Ata (a unit of length used in ancient China and Japan) (咫)
Ata is a length unit used both in China and Japan.
In China
On the section of 'ata' (in pinyin, 'zhi') in Shuowen Jiezi (Analytical Dictionary of Chinese Characters), there exists a description that '8 sun ('cun' in pinyin; 1 sun is about 3.03 centimeters) of shu ('zhou' in pinyin) was equalized with 1 ata, and that 10 sun of shu was equalized with 1 shaku ('chi' in pinyin; 1 shaku is about 30.3 centimeters).'
Here, 'shu' meant the circumference of a circle, and 'ata' was its unit. The ratio of the circumference of a circle to its diameter is about 3.14, and provided that we round off the second decimal place and regard the ratio as 3.2, the circumference of a circle whose diameter is 1 shaku becomes 3.2 shaku, and in ancient China, this 3.2 shaku was equalized with 4 ata. Therefore, 1 ata became 0.8 shaku.
The Japanese Yata no Kagami (the eight-span mirror; one of the Imperial regalia) has 8 ata of circumference, in other words, the diameter of the mirror is 2 shaku (23.04 centimeters). 1 shaku in the Later Han Dynasty China was set at 23.04 centimeters (2 shaku), so it means the mirror of about 46 centimeter in diameter, and by now 4 circular mirrors of this size have been excavated in China.
In Japan
Ata' is the nominalized word of the Japanese verb 'atsu,' which means 'measure with one's hand opened.'
Concretely, ata is the length from the tip of the middle finger to the tip of the thumb when they are opened, and shaku was originally defined to have the same length. In fact, the Chinese character 咫 (reads as ata) seen in Nihonshoki (Chronicles of Japan) was replaced with 尺 (reads as "shaku") in Kojiki (The Records of Ancient Matters), which is exemplified by Yata no kagami (八咫鏡) replaced with 八尺鏡. In Japanese mythology, the length of 'yata' (八咫; 8 ata) is often seen such as, in Yata no kagami (八咫鏡) and in Yata garasu (八咫烏; the sacred crow with the span of 8 ata).
Ata' is also read as 'saka.'
Yasakani no magatama (comma-shaped jewel) is written as 八尺瓊勾玉 in Chinese characters, and the second character 尺 was originally written as 'ata' (咫).