Interesting Machines-2 Orrery

What is an Orrery?

Simply put, an Orrery is a functional mechanical model of the solar system. But the definition is the only simple part of this beautiful mechanical system.

Image Source: Wiki

Antikythera Mechanism

Image Source: Wiki

Robert Brettell Bate, circa 1812. Now in Thinktank, Birmingham Science Museum.

Image Source: Wiki

A 1766 Benjamin Martin Orrery, used at Harvard

History

The Antikythera mechanism, which is dated to ca. 150 – 100 BCE, may be considered the first orrery that is still in existence. Discovered in the wreck of a ship in 1900 off the Greek island of Antikythera (hence the name), this device consisted of hand-driven mechanisms that represented the diurnal motions of the Sun, the Moon, and the then-known five known planets (Mercury, Venus, Earth, Mars, Jupiter).

During the 16th century, two astronomical clocks were built for the court of William IV, Langrave of Hesse-Kassel (in modern day Bavaria, Germany). These showed the motions of the Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn based on the Ptolemaic system.  These clocks are now on display at the Museum of Physics and Astronomy and the Royal Cabinet of Mathematical and Physical Instruments (in Kassel and Dresden, respectively).

Clock makers George Graham and Thomas Tompion built the first modern orrery around 1704 in England. Graham gave the first model, or its design, to the celebrated instrument maker John Rowley of London to make a copy for Prince Eugene of Savoy. Rowley was commissioned to make another copy for his patron Charles Boyle, 4th Earl of Orrery, from which the device took its name in English.

Design

As a functional model of the Solar system, the Orrery is designed to replicate the movements of the planets and their moons around the Sun. The rotational motion of the planets and their moons is controlled by gears. Each planet and its moon has its own gearing powered by a common hand crank, in most cases. The gear ratios are set according to the number of days it takes for the planets to move around their own axis and around the sun. 

There are different types of Orreries. A model that only includes the Earth, the Moon and the Sun is called a tellurion or tellurium, and one which only includes the Earth and the Moon is a lunarium. A jovilabe is a model of Jupiter and its moons.^[12]The below table gives the list of planets along with the time periods: Source: Wikipedia

Planet	Avg. Distance from Sun	Diameter	Mass	Density	No. of moons	Orbital period (years)	Inclination to ecliptic	Axial tilt	Rotational period (sidereal)
Mercury	0.39 AU	0.38 Earth diameter	0.05 Earth mass	5.5 g/cm³	0	0.24	7.0	0	59 days
Venus	0.72	0.95	0.82	5.3	0	0.62	3.4	177	-243 days
Earth	1.00	1.00	1.00	5.5	1	1.00	0	23	23.9 hours
Mars	1.52	0.53	0.11	3.9	2	1.88	1.9	25	24.5 hours
Jupiter	5.20	11.21	317.9	1.3	67	11.9	1.3	3	10 hours
Saturn	9.54	9.45	95.2	0.7	62	29.5	2.5	27	11 hours
Uranus	19.2	4.01	14.5	1.3	27	84	0.8	98	-17 hours
Neptune	30.1	3.88	17.1	1.6	14	165	1.8	28	16 hours
Pluto	39.4	0.18	0.002	2	5	248	17.1	122	-6.4 days

Given below is the gear train arrangement of the Ken Condal orrery

Construction and Craftsmanship

Calculating the time taken for planets and their satellites to move around the sun and their respective axes is not the tough part in making an orrery. As mentioned above an orrery depends on its gear train to move the planets around the sun. Getting the number of teeth and diameter right to ensure the correct variations in rpm for each planet is only the beginning of the complexity. The real complexity is in getting the gear train tolerances right and balancing the system for smooth movements of all planets, 

Fabrication of the gears to minimum backlash at the same time not too tight to increase manual effort when using the crank. Too much backlash increases rotation movement losses thereby increasing the inaccuracies in the orbit time. 

I don't deny there are more complex gear trains with better accuracies being manufactured. We still have to remember Orreries are made by individuals with very limited access to high end machining technologies and these are not mass manufactured gears but one-offs. It is a very demanding process to get the tolerancing right during design and to get the gears manufactured accordingly. 

Gears are usually made of whatever material the creator finds suitable, can be brass, steel, wood, etc. The Orrery has to look beautiful. Its probably one of the few machines in the world whose Form and Function are important in equal parts. Makes the machine all the more fascinating.A lot of respect to the people making these beautiful machines.

Sanderson Orrery: The Orrery of Professor Jules Verne.

One of my favorite machines and definitely one of the most beautiful machines in the world. Powered by a steam engine. The video is with the Mozart Clarinet is a wonderful tribute to this timeless beauty.