The sun, unit of length
The sun measures exactly one thirty-third of a meter, that is 30.3 millimeters, a tenth of the shaku. It is the unit in which a Japanese craftsman states a thickness or a small width.
It survives in a place metrication never reached: the Sword and Firearms Control Law of 1958 sorts blades by length, and the trade still quotes those lengths in shaku and sun. Under one shaku a blade is a tantō, between one and two a wakizashi, over two shaku, that is over 60.6 centimeters, a katana. A blade described as two shaku three sun is 69.7 centimeters, and no catalogue writes it any other way.
The sun in other units
| 1 sun | 0.000030303 km |
| 1 sun | 0.030303 m |
| 1 sun | 3.0303 cm |
| 1 sun | 30.303 mm |
| 1 sun | 0.0000188294 mi |
| 1 sun | 0.0331398 yd |
| 1 sun | 0.0994194 ft |
| 1 sun | 1.19303 in |
| 1 sun | 0.0000163623 nmi |
| 1 sun | 0.30303 dm |
| 1 sun | 30,303 µm |
| 1 sun | 0.0165699 ftm |
| 1 sun | 0.1 shaku |
| 1 sun | 0.0166667 ken |
| 1 sun | 0.00000771605 ri |
Frequently asked questions
Is the sun the Japanese inch?
It is translated that way, and it is markedly larger: the sun is 1.19303 inches, close to a fifth more, and the inch is only 0.8382 sun. A board sold as one inch thick in an American catalogue and as one sun in a Japanese one differ by nearly five millimeters, which shows at the joint.
What are the bu and the rin?
The tenth and the hundredth of the sun, 3.0303 and 0.30303 millimeters. The bu is the one still in use: Japanese chisels and plane irons are sold by the bu, an eight bu chisel having a blade 24.2 millimeters wide. The word does not name a length but a part, and it names the tenth on other scales too, from an interest rate to the fineness of an alloy.
Is the Chinese cun a sun?
It carries the same character and not the same value: Chinese metrication fixed it at a thirtieth of a meter, 33.33 millimeters, a tenth of the chi. And in acupuncture it stops being a fixed length at all. There the cun is proportional to the patient, taken from the width of the thumb or from a twelfth of the span between two bony landmarks, so a point three cun above the ankle does not fall in the same place on two bodies.