八咫鏡

Etymology
Phrase consisting of.

The name is a description of the mirror's size. A circle with a diameter of one would have a circumference of four ata. A circumference of eight ata would thus mean a mirror with a diameter of two shaku. The shaku at that time was roughly 23 centimeters, so the mirror would have been roughly 46 centimeters across.

Proper noun

 * 1) the "Eight-Hand Mirror", one of the three sacred treasures comprising the Imperial Regalia of Japan

Usage notes

 * The ata of that time period was around 18 centimeters or 7.25 inches, so note that the in the English name is not the same as the four-inch unit of measurement.