Astronomical Models continued

To correctly represent the motion of the earth and the moon you have to allow for the differences in their axes of rotation.

Fig. 1 Motion of Earth around Sun

Motion of Earth around Sun

Fig. 2 Motion of the Moon

Motion of Moon

The movements this model represents are shown diagrammatically in Figures 1 & 2 on the left, and in the table below.  When the model is in motion, the seasons of the earth are easily appreciated, and eclipse of the sun and moon are also correctly represented.  Because the Moon's orbit is tilted at 5 degrees, it rises and falls, approximately once a month, through the plane of the earth's orbit round the sun.  Only when the moon is actually in this plane at the same time as being in line with the sun and the earth can an eclipse occur, which is why this plane is called the ecliptic.

Movements shown in this model

1. Earth revolves the sun once per year.  The plane in which it moves is the ecliptic.

2. Earth rotates on its own axis, once per solar day, 365.26 days per year (in model 364.50).

3. Earth's equator is inclined at 23.5 degrees to the ecliptic.   Its axis points in a constant direction in space, to the Pole star.

4. Moon revolves round Earth in 29.5 days (in model 29.6 days).

5. Plane of moon's orbit is inclined at 5 degrees to the ecliptic.   This plane is nearly constant in space, but its pole rotates slowly backwards, one revolution in 18.6 years (in model 19.0 years).

A very complex structure has to be assembled to correctly represent the Earth and Moon motions.  Their drives both move up the central axis.  There are two other drives for the lunar orbit cages.

Lunar Orbit Cages

Main Gearbox of Orrery


The main gear box of the model. 

Orreries were once fashionable adult toys of the 18th century. Usually an Orrery consisted of a set of concentric tubes, one for each planet with outer arms supporting balls representing the planets.  They fell out of fashion as they are only approximate representations, the orbits have to be circular rather than elliptical.   They still remain a favorite model for Meccano engineers.  Click here for an illustration of Joseph Wright of Derby's, "A Philosopher giving that Lecture on the Orrery".

Six of part 143, illustrated on the right, have to be used to make up the various sub-assemblies that carry the orbit cages.

Modelplan No. 59 by Alan Partridge which has complete instructions for building this model, may be purchased from the publishers MW Models.

My constructions inspired Paul McMahon to build a model of his own.

Circular Girder part 143Part 143

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