Time in History: Views of Time From Prehistory to the Present Day
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Although in everyday life we find it convenient to determine time by the position of the earth relative to the sun, in practice it is more accurate to determine the times when stars cross 'the meridian, which is the projection on to the sky of the circle of longitude through the place on the earth's surface where the observations are being made. The interval
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between successive transits of the same star, or group of stars, across the meridian is called the 'sidereal day'. Since there is one more sidereal day in the year than there are solar days, the solar day is about four minutes longer than the sidereal day, which can be converted into the solar day by a numerical formula given to ten decimal places by observations extending over two hundred years.
The unit of time on which the seasons and the calendar depend is called the 'tropical year'. It is the time between two successive passages of the sun through the point at which its annual path against the general background of the stars (the 'ecliptic') crosses the celestial equator at the spring equinox. It is not the same as the time between successive passages of the sun through the same fixed point on the sky, because the equinox has a retrograde motion of just over 50 arc-seconds a year. This 'precession of the equinoxes', as it is called, is due to the gravitational pull of the sun and moon on the earth's equatorial bulge, thereby causing the earth's axis to precess, like the axis of a spinning top, with a period of about 25,800 years. The discovery of the precession of the equinoxes was made in antiquity by the Greek astronomer Hipparchus, and correct knowledge of it is required for the precise determination of the calendar. In antiquity the length of the tropical year was determined with the aid of a gnomon at noon at successive summer solstices (at the same place) when its shadow is shortest; whereas the length of the sidereal year was obtained from successive heliacal risings of the same bright star. According to modern measurements the tropical year is equal to about 365.2422 mean solar days, and the sidereal year to about 365.2564 of these days.
The estimate of the tropical year that was the basis of the Julian calendar, 365.25 days, was just over eleven minutes too long, equivalent to an extra day every 128 years. Consequently, by 1582 the spring equinox, which in Julius Caesar's day fell on 25 March, had retrograded to 11 March. Moreover, Easter, which by the decision of the Council of Nicaea in the year 325 should be celebrated on the first Sunday after the spring full moon (i.e. the full moon occurring on, or immediately after, 21 March) had got steadily further away from the full moon. To bring the equinox to 21 March, Pope Gregory XIII, acting on the advice of a special commission that included the distinguished Jesuit Papal astronomer Christopher Clavius, directed that the day after 4 October 1582 be designated 15 October (October was chosen because it was the month with the fewest saints days and other special ecclesiastical days), and that the leap year intercalary day be omitted in all centenary years except those that are multiples of 400. Thus 1600 was a leap year and
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Fig. 5 The celestial sphere
. Since they are so remote, the so-called 'fixed stars' (as distinct from planets, satellites, comets, and so on) can be depicted as fixed points on a sphere with a radius that is very large compared with the radius of the earth's annual orbit around the sun. The lines from all observers on earth to any given star will therefore cut this sphere, known as the 'celestial sphere', at the same point. Consequently, any observer O can be regarded as being at the centre of the celestial sphere. Because of the earth's diurnal rotation, the celestial sphere appears to rotate daily about the line joining the earth's north and south poles. In accordance with standard mathematical terminology, any circle on the celestial sphere which has its centre at O is called a 'great circle'. (Lines of longitude on the earth's surface are also great circles, whereas lines of latitude, except the equator, are not, being only so-called 'small circles'.) In the above figure the great circle ENWS represents the horizon of O, with its pole at the zenith- point Z. Similarly, the great circle ELWM represents the celestial equator (projection of the terrestrial equator on to the celestial globe). Its north pole is at P, the so-called 'Pole Star' being close to it. V represents the vernal equinox (first point of Aries). The ecliptic (projection on to the celestial sphere of the sun's apparent annual path against the general background of the stars, due to the earth orbiting the sun) is the great circle through V that lies in the plane cutting the plane of the celestial equator at an angle of approximately 23½ degrees. The earth's axis of diurnal rotation is in the direction OP. The celestial sphere rotates about the line joining O to the poles of the ecliptic in a top-like motion. This 'precession of the equinoxes' has a period of nearly 26,000 years.
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2000 will be too, but the intervening centenary years were not. Moreover, it was decreed that the year should begin on 1 January. The new calendar, had been suggested by a medical lecturer in the University of Perugia, Luigi Giglio (latinized as Aloisius Lilius). Lilius died in 1576, but his brother Antonio presented his scheme to the Pope. Unfortunately, Lilius's manuscript was never printed and is now lost. Clavius regarded Lilius as a man entitled to immortality because 'he was the principal author of such an excellent correction.' For a more detailed account of these questions and of the dating of Easter, see the Appendix.
