China 1955 – Zhang Heng, Zu Chongzhi,
Zhang Sui
Zhang Heng (AD 78-139), also spelled Chang Heng, was court
astrologer to the Emperor of China, although he had originally
made his name as a poet and writer. In his role as court
astrologer he adjusted the calendar to bring it into line with
the seasons as determined by his observations. His greatest
invention was a seismograph for recording earthquakes (which,
incidentally, is illustrated on a Chinese stamp from 1953). Another great
invention was an improved form of armillary sphere turned by
water power, a precursor of water-powered clocks. Zhang also
studied mathematics but his reputation in this was not as good
as in astronomy.
Zu Chongzhi (429–500), also spelled Tsu
Ch’ung-Chih, made many improvements to astronomy and
mathematics. He calculated the value of precession, although
his result was nearly twice the true amount. He was more
accurate with his measurement of the lunar month and the length
of the year which allowed him to predict the occurrence of
eclipses. In mathematics, he calculated the true value of pi to
several decimal places and introduced the now-familiar fraction
of 22/7 as a good approximation. He also devised the Daming
calendar, which took into account the effect of precession, but
it was not adopted until after his death.
Zhang Sui (683-727), also known as Yi Xing or
I-Hsing, developed Zhang Heng’s work to create a
water-powered celestial sphere with an escapement that struck
the quarter hours and hours as the sphere turned once every
day. Two additional wheels outside the sphere carried small
globes representing the Moon and Sun. By comparing his
observations of star positions with those of earlier
astronomers, he is credited with discovering the phenomenon of
proper motion. He also drew up a new and more accurate version
of the calendar – in the Chinese calendar, as in the
Jewish calendar, months are based on the cycle of lunar phases
and how these fit into the solar year is governed by certain
rules which were modified by a succession of calendar
reformers.
The above three feature in a set of four
commemorating scientists of ancient China. The set is rounded
out by Li Shizhen (Li Shih-Chen) (1518–1593), a physician
and pharmacologist.
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