K.P. Cheung, Department
of Architecture, The University of Hong Kong
This page started in July 1997
Although sundials existed in some form in China in Zhou Dynasty at about 800 B.C., and a very sophisticated water clock was made in Song Dynasty at about 1000 A.D., equatorial sundials and polar sundials commonly found in Europe and Arabian lands since Renaissance never existed in China till late Ming Dynasty at about 1600 A.D. when Catholic missionaries introduced them to the astronomical officials of China.
| Photo P1, P2 - The equatorial sundial and its description erected inside The Forbidden City, Beijing (Photo taken in April 1997, courtesy of Dr Sam C.M. Hui) | |
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In Song Dynasty in China (about 1000 - 1400 A.D.), there existed a popular type of portable "sundial" (See the replica photos).
| P3, P4 - A replica of Song Chinese portable sundial made of wood (by courtesy of replica owner Rev. C.M. Kwok). | |
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The dial surface is adjusted to incline at varying angles to the horizon by pivoting it at 13 different notches engraved on the base plate, determined according to the starting days of the 24 fortnight periods of The Old Chinese Agricultural Calendar (See Joseph Needham, Science and Civilisation in China, CUP, 1970, Volume 3 Section 19-25). Do you know why 13 notches were used to mark 24 fortnight periods? (Hint: Look at the Solar Declination Table and note that the South-most notch is for Winter Solstice, and the North-most notch is for Summer Solstice.)
The Compass is for setting the North-South alignment of the sundial. The dial surface is slanted up to face North generally. The shadow cast by the style on the circular hour division of the slanted dial surface tells the hour of the day. This dial looks like an equatorial sundial, but it is not. For an equatorial sundial (See the earlier Photos P1, P2 taken at The Forbidden City), the upper slanting dial surface facing the sky is for reading local solar time (URL: http://www.crest.org/staff/ceg/sunangle/) from Spring Equinox at 21 March, via Summer Solstice at 21 June, to Autumn Equinox at 23 September, and the lower slanting surface is for use in the other half of the year. The Song portable dial shown above uses only the upper dial surface for the entire year. for use in different latitudes, this sundial will give different hour readings of the same solar position. If I have time, I will use an accurate sundial tester, which is in fact the Sunlight-2 Heliodon, to test this Song portable sundial. I may expedite this test if somebody would request it for academic reasons.
