of the knitting needle, which has been pushed through the bung, so that the sphere is at the centre of the flask. Partially fill the flask with water so that, when it is inverted, the water level passes through the centre of the flask and the plastic ball is half immersed. Stick a circular piece of coloured Sellotape on to the 'bottom' of the flask, marking the end of the axis through the neck of the flask and the ball. This also represents the Pole Star.

 

 

Procedure


a Mark the celestial equator and place the flask, with its neck slanting downward, in a chemistry tripod with a round (instead of triangular) top; or in a horizontal ring on a retort stand. Or support it in a clamp (it may be necessary to use a second boss to prevent the clamp turning under the weight of the model) so that the polar axis is inclined at about 50° to the horizontal.
 
The level of the water provides a horizon whilst the small sphere represents the Earth at the centre of the celestial sphere.

flask model of celestial sphere  
b Add a band of coloured tape to represent the Zodiac, within which the ecliptic path will be included. This band should be at 23 1/2° to the equatorial plane of the model.
 
Rotate the model about its axis to provide a model of the repetitive, daily motion of the stars.

 

Teaching notes


diagram of model showing support rod representing the axis round which the Earth spin Rotating the flask models the daily motions of the Sun and other stars. In this diagram, an observer standing centrally, in the horizon plane, is modelled by the signpost.
1 The model: The Earth support rod represents the axis round which the Earth spins.
 
The plane of the celestial equator is at right angles to the ‘Earth’s support’ rod through to the Pole Star. It is the ring where the plane of the Earth’s equator meets the celestial sphere. The celestial sphere revolves around an axis through the Pole Star.
 
In a single night and day each star makes a complete circle around the Pole Star. Some stars are always above the horizon. Others dip down below the horizon for part of their journey. These stars rise and set.
 
The model does not provide a description of the lagging and wandering motion of the Sun, the Moon and the planets. These are best explained by ignoring the Earth's diurnal rotation.
 
2 You can stick twelve yellow stickers on the ecliptic band to represent the position of the Sun at monthly intervals. To early astronomers the ecliptic was the path followed by the Sun; today we know it represents the Earth’s yearly orbit round the Sun.
 
With the daily motion frozen we can see the Sun crawling backwards from west to east along the ecliptic, which is inclined to the celestial equator at 23.5°. The Sun’s speed round the ecliptic varies; in the northern winter the Sun travels a little faster (the Earth is closer to the Sun) and so the four seasons are not exactly equal in length.
 
3 At the equinoxes, the daily path of the Sun makes just half a circle slanting above the horizon plane between sunrise, due east, and sunset, due west. The Sun, which is always on the ecliptic, is also on the celestial equator. People living on the Earth’s equator will see the Sun directly overhead at noon.
 
This experiment was safety-checked in April 2007

 

Related guidance


The motion of the Sun