The electronic computer has become a pervasive presence in our lives. But how and why did they become so important in such a short time? The answer is that they produce something humankind has always craved: numbers. Over the course of this programme, Dr Luca Di Mare will give an account of the history of computers – which, it turns out, have a surprisingly long history. This month, we look at the origins of computers in the pre-modern world.
In the ancient world, the need for computation was related to astronomy because farming work was scheduled according to astronomical events such as solstices, or the phases of the moon. Astronomy was the only human endeavour able to produce a substantial body of quantitative data that could be modelled and predicted via calculations. The ancient people form Mesopotamia – nowadays Iraq – had quite a knack for algebra. They performed fairly sophisticated calculations using a numbering system in base 60. We use numbers in base 10 but the way we represent angles in degrees, primes and seconds comes from the Mesopotamian numbering systems. The Plimpton 322 clay tablet from Babylon (Figure 1) is 3700 years old and contains tables used to aid calculations of the position of celestial bodies in the sky. The values on Plimpton 322 are believed to be triplets of numbers obeying Pythagoras’ theorem and can be regarded as the oldest trigonometric table in the world.
The partnership between science and computation in the ancient world really blossomed in Greece in the fifth century BC because of the interest of Greek philosophers in cosmology and in questions of number theory. Greek philosophers devised geometric models of the universe, where the planets, including our own, the Sun, the Moon and the visible stars travelled along circular trajectories combined to match their observed position in the sky. These models allowed predictions to be made of the positions of celestial bodies. The final expression of the complex astronomical models formulated by Greek philosophers and their successors in Europe, Asia and Northern Africa during the Middle Ages were the large astronomical clocks adorning European cathedrals and public buildings since the late 1300s. These clocks were the supercomputers of their time. Their complex mechanical movements could display not only the time of the day but also the phases of the moon, the positions of stars and other astronomical data of interest to the city they served. One of the oldest working surviving clocks is visible today in Salisbury’s cathedral (Figure 2), where it was moved in the late 1700s from a now demolished tower.
Cathedral clocks used a mechanical process – transfer of motion between wheels of different diameter – to perform a simple algebraic operation: multiplication. Computers that use physical phenomena to perform calculations and produce results that are measured by a physical quantity – e.g. the displacement of a dial, or a voltage – are known as analogue computers.
One of the most important types of analogue computers were the tide-predicting machines that appeared from the 1870s based on a design by William Thompson, Lord Kelvin. One of these machines produced the predictions of tidal flows in the English Channel and Normandy coast for 6th June 1944: D-Day. The machine, known as TPM No. 2, had been commissioned in 1896 because its predecessor had worn down from constant use. TPM No 2 did not become operational until 1910. Analogue computers were still in use in the 1970s and bore the bulk of computing tasks for early space missions.
Dr Luca Di Mare is Tutorial Fellow in Engineering Science at St John’s College
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