1421: The Year China Discovered the World

by Gavin Menzies

  The Chinese determination of elapsed time (GM)

  An essential requirement for determining longitude was a precise measurement of elapsed time. The Chinese measured the passage of time by the sun’s shadow.

  The most famous existing observatory is the Zhou Gong Tower, built seven centuries ago. It is a truncated pyramid measuring twenty-five feet square at the top. Stairways lead from ground level to the platform on the top, upon which stands a three-roomed building with a good view to the north of a forty-foot gnomon, or vertical pole. The observatory, too, has a thin vertical rod for observation of meridian transits, and one of the rooms is equipped with a clepsydra, or large water clock.

  Lying on the ground to the north of the tower and extending for 120 feet is the device for measuring the sun’s shadow. To ensure this device was level, two parallel troughs of water extended along its length, enabling its stones to be laid precisely parallel with the water.

  The gnomon itself extended forty feet into the sky. This enabled the sun’s shadow thrown by the pole to be measured. As an illustration, at the equinox on the equator the sun rises in the east and sets in the west. At midday it is exactly above the observer and hence casts no shadow – it is a dot. The longest shadows are cast at sunrise and sunset. The length of the shadow will tell the time on that particular day at that particular place.

  Back in ad 721, the Chinese realized that the length of the sun’s shadow varied not only according to the time of day but for every day of the year, and depended on the observer’s latitude. They conducted an experiment between latitudes 17°209N and 40°N. Along this meridian line, thousands of miles long, they measured simultaneously the length of shadows at the summer and winter solstices using a standard eight-foot gnomon. This showed that shadow lengths varied just over 3.56 inches for each four hundred miles of latitude. They could thus make corrections for their position.

  They also appreciated that the length of shadow varied with the seasons. In one celebrated measurement, it was 12.3695 feet at the summer solstice and 76.7400 feet at the winter. This enabled them to make corrections for each day of the year as well as for different positions on the earth’s surface.

  The final adjustment was to correct the irregular motion of the earth around the sun occasioned by the eccentricity of the earth’s orbit and the difference between the equator and the ecliptic – this is known as ‘The Equation of Time’. It causes differences between absolute and solar time, reaching a maximum positive difference of fourteen minutes and thirty seconds in February and a maximum negative difference of sixteen minutes and thirty seconds in November. So accurately did the Chinese determine this equation of time that the great mathematician Laplace wrote: ‘The [Chinese] observations made from 1277 to 1280 are valuable on account of their great precision and prove incontestably the diminution of the obliquity of the ecliptic and the eccentricity of the earth’s orbit between then and now’ (Needham, 1954, vol. 3, p. 398). This outstanding precision is illustrated by their estimate of the length of lunation at 29.530591 days – an error of less than one second in a month.

  Chinese observatories (GM)

  The Chinese replicated the Zhou Gong Tower, first in Nanjing, then, when the capital was moved north in 1421, in Beijing. Later, as noted in chapters 4 and 8, they built observatories around the world. We know what equipment was in the observatories from an inventory listed in History of the Yuan Dynasty (1276–9) (Needham, 1954, vol. 3, p. 369). Here are the principal pieces of equipment:

  Hun thien hsiang – celestial globe (Ricci’s first instrument)

  Yang i – hemispherical sundial

  Kao piao – lofty gnomon, forty feet, as at Yang Cheng

  Li yun i – theodolite

  Cheng li – verification instrument to determine exact positions of sun and moon near eclipse

  Ching-fu – shadow amplifier

  Jih yueh shi yi – instrument for observation of solar and lunar eclipses

  Hsing kuei – star dial

  Ting shih – time-determining instrument

  Hou chi – pole-observing instrument

  Chiu piao hsuan – plumb lines

  Chengi – rectifying instrument

  As may be seen, the list has instruments for recognizing stars in the sky (celestial globe); for measuring the length of the sun’s shadow (lofty gnomon); for determining exact positions of the sun and moon at eclipses (cheng li); for amplifying the sun’s shadow (ching-fu); for observing lunar eclipses (jih yueh shi yi); and for observing the Pole Star (hou chi).

