Antikythera Mechanism - Mechanism - Gearing - Proposed Planet Indication Schemes

Proposed Planet Indication Schemes

Because of the large space between the mean sun gear and the front of the case and the size of and mechanical features on the mean sun gear it is very likely that the mechanism contained further gearing that has either been lost in or subsequent to the shipwreck or was removed before being loaded onto the ship. This lack of evidence and nature of the front part of the mechanism has led to numerous attempts to emulate what the Greeks of the period would have done and of course because of the lack of evidence many solutions have been put forward.

Evans et al. proposal. Freeth et al. proposal.

Michael Wright was the first person to design and build a model with not only the known mechanism but also with his emulation of a potential planetarium system. He suggested that along with the lunar anomaly the deeper understood solar anomaly would also be indicated. He achieved this by the attachment of three meshing and equally sized gears to one of the spokes of the b1 mean sun gear. The furthest gear away from the central spindle was fitted with an offset pin over which an arm with a slot was fitted which in turn attached to the sun spindle, causing anomalous movement indicative of the solar anomaly.

The inferior planets are indicated using more gears attached to b1 or attached to a plate which is in turn attached to pillars which evidence suggests existed on b1 at one time, which are evidenced in scans of the mechanism. These gears ultimately drive disks upon which are pins onto which arms with slots are placed. The arms are attached to the relevant planet indication spindle and a combination of both the rotation of b1 and the action of the pin and slot mechanisms the planets' motions are synthesised and indicated on the front dial.

The superior planets are much more complex and their mechanisms require significant extra hardware. Each superior planet system is mounted on a separate 223 toothed main gear (this has the same tooth count as b1) which is mounted on a rectangular plate with wooden spacer blocks on each short end, these are then attached to the mechanism as a whole. The individual main gears are driven by smaller coaxial transfer gears driven by b1, as all of these gears share the same tooth counts the ratio between b1 and superior gear is 1. Each superior system is very similar with the only differences being the size of the gears. The main gears are free to rotate, the upper plate is free to rotate, the spindle gear is fixed. The main gear is driven by the b1 transfer gear and drives the smaller coaxial gear attached to its surface. This gear drives a larger transfer gear which drives two smaller gears, one of these is coaxial and on the other side of the upper plate, the other is on the same side of the upper plate and drives the pin carrier wheel which is on the other side of the upper plate. The smaller driven gear then drives the fixed gears on the top of the upper plate, the smaller of those (or in the case of the Mars mechanism the only one) drives the fixed spindle gear. Attached to the spindle is an arm with a slot which engages with the aforementioned pin carrier wheel. This whole system rotates with the mean sun gear and subtracts from that gear's angular velocity to make the required ratio and indicate it on the front face.

Evans, Carmen and Thorndike published a solution with significant differences to Wright's. Their proposal centred on what they observed as irregular spacing of the inscriptions on the front dial face which to them seemed to indicate an off centre sun indicator arrangement, this would simplify the mechanism by removing the need to simulate the solar anomaly. They also suggested that rather than accurate planetary indication (rendered impossible by the offset inscriptions) there would be simple dials for each individual planet showing information such as key events in each planet's cycle, initial and final appearances in the night sky and apparent direction changes. This system would lead to a much simplified gear system, with much reduced forces and complexity.

Their proposal used simple meshed gear trains and accounted for the previously unexplained 63 toothed gear in fragment D. They proposed two face plate layouts, one with evenly spaced dials and another with a gap in the top of the face to account for criticism regarding their not using the apparent fixtures on the b1 gear. They proposed that rather than bearings and pillars for gears and axles they simply held weather and seasonal icons to be displayed through a window.

In a paper published in late 2012 Evans et al. proposed a system of epicyclic gearing with pin and slot followers.

Freeth and Jones published their proposal in 2012 after extensive research and work they came up with a compact and feasible solution to the question of planetary indication. They also propose indicating the solar anomaly on a separate pointer to the mean sun wheel's date pointer. Their front panel display would be essentially the same as Wright's although instead of pointers with text they would use semi-precious stones for each of the indicated bodies. The materials to be used are in order from the centre outwards:

• Moon (silver)
• Mercury (turquoise)
• Venus (lapis lazuli)
• Sun (gold)
• Mars (red onyx)
• Jupiter (white crystal)
• Saturn (obsidian).

Unlike Wright's model however, this model has never been built and has only been operated as a computer simulation.

The system to synthesise the solar anomaly is very similar to that used in Wright's proposal. Three gears, one fixed in the centre of the b1 gear and attached to the sun spindle, the other fixed on one of the spokes (in their proposal the one on the bottom left) acting as an idle gear and the final positioned next to that one, the final gear is fitted with an offset pin and over said pin an arm with a slot which is in turn attached to the sun spindle inducing anomaly as the mean sun wheel turns.

The inferior planet mechanism is again similar to Wright's mechanism however it uses fewer gears. A gear is attached to the centre of b1 and meshes with another gear which uses b1 as an epicyclic platform (Venus uses the upper left spoke and Mercury the lower right). Attached to this gear is a plate to which a pin is fixed, a slotted arm goes over this pin and is attached to the indicator spindle, the spindle is rotated freely about the centre of b1 with anomaly induced by the pin and slot mechanism.

The superior planet mechanisms differ from Wright's but perform the same function using fewer gears. They all follow the same general principle of the lunar anomaly mechanism. All planet systems contain four gears: the input gear which is fixed, the pin gear, the slot gear and the output gear attached to the indicator shaft. Two mounted on offset axes using pin and slot systems and two mounted on the same axis, one driving and one being driven. All systems use ratios related to Babylonian astronomy. The superior planet gears are mounted on a sub-plate using metal bridges, which is in turn located in front of the inferior systems and attached to the wooden frame, the front dial plate is fixed on top of this.

There are in total 8 coaxial spindles of various sizes to transfer the rotation of the mechanism to the pointers. In total these additions require 18 new gears and because of the application of the sun anomaly the addition of a separate date pointer and of course the extra planet pointers giving a total of 47 gears and 8 pointers.