The Antikythera Mechanism Episode 8 - Making The Mean Lunar Sidereal Train

after many generations of watching the night sky it was clear to the ancient observers that there was an underlying order and structure to the cosmos in addition to the cycle of night and day and the phases of the Moon when measured against the background of stars they found that the objects they were watching had a repeatable and predictable motion and a good example of this is the motion of the moon if an observation is made when the moon is adjacent to a prominent star then when it returns to that star an interval of time can be calculated this is known as its side aerial period or in the case of the moon we simply call it the side aerial month now the ancients were well aware that this was merely an average representation of the time interval they might not have had our modern understanding of exactly why it was so but they certainly knew that the moon sped up and then slowed down during its trip back to the same reference point and that even that small oscillation was itself continuously moving throughout each cycle it's perhaps the most remarkable aspect of the device that such subtle observations were mechanically represented within its keyring and it began with the calculation of the mean lunar side aerial month as a further resonance of the metonic cycle it so happens that 254 sidereal months also almost exactly equals 19 solar years so much like the metonic gearing the side aerial period of the Moon can be given a convenient mathematical expression now of course many aspects of this machine were pushing the engineering limits of the day at this part of the train in particular takes it closer to the edge than the rest and it's those engineering limitations that I'd like to go through now to begin with the gearing rapidly steps up in this part of the train demanding relatively fast rotations from assemblies with considerable inertia and this puts a heavy load on some very small components for the most part the high loads are dealt with in the same manner as the rest of the mechanism square arbors fitting into square holes well peened in the position but the teeth are at their greatest risk of damage in this part of the train and there's no better example of the material limitations experienced in the mechanism much like the EM assembly the D assembly must be capable of removal from the main plate additionally the maker elected to run at flush against the underside of the assembly platform and in so doing created the requirement that the D assembly be fastened with a recessed retaining pin now you'll have noticed that the wheels and pinions in the mechanism are quite thin in most cases between one and two millimeters thick and they obviously need to stay well meshed to function correctly in modern clock making the meshing is ensured by setting a limiting end shake with the pivot shoulders and to some extent by using relatively long pinions that can tolerate a small amount of longitudinal shift between the components but the sleek design of this machine rules out the option of using longer pinions and the concept of shouldered pivots had apparently not yet been invented generally the issue was solved by allowing a small clearance on the inside of the arbors a bit like a simple axle it provides the required limits to the movement at the cost of a higher friction penalty a good example is the way that the EM assembly was fabricated but in this higher load section of the train the maker discovered that method was unsuitable and was forced to come up with a more reliable solution it shares some of the features of a modern bridge but it might be more correctly described as a sort of thrust plate that captures the small pinion while still permitting its free rotation whatever you choose to call it it performs almost exactly the same function as the shoulder of a modern pivot albeit with more friction locking the longitudinal travel of the assembly and ensuring a very limited and precise end shape but as effective as this idea is it still leaves one part of the problem unsolved a natural limitation of the larger wheels due in part to the simple square seating is the tendency for them to very slightly tilt out of the horizontal plane for the low load assemblies this isn't really an issue but for the high loads in this part of the train and elsewhere in the mechanism like the main solar drive wheel this dramatically affects their efficiency and at worst can quickly lead to them becoming damaged so it was clear that they required stabilisation and for that the maker came up with the idea of these small stabilizing strips known as curves carefully set to the correct height they acted as a guiding surface that limited the extent to which the wheels could twist out of alignment now as it happens there's an interesting combination of methods used to fasten these curves to the main plate some are fastened as you've seen here using pins but others were soft soldered into position and that's a process that I figured was worth looking at in more detail so be sure to check out that video if you haven't already done so the stud for the SI assembly is the final component to be fabricated for this part of the train with a small notch file to provide clearance with a d' assembly hub and a pin used to maintain its orientation with the main plate and I should also mention that there are two final curbs to install underneath the main solar drive wheel that if installed now would partly obscure the internal view of the mechanism so I've decided to hold off fixing those until a later stage of the build in any event has unorthodox as the solution appears the curbs turned out to be quite effective the friction penalty has certainly increased but it is quite acceptable and importantly the objective has been achieved because the assemblies are now constrained into their operating planes as required to the extent that the tiny wheel known as e 2 which is now rotating at the mean side aerial period of the moon is now a reliable input to the next part of the mechanism the epicyclic been and slight gearing thanks for watching I'll see you later you

Channel: Clickspring

Views: 267,623

Rating: 4.9798117 out of 5

Keywords: Antikythera, antikythera mechanism, hellenistic, greek, ancient greece, clockmaking, clickspring, bronze, brass, the antikythera mechanism, sidereal, sidereal month and synodic month, sidereal month, sidereal month vs synodic month, synodic month

Id: OBI54xujkN0

Length: 8min 37sec (517 seconds)

Published: Fri Sep 21 2018

Reddit Comments

[u/ballards_anus_blood]

This is exactly what I come to this sub for. First: amazing craftsmanship. Second: Well shot and produced. Third: Interesting and informative. And lastly: it has created a desire to purchase brass in bulk and start machining stuff by hand at home.

To be clear there's no way I'm going to do that, but... I want to.