  Some of the instruments need explanation. The Chinese had long known that the longer the sun’s shadow (i.e. the bigger the gnomon), the more accurate the measurement of time. However, the longer the shadow got, the more attenuated and fainter it became. In the early Ming era they devised a ‘camera obscura’, a hole in the top of the roof of the observation chamber which resulted in a sharper shadow. They intensified this with a type of magnifying glass. The upshot was that a long shadow could now be measured to within one-hundredth of an inch.

  The measurements of time so far described would only work when the sun was out. Measuring time in darkness was accomplished using various types of water clock – clepsydras – which themselves were calibrated by day against the gnomons. There were several types of clepsydra; one of the best known was a steelyard type (chheng lou) which had compensating mechanisms to take account of both air pressure in the atmosphere and the height of water in the clock itself. One of these was found in the Pandanan junk wreck. We can see and marvel at the ingenuity of these astonishing devices for the polyvascular type is illustrated and explained in the Chinese encyclopedia printed in 1478 (Shi Lin Guang Ji) now in the Cambridge University Library. We can summarize by saying that by the end of the voyage of 1421–3 the Chinese had the ability to measure time from their observation platforms which by then straddled the globe.

  Eclipses (GM)

  Eclipses of the earth’s moon and the sun, that is solar and lunar eclipses, occur when the sun, moon and earth are in line with one another and when the moon’s orbit around the earth is in the same plane as the earth’s orbit around the sun. When these planes differ the result is a new or full moon rather than an eclipse.

  Solar eclipse

  In an eclipse of the sun, the line-up is like this:

  The moon’s shadow blots out the sun over a small portion of the earth. It becomes night for a very short period. The spot of darkness, the umbra, travels across the earth as the moon rotates around the earth, and the earth itself rotates. Thus, observers in different locations see the solar eclipse at different times.

  Lunar eclipse

  In a lunar eclipse, the earth is between sun and moon. Because the earth is so much bigger than the moon, the earth’s shadow blots out the moon. The great difference, insofar as astronomical observations are concerned, is that in a lunar eclipse the event may be seen simultaneously by observers across half the earth, whereas in a solar eclipse the event occurs only above a small piece of earth at any one time.

  The key to using a lunar eclipse to determine longitude is (i) that the event is seen across the world simultaneously; and (ii) while the event is seen, the earth is rotating, which has the effect that the heavens appear to rotate in the opposite direction to the earth.

  Passage of events during a lunar eclipse (JO and MP)

  There are four distinguishable events during an eclipse: U1 – first contact, when the moon enters the dark umbral shadow; U2 – second contact, when the moon has just fully entered the umbra (totally covered); U3 – third contact, when the moon first starts to emerge; and U4 – fourth contact, when the moon has just fully emerged. These events can be observed across almost 180 degrees of longitude (east to west).

  Longitude determined by elapsed time during a lunar eclipse (JO and MP)

  With their gnomons and water clocks, the Chinese were able to determine the passage of time, minute by minute, throughout the day and night. They could also forecast when a f
ull lunar eclipse would occur – about every six months somewhere across the globe. The instruction to the navigators and astronomers was as follows: ‘After landing in unknown territory, when the next total eclipse begins, wait until the third event [U3] occurs and the last bit of darkness disappears. Just when the first sliver of light appears as the moon starts to come out of its eclipse [U3], both the observer in the new territories and the astronomer in Beijing look into the night sky and determine which major star is transiting the local meridian.’ The local meridian is an imaginary line on the celestial sphere which starts at the celestial pole north of the observer, extends directly over the observer’s head (the observer’s zenith), and ends at the celestial pole south of the observer. Along this imaginary line, the observer selects a known star crossing the line. This is both observers’ key sighting at this point.

  When the astronomer in the newly discovered territories has returned to Beijing, he and the astronomer at Beijing’s observatory compare notes. The one who has returned from abroad relates that at event U3, star alpha was transiting his local meridian. The Beijing observer relates that at that U3 moment, star beta was transiting his local meridian, both well-known stars. They now get out their timekeeping device. This has been calibrated from the gnomons. They wait until star alpha crosses the zenith and then start counting with their time-measuring device until star beta crosses their zenith. The time elapsed between star alpha and star beta crossing the zenith is the distance the earth has rotated between the two observers – the one in Beijing, the other in the newly discovered lands. The earth rotates 360 degrees every twenty-four hours. Thus, if we assume that the time elapsed between the transit of star alpha and star beta was exactly six hours (i.e. a quarter of the time it takes the earth to rotate), then the difference in longitude between Beijing and the new territory is also a quarter of the total longitude around the world, i.e. one-quarter of 360 degrees, or 90 degrees.

