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This orrery is made primarily from plywood and mdf using a cnc router.

It was not intended to make an instructable and was designed using rough sketches so I apologise for the lack of good drawings and construction photos!

It incorporates :

- 8 planets including earth rotating around the sun.

- The earths axis maintains its orientation in space as it orbits, giving the correct seasonal variation.

- A disk giving the International Astronomy Union constellation boundaries centred on the earth which also maintains its orientation in space as the earth orbits. This gives (I hope) the constellation in which any of the planets or the sun lie, as viewed from earth.

- A moon rotating around the earth which can be used to judge the moon phase

- A 100 year disc to indicate the year and a yearly scale to indicate the date.

-The orrery is rotated by hand using a knurled knob.

Step 1: Video of Orrery in Operation

Step 2: Planetary Gearing

The planetary gearing is contained in a 12 sided case (each side corresponding to a tropical zodiac sign). Some of the sides are removed for access.

The gears are cut from 10mm ply to modulus 2.

There is an 8mm input shaft, turned by a knurled disc, and a series of concentric tubes (made from a stainless steel extending pointer) drive the planets

The 100 year disc is driven by the input shaft but is mounted on a third shaft to bring it closer to the edge and improve its visibility.

On the input shaft, only the 77t, 59t and 16t gears are fixed to the shaft.

The {39t, Knurled disc, 53t, 79t } and {78t, 53t, 26t, 22t, 15t} are two blocks of gears which are free to rotate on the input shaft.

On the concentric driven shafts, the {41t, 78t} and {101t, 39t} are solid blocks, the first fixed to the earth tube and the second is free to rotate.

Above the deck, the planets are mounted using a bicycle spoke fitted radially into a tight fitting wooden hub on the relevant concentric tube. The axis of the planets are aligned with their spin axis in space.

Step 3: Earth Control

The earth axis is mounted on an arm. Three equal gears below the arm, the central one fixed to the deck, keep the earth axis aligned in space.

A similar system above the arm maintains the alignment of the I.A.U. constellation scale. In this case the middle gear is fixed to the shaft of the lower middle gear , removing the need for the first central fixed gear.

This arrangement is used to accommodate the moon shaft which is concentric with the earth axis.

Step 4: Moon Control

The moon shaft is concentric with the Earth's axis.

A ratio of an 8 tooth pinion and 99 tooth internal ring gear ( modulus = 2.26) is used This would give a moon orbit every 29.5 days, but with the earth orbiting the sun annually we get an extra moon orbit reducing its orbital period to the correct value of 27.3 days.

As the ring gear is outside the pinion another reversing pinion is necessary between them to give the moon the correct rotational direction. Another 8 tooth pinion is used for this purpose.

Step 5: Accuracy (inaccuracy!)

Obviously neither the scale nor the shape of the orbit are accurate.

The orbital period and % inaccuracy are given in the attached table.

Step 6:

<p>That's really neat! These are so fun :)</p>
<p>Thank you Swansong, Yes I enjoyed making it and refer to it regularly to check the moon phase and the positions of the nearer planets. </p>

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