At first only Catholic countries adopted the Gregorian calendar, since in Protestant lands, despite some influential support for it, the feeling became widespread that the Pope 'with the mind of a serpent and the cunning of a wolf' was stealthily seeking by means of the calendar to dominate Christendom once again.1 Although this point of view now seems ludicrous, it was not thought so at the time. For Gregory XIII was not only a powerful promoter of the Counter-Reformation but had fully supported Philip II in his ruthless campaign against the Protestants in the Spanish Netherlands and had celebrated the St Bartholomew's Day massacre of the French Huguenots in 1572 by having a medal struck to commemorate it. Nevertheless, there were Protestant astronomers, notably Tycho Brahe and Kepler, who approved of the Gregorian reform, although others felt that Clavius had not applied sufficient scientific rigour in his investigations concerning it. In 1613, at the Diet of Regensburg, Kepler (who, in supporting Clavius, made the point that 'Easter is a feast and not a planet. You do not determine it to hours, minutes and seconds') argued that the Gregorian calendar did not involve the acceptance of a papal bull but only the results of calculations by astronomers and mathematicians.2 Nevertheless, the Protestant states maintained their opposition until 1700, when most of them decided to adopt the Gregorian calendar. In England and Ireland, however, anti- Catholic feeling, which was as much political as religious, successfully prevented its introduction for another fifty years, until the inconvenience of using a different date from that employed in the greater part of Europe could no longer be tolerated. Already in 1583, however, Queen Elizabeth's favourite mathematician, astrologer, and secret agent, John Dee, using data from Copernicus, had produced an eleven-day correction which he claimed was more accurate than the Gregorian ten-day correction due to Clavius. An English mathematical committee, composed of the astronomer Thomas Digges, Sir Henry Savile, and a Mr Chambers, agreed with Dee but recommended, to his disgust, that it would be more
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convenient in practice to adopt the same calendar as the Continent. Queen Elizabeth's ministers Burghley and Walsingham approved of Dee's plan, but nothing came of it because of the violent opposition of the bishops, who argued that the new calendar showed the influence of Papism. When at last, in September 1752, the change was made Dee's correction was adopted, 3 September becoming 14 September.
In England 25 December was taken as the beginning of the year during the Middle Ages until the latter part of the twelfth century, when 25 March was chosen instead. The Church decided to begin its year on that day (Lady Day) because it was the day of the Annunciation, being exactly nine months ahead of Christmas Day. In England the year beginning on 25 March was called the 'Year of Grace'. Although January appeared as the fi
rst month of the year in calendars and almanacs, all official documents followed the dating of the 'Year of Grace' until 1751. In that year the official year began on 25 March and ended on 31 December. From then onwards the official year began on 1 January. These changes were authorized by an Act of Parliament of 1750. Only a minimal change, however, was made in the tax year, which still ends on 5 April. That date in the new style calendar corresponded to 25 March in the old style calendar. In Scotland the year has begun on 1 January since 1600.
Confusion can easily arise when we try to compare dates between 1582 and 1752 according to the Julian calendar that prevailed in England with the corresponding dates in the Gregorian calendar used in some of the principal European countries. For example, it has sometimes been asserted that Cervantes died on the same day as Shakespeare. Unfortunately, this remarkable coincidence did not occur; Cervantes died in Madrid on Saturday, 23 April 1616, according to the Gregorian calendar already in use there, whereas Shakespeare died at Stratford-upon-Avon on Tuesday, 23 April 1616, according to the Julian calendar still current in this country, the corresponding Gregorian date being Tuesday, 3 May 1616, and so Shakespeare in fact outlived Cervantes by ten days.
The greatest opposition to the new calendar arose in the eastern Churches, and was forcibly expressed by the Patriarchs of Constantinople, Alexandria, and Armenia. Not until 1923 did the Orthodox Church in Greece, Romania, and Russia adopt it. The monks on Mount Athos (in north-eastern Greece) have still not accepted it. Nearly all the monasteries there adhere to the Julian calendar, which is now thirteen days behind the Gregorian. Moreover, at one monastery they still reckon the time of day according to the original Georgian style, with sunrise always occurring at twelve o'clock. Everywhere else on the 'Holy
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Mountain' they follow the old Turkish system, with sunset at that time. This system is said to go back to the Byzantines and at least has the advantage that the traveller knows from his watch how many hours of daylight there are left.3 The island of Foula, twenty miles west of Shetland, still retains the Julian calendar for its festivals, such as Christmas and Hogmanay.
Although civil time is based on natural phenomena, we have seen that not only religious but purely political considerations can influence the construction of a calendar, as in the case of ancient Rome. A much more recent instance of this occurred when, after deposing Louis XVI, the National Convention, or French Parliament, decided to introduce a completely new calendar. It was decreed that Year I should begin on what would otherwise have been 22 September 1792, the day the Republic was proclaimed. New names, such as Germinal, Prairial, and Thermidor, were devised by the dramatist Fabre d'Eglantine for the twelve new months of thirty days each, divided into three 'weeks'. each of ten days. At the end of the year there were five days of festival called Sansculottides, or 'Trouser-days'. (The culotte, or breeches, was regarded as an aristocratic garment, and the common people wore trousers. Sansculotte was originally a derogatory term applied by the upper classes to their lower-class opponents.) The sixth extra day in leap year was to be 'The Trouser-day' when, according to Fabre, Frenchmen 'will come from all parts of the Republic to celebrate liberty and equality, to cement by their embraces the national fraternity, and to swear, in the name of all, on the altar of the country, to live and die as free and brave Trousermen.'4 Fabre also devised names for each day of the year, many referring to fauna, flora, minerals, and agricultural implements. The new calendar, which was described by the American statesman John Quincy Adams as an 'incongruous composition of profound learning and superficial frivolity, of irreligion and morality, of delicate imagination and coarse vulgarity',5 had a short life. It was officially discontinued by Napoleon, and on 1 January 1806 the French reverted to the Gregorian calendar, which despite its imperfections is still the most widely used calendar in the world.