  Note by GM: Refinements can be introduced by conducting this procedure four times at U1, U2, U3 and U4 and then applying the averages to reduce errors.

  Proof of the theory (JO and MP)

  We decided to test our theory by observing the lunar eclipse of 16 and 17 July 2000. We positioned our team across the Pacific from Tahiti to Singapore. By a happy coincidence, we chose the same positions on which the Chinese had erected observation platforms.

  Table 1: Observations of the 16/17 July 2000 lunar eclipse. As can be seen, the error of a single observation was typically ±1.5° or better. Since one degree is the equivalent of four minutes of time, this error is the equivalent of about six minutes of time. The error of the combination of two observations would be better by and would thus be about ±1°.

  The observations list the celestial longitude measured from the vernal equinox of whatever star was transiting the local meridian – that is, the straight line passing from north over the observer’s head to south. The celestial longitude is measured along the equator of a star map. Thus, 339° (Tahiti) measures the position of a rotating cylindrical star map. The time elapsed between U2 and U3 enabled that cylindrical star map to rotate for 339° past 360° to 8° – about two hours. Average errors were: Tahiti 1.1 degrees, New Zealand 0.1 degree, Melbourne 0.1 degree, Singapore zero degrees. Our observers were amateurs; with more training and experience than we could provide the errors could be reduced.

  Practical implementation (GM)

  The result has startling implications, for longitude has been calculated from Tahiti in the east with sixty-six nautical miles’ error to Singapore in the west with no longitudinal error. There is six nautical miles’ longitudinal error between Singapore and New Zealand, none between New Zealand and Australia. Longitude has been calculated across nearly one-third of the world correct to within sixty-six miles.

  The Chinese could have determined longitude just as accurately as Professor Oliver’s team did. The brilliance of this method is that, unlike calculations for latitude, no sextant is required. Neither is a clock; the only instrument needed is one to accurately determine elapsed time, a role fulfilled by the gnomon.

  Having accurately determined the longitude of Malacca (Singapore), the Chinese fleets could now use observation platforms and gnomons on their bases around the Indian Ocean – at Semudera (Sumatra), the Andamans, Dondra Head (Ceylon/Sri Lanka), Calicut on the Malabar coast of India, Zanzibar in East Africa, the Seychelles and Maldive archipelagos – all of which appear on the Wu Pei Chi charts which credit Zheng He with providing the information. There is no reason why longitudes across the whole Indian Ocean should not have been determined in one eclipse, provided a sufficiently large fleet was deployed. This, I think, happened with the results seen in the Cantino, where the coast of East Africa appears as if drawn with the aid of satellite navigation.

  The brilliance of Zheng He’s astro-navigation arises both from its simplicity and because each part contributes towards a composite whole greater than its parts.

  Establishing the right ascension and declination of Canopus and of Crucis Alpha and Beta (Southern Cross) had enabled them to be cross-referenced to Polaris (see Wu Pei Chi chart for passage between Dondra Head and Sumatra). Measuring Polaris’s altitude as they sailed north would have enabled Chinese navigators to calculate half the circumference of the earth. Sailing due north between the equator and 40°N was 2,400 nautical miles (10,000 li); thus, continuing to the North Pole would be a further 50° or 12,500 li; thus, the earth’s circumference must be 100,000 li. Because they knew the position of Canopus and the Southern Cross, they could use the size of the earth to determine the true position of the South Pole (the centre of the cirumpolar stars – Canopus becomes circumpolar below 68°S). Hence, they could determine the position of the South Magnetic Pole so as to establish true south and north.

  The Chinese now had everything needed to accurately chart the world – latitude, longitude, size, direction. They went on to chart every continent with great accuracy. The fruits of their labours reached Europe via Niccolò da Conti and enabled Europeans to set sail on their voyages of discovery with maps based on Chinese cartography.

