Marking the Noon Hour

A "Sun Clock" at Kochi Castle, Japan

By Steve Renshaw and Saori Ihara

March, 1997

As with many buildings which are considered national treasures in Japan, entrance to the west door of the main "house" on the citadel of Kochi Castle requires removal of ones shoes. Near this entry area, in a small yard, a "sun clock" made in the Edo Era (1603-1867) can be seen. Most visitors, both local and foreign, pass the little "instrument" without notice; yet at one time, this simple string gnomon and stone arrangement had more impact on the lives of Kochi citizens than anyone could imagine.

[Kochi Castle Sundial 1]
"Sun Clock" in front of Kochi Castle; moved during a period of repairs in the late 1940's, the linear depression in the middle of the stone is aligned with magnetic north. The angle of the "gnomon" is about 40 degrees.

A sun "dial" consists of two major components: a gnomon and a set of incremental markings generally called a "dial". To be a relatively accurate measure of time throughout the year, the alignment direction and angle of the gnomon should be such that it always points at celestial north. The angle of the gnomon should thus equal the latitude of the location of the instrument. The "sun clock" at Kochi Castle, similar to ones which can be found throughout Japan, looks somewhat different from sun "dials" we usually see in the West. However, its basic function was similar. Instead of the solid gnomon we are used to seeing, the gnomon of the castle's "sun clock" is composed of a post at the northern end of the stone with a string attached to its top and stretched to the southern base. Instead of a "dial", the "sun clock" has but one indented line in the middle parallel (lengthwise) with the stone.

The angle of the gnomon (string to stone base) is about 40 degrees. The actual latitude of Kochi is a bit over 33 degrees, so the gnomon of the clock is obviously not parallel with the earth's axis. The indented north-south "line" of the stone is aligned at present with magnetic north, thus placing it out of alignment with "true" celestial north. At best, the instrument gives a somewhat inaccurate indication of the noon hour. What then, in a culture known throughout its history for precise engineering, was the function of this rather crude instrument?

As in every culture, knowledge of time was an important aspect of day-to-day life throughout the history of Japan. While records are scarce regarding means of determination prior to influences from China and Korea, there is evidence that by the latter part of the 7th century, a clepsydra (water clock) based on Chinese principles of time keeping was in use for "knowing the hours" in the Yamato (present Nara/Asuka region) of Japan. [It should be noted that from early times of Chinese influence, hours were based on a system of Chinese Geomancy. In East Asian countries influenced by China, one hour was equal to what would be considered two hours in the West. Hours were also named according to the familiar animals of the Chinese Zodiac rather than on a numerical basis. For more on this, see The Sun, the Moon, and Happy New Year in Japan.]

Despite virtual isolation from the outside world during the Edo era, mechanical time pieces (such as pendulum clocks) were in use in much of Japan at least by the 18th century. However, for many centuries (even until the late 19th when time pieces were common), arrangements for meetings, appointments, and social gatherings were sometimes based on a relatively loose interpretation of coincidence of celestial phenomena with geophysical landmarks. For example, people might decide to meet "tomorrow when the sun has reached the top of that mountain in the southeast", or lovers might decide to meet "when Subaru is at the position of the horse" [meaning the asterism Subaru's transit overhead at night]. Obviously, such means of determining times for engagement fulfilled many informal needs, but the Shogunate along with daimyos from the various feudal domains of the Edo Era saw the need to have a more standardized method for setting and coordinating functions of time pieces as they became more available to citizens in each domain.

While it is possible that the "sun clock" at Kochi Castle was an educational or amateur instrument of the daimyo or one of his children, it is more probable that it was the instrument used to determine the "official" noon hour for Kochi City in Edo times. As in the West, determination of standard time was pretty much a local matter up until latter parts of the 19th century. No standard such as that established in Greenwich with its time ball in the early 1800's was instituted by the Tokugawa Shogunate in Tokyo, and virtually every feudal domain had to develop its own standard for determining "correct" time. Most likely, given its place on the castle grounds, the "sun clock" was the instrument used to provide citizens of Kochi with a standard for adjusting their time pieces. At the moment when the string's shadow was perceived to be exactly coincident with the linear groove in the stone, on signal, temple bells were probably sounded to alert citizens that the hour of noon was "at hand". Before the present era, Kochi was known as Tosa. Thus, the stone probably functioned to establish "Tosa Standard Time"... TST.

[Kochi Castle Sundial 2]
Photo of the "sun clock" taken at noon; note the shadow of the string roughly coincides with the linear marking on the stone.

The style of "sun clock" seen at Kochi Castle was introduced to Japan by Shihei Hayashi at the end of the 18th century and was in widespread use throughout the country in latter Edo times. We really do not know what the original alignment of the stone or angle of its gnomon was. There certainly were calendar scholars in Kochi throughout the Edo Era, and it is clear that they not only had a relatively precise measure of the city's latitude, but could also easily determine alignment with celestial north. Whether or not one of them assisted in the original placement of the stone and angular "setting" is not known. Given the generally accepted attitude of the shogunate and respective daimyo of the time, it seems likely that such precision was not seen as necessary. Rather, the simple establishment of a standard that the daimyo could "rely on" was sufficient for regulating the temporal affairs of the city. The Meiji Era, of course, saw the demise of shogunate rule and a movement to follow GMT as a base for time standardization throughout Japan.


Aston, W.G. trans. (1972) Nihongi; Chronicles of Japan from the Earliest Times to A.D. 697. Charles E. Tuttle Co. Ltd., Tokyo.

Okamura, K. (1995) Tosa Tenmon Sanpo. (Tosa Astronomical Promenade). Kochi Shinbun Co. Ltd., Kochi, Japan.

Sansom, G.B. (1931) Japan; A Short Cultural History. Charles E. Tuttle Co. Ltd., Tokyo. Revised 1993.

Schutte, K. (1975) Modern Sundials. In G.D. Roth (Ed.) and A. Beer (Trans.) Astronomy; A Handbook. Springer-Verlag, New York; Sky Pub. Corp., Cambridge, Mass.

Sugimoto, M. and D.L. Swain (1989) Science and Culture in Traditional Japan. Charles E. Tuttle Co. Ltd., Tokyo.

Uchida, M. (1975) Koyomi to Nihonjin (The Calendar and Japanese). Yuzankaku, Tokyo.

Uno, S. et al (1991) Nihon Zenshi (Japan Chronik). Kodansha, Tokyo.

Walters, D. (1989) Chinese Geomancy. Element Books Ltd., Longmead, Shafttesbury, Dorset.

Whitrow, G.J. (1988) Time in History; Views of Time from Prehistory to the Present Day. Oxford University Press, Oxford.

Please send comments to

Steven L. Renshaw

Return to Astronomy in Japan Home Page