The pendulum clock and the clocklike universe
Although medieval scholars were not, as a rule, concerned with machines, they became more and more interested in mechanical clocks, particularly because of their connection with astronomy. It was generally believed that a correct knowledge of the heavenly bodies and
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their motions was necessary for the success of most earthly activities. The theory of astral influences was accepted by most Christian thinkers until the seventeenth century. That is why medical students were required to study astronomy and astrology, so that a horoscope could be cast of the hour when the patient fell ill and the propitious hour for the appropriate treatment, such as surgery, be determined. A present-clay relic of the influence of astrology on medicine is our use of the Italian word 'influenza' for the viral infection which was thought in former times to be due to a malevolent flow coming down to the sufferer from an evil star. Another etymological relic is the word 'disaster': originally this referred to the unfavourable aspect of a star (Latin astrum). Carlo Cipolla has drawn attention to the assertion by a writer in 1473 that the public clock in Mantua served the purpose of showing 'the proper time for phlebotomy, for surgery, for making dresses, for tilling the soil, for undertaking journeys and for other things very useful in this world.'6 In particular, people believed that a star 'born' when it first appeared on the horizon influenced the life of a child coming into this world at that moment, and that a star just setting at the moment of a child's birth had implications for the circumstances of his or her death.
The invention of clockwork and its application to mechanical models of the universe, such as de' Dondi's, made a powerful impact on many minds. It is, therefore, not surprising that the clock metaphor came to be used in a variety of contexts. For example, Jean Froissart, in his poem 'Li orloge amoureus' (c. 1380) presented an elaborate allegory in which various aspects of chivalrous love were compared with the different parts of a mechanical clock, the verge-and-foliot escapement being associated with the virtue of moderation, since self-control was the highest in the canon of virtues of the medieval knight.7 No doubt Froissart had an actual clock in mind when he wrote this poem, and if so it may well have been Henri de Vic's clock at the royal palace in Paris. Sadly, that famous clock later became an object of derision, as is evident from the scurrilous rhyme:
C'est I'horloge du Palais;
Elle va comme ça lui plaît!
A particularly interesting indication of the way in which the invention of clockwork began to influence philosophical thought occurs in a treatise by Froissart's contemporary Nicole Oresme ( 1323-82) on the question of whether the motions of the heavenly bodies are commensurable or incommensurable. Part of the treatise is in the form of an
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allegorical debate between Arithmetic who favours commensurability and Geometry the opposite. Arithmetic argues that incommensurability and irrational proportion would detract from the harmony of the universe. 'For if anyone should make a mechanical clock would he not move all the wheels as harmoniously as possible?'8 This is an early example of the mechanical simulation of the universe by clockwork suggesting, at least implicitly, the reciprocal idea that the universe itself is a clocklike machine.
This idea came to the fore in the scientific revolution of the seventeenth century. Early that century Kepler specifically rejected the old quasi-animistic magical conception of the universe and asserted that it was similar to a clock. Among others who drew the same analogy was Robert Boyle ( 1627-91). In a passage in which he maintained that the existence of God is not revealed so much by miracles as by the exquisite structure and symmetry of the world--that is by regularity rather than irregularity--he argued that the universe is not a puppet whose strings have to be pulled now and again but it is like a rare clock, such as may be that at Strasbourg, where all things are so skilfully contrived, that the engine being once set a-moving, all things proceed according to the artificer's first design, and the motions . . . do not require the particular interposing of the artificer, or any intelligent agent employed by him, but perform their functions upon particular occasions, by virtue of
the general and primitive contrivance of the whole engine.9
Boyle's words clearly imply a conception of nature from which all traces of the animistic world-view, such as was still evident at the beginning of the seventeenth century in Gilbert's book on the magnet, have been banished. In the development of the mechanistic conception of nature in the course of that century the mechanical clock played a central role. It was surely no coincidence that the greatest practitioner of the mechanical philosophy in its formative period, the Dutch scientist Christiaan Huygens ( 1629-95), who in the first chapter of his Traité de la lumière declared that in true philosophy all natural phenomena are explained 'par des raisons de mechaniques', was also responsible for converting the mechanical clock into a precision instrument.
This development was based on the discovery of a natural periodic process that could be conveniently adapted for the purposes of accurate time- keeping. As the result of much mathematical thinking on experiments with oscillating pendulums, Galileo ( 1564- 1642) came to the conclusion
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that each simple pendulum has its own period of vibration depending on its length. (Historians of science now ascribe priority in this important discovery to the French scientist Marin Mersenne ( 1588- 1648).) In his old age Galileo contemplated applying the pendulum to clockwork which could record mechanically the number of swings.