  OBSERVATION PLATFORMS USED BY THE CHINESE 1421–3

  NOTES

  Chapter 1: The Emperor’s Grand Plan

  1. Anlui province on the north bank of the Yangtze in eastern central China.

  2. Chinese emperors were known not by their personal name but by their title, and, after death, a ‘temple name’, such as ‘Sincere Emperor’, reflecting the course of their life.

  3. Mary M. Anderson, Hidden Power: The Palace Eunuchs of Imperial China, Prometheus, Buffalo, New York, 1990, pp. 15–18, 307–11.

  4. R.H. Van Gulik, Sexual Life in Ancient China, Leiden, 1961, p. 256.

  5. Anderson, op. cit.

  6. Dorothy and Thomas Hoobler, Images across the Ages: Chinese Portraits, Raintree, Austin, Texas, 1993.

  7. Confucius, as quoted by F. Braudel in A History of Civilisations, trs. R. Mayne, Penguin, Harmondsworth, 1994, p. 178.

  8. The dragon was credited with miraculous powers and was used as a metaphor for people of great virtue and talent. Almost all the items and artefacts closely connected with the emperor – his throne, his robes, his bed, etc. – were prefixed with ‘dragon’ or ‘phoenix’, the phoenix being another mystical creature with extraordinary powers.

  9. In early 2002 the Chinese government announced ambitious plans to restore the dry-docks and build a full-size replica of one of Zheng He’s junks.

  10. Ming Tong Jian, Comprehensive Mirror of Ming History, 1873, Ch. 14, quoted in Louise Levathes, When China Ruled the Seas, Simon & Schuster, 1994, pp. 73–4.

  11. Ahmad ibn Arabshah, Miracles of Destiny in Timur’s History, 1636.

  12. Shun Feng Hsiang Seng (‘Fair Winds for Escort’), anon., c. 1430, Bodleian Library.

  13. Miles Menander Dawson, The Wisdom of Confucius, Boston, Mass., 1932, pp. 57–8.

  14. Quoted by Edmund L. Dreyer in Early Ming China: A Political History 1355–1
435, Stanford University Press, Calif., p. 204.

  15. Hafiz Abru, A Persian Embassy in China, 1421, trs. K.M. Maitra, Lahore, 1934, p. 55.

  16. Emperor Zhu Di’s instructions to Zheng He, paraphrased from the two stone steles of 1431.

  17. The number of voyages made by the treasure fleets is, and will continue to be, a matter of dispute. The inscriptions on commemorative stones erected by Zheng He before his final voyage claim that his fleets had, until then, made seven. Most authorities classify his fourth and fifth voyages as one. I have adhered to this classification; the voyages beginning in 1421 were therefore the sixth.

  18. Hoobler, op. cit.

  19. L. Carrington Goodrich (ed.), The Dictionary of Ming Biography, Columbia UP, New York, 1976, p. 1365.

  20. N.I. Vavilov, ‘The Origin, Variation, Immunity and Breeding of Cultivated Plants’, trs. K.S. Chester, Chronica Botanica, Vol. 13, Waltham, Mass., 1949–50; and J. Needham, Science and Civilisation in China, Vol. VI, Pt 2, sec. 41, p. 428.

  Chapter 2: A Thunderbolt Strikes

  1. Hafiz Abru, A Persian Embassy to China, 1421, trs. K.M. Maitra, Lahore, 1934, pp. 113–15.

  2. Ibid., p. 115.

  3. Ibid., pp. 115–17.

  4. Ibid., p. 117

  5. Shang Chuan, Yongle Huang Di, Beijing, 1989, pp. 214–15, citing the Cochin tablet ‘Taizong Shi Lu’, ch. 236.

  6. S.W. Mote and Denis Twitchett (eds), The Cambridge History of China, Vol. 7, The Ming Dynasty, Cambridge UP, Cambridge, 1988, p. 292.

  7. Abru, op. cit., p. 108.

  8. Quoted in Louise Levathes, When China Ruled the Seas, Oxford UP, Oxford, 1994, p. 157.