** Meccano Model Images**

**The images on this page are a subjective collection of
images of various Meccano \ Erector models that I like.**

Below are 6 pictures of Paul McMahon's orrery. Inspired by the astronomical
models on this site, he wanted to design his own.
It's a sun, moon and earth orrery.

It produces exactly 365.25 days in a year (i.e. accurate to within 0.002% of the
standard Gregorian calendar year). The moon rotates around the earth every 29.65
days (i.e. ~0.3% accuracy), and the earth is inclined at ~23 degrees to the
ecliptic to demonstrate the seasons.

It uses only standard Meccano gears and sprockets. It's a bit different from
some of the other orreries as the drive motor is in the base. This gets around
the problem of providing power to the rotating arm. It also allows more
flexibility in the gearing to produce the peculiar ratios required, without
resorting to special gears. There is a separate battery box to provide power for
the light (spacebase set) which shines through the centre of the sun.

Here's a list of the gears used:

1. Rotating the earth around the sun, and rotating the earth itself

15t x 1

19t x 6

25t x 3

60t x 1

95t x 2

worm x 1

contrate 50t x 2

bevels 16t,48t 1 pair

133t x 1

2. moon drive (and final drive to earth)

15t x 3

19t x 4

57t x 1

60t x 3

14t sprocket x 1

18t sprocket x 1

3. Keeping the axis of the earth inclined at a constant angle sprocket 36t x 2
(although not critical - any two the same size will do)

168 bearing complete x 1

73t 4" toothed disc x 1

A couple of other gears are used to reduce the speed of the motor (11t pinion
& 50t contrate) but these aren't critical.

The earth globe is a novelty pencil sharpener. With the sharpener removed, it
fits nicely on a small flanged wheel.

Since the drive is fixed in the base, the calculations to provide the correct
speeds are a little unusual. Firstly the earth has to rotate 366.25 times a year
to provide 365.25 days. The target period for the moon is the sidereal month of
27.32 days, which is equivalent to 29.53 days for the (synodic) month, as viewed
from earth. In the model, the sidereal month is 27 3/7 days which equates to
29.65 days for the standard lunar month.