1977 年 86 巻 5 号 p. 273-284
In 1872 the Meiji Government adopted the solar calendar, the twenty-four hours system and the new date, and in 1886 decided the Japanese Standard Time based on the time of the meridian of 135°E., running through Akashi in Hyôgo Prefecture near Kobe. Since then, the date and the time have entirely changed from those of the Edo period when the lunar-solar calendar and many local times prevailed throughout Japan.
The conversion of the old date into the new one is not so difficult task, but on the contrary, that of the old times into the new ones has complicated problems, because the old calendar system was different in many respects from the new one and also each clan adopted the local time derived from the time of each castle-town.
Firstly, the structure of the old calendar, which was introduced from China, was basically different from the new one. The length of the hour-units of daytime and nighttime differed according to the twenty-four short seasonal unit days, including the day of autumnal equinox, of vernal equinox, of summer solstice and of winter solstice. The beginning hours of daytime and nighttime were partly improved by the astronomers of the Tokugawa Government ; in the early part of the Edo period they were the times of sunrise and sunset of the castle-towns, and later they were more improved.
However, in 1797 TAKAHASHI-Yoshitoki, a young but an excellent astronomer, who studied the Western astronomy through the books edited by the European missionaries stationed in China and brought up INÔ-Tadataka, the greatest land-surveyor in the Edo period with the aid of astronomical observation, decided the times scientifically. He got the accurate position of the sun by spherical trigonometrical calculation, and cleared up the beginning and the end of twilight by the angle of depression of the center of the sun on the days of autumnal and vernal equinox at Kyoto the result was 7°21′40″. The new definition had been used until 1871.
This paper deals with some ways of conversion of the old times into the new ones, and illustrated, taking outstanding examples of castle-towns in the Edo period, how the converted times were different from one another throughout Japan in accordance with the latitudes and the longitudes of the towns.