ASTRONOMICAL CLOCK

By Adrian Garner

After some four years Adrian Garner has finished the design and construction of his astronomical clock. MEWS readers may have seen this clock under construction at a Bristol or a Model Engineer Exhibition where Adrian showed the mechanism under construction as part of the Society of Model and Experimental Engineers display. 

Adrian tells MEWS that at those exhibitions people seemed far more interested in the mechanism at the back of the clock – a remontoire which rewinds every three complete oscillations of the pendulum – than the front of the clock showing the time! One of the pictures above shows the back, but without the pendulum and with a locking bar to prevent movement.

A remontoire  is a small secondary source of power which runs the timekeeping mechanism and is periodically rewound by the  mainspring. It increases the accuracy by evening out variations in drive force caused by uneven friction in the gear train.

Adrian also tells us that he consciously splits the design and construction work. He makes a full set of 2D drawings using AutoCad LT. The drawings are being made available.

Astronomical clocks include special mechanisms which display information such as relative positions of the earth, sun, moon and sometimes more. They date back to the 14th Century and can be found around Europe and adorn many churches and public buildings.

Adrian’s clock is on a domestic scale and one of a very few made by model engineers. As well as including an orrery, the clock also tells sidereal time.

Sidereal time is used by astronomers use to locate a star. It is a time scale that is based on Earth's rate of rotation measured relative to the fixed stars. John Harrison famously used sidereal time with ‘equation of time’ tables to convert to normal (24 hours a day) time to check the accuracy of his clocks.

Viewed from the same location, a star seen at one position in the sky will be seen at the same position on another night at the same sidereal time. Time on a normal clock measures a slightly longer cycle, accounting not only for Earth's axial rotation but also for Earth's orbit around the Sun. A sidereal day is approximately 23 h 56 min 4.1sec.

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