Yata no Kagami (八咫鏡)

Yata no Kagami is a mirror and one of the Sanshu no Jingi (Three Imperial Regalia).
In the "Nihonshoki" (Chronicles of Japan), there is a description stating that 'the mirror was also referred to as Mafutsu no Kagami mirror.'

Summary

According to Japanese mythology, the mirror was made by the goddess Ishikoridome no mikoto when Amaterasu Omikami (the Sun goddess) was hiding behind the Ama no Iwato, the Gate of the Celestial Rock Cave. When Amaterasu Omikami made a narrow opening in the rock gate, she saw a reflection of herself in the mirror. Attracting her interest, the reflection made her come out of the cave and the world became bright once more. During Tensonkorin (the descent to earth by the grandson of the sun goddess), Amaterasu Omikami gave the mirror to her grandson Ninigi. She issued a Shinchoku, a divine decree (Hokyo Hosai no Shinchoku, literally meaning "a decree to enshrine the treasure mirror") to have the mirror enshrined with her own reflection.

An 'ata' (the 'ata' coming after the 'y' of 'Yata') was a unit for measuring circumference, with one ata being equivalent to approximately twenty-four centimeters.
Having a diameter of twenty-four centimeters was regarded as a circumference of four 'ata.'
This means that the Yata no Kagami was a round mirror forty-six centimeters in diameter. Yata no Kagami is thought to have originally been a common noun signifying a mirror used in Shinto rituals, then later becoming a proper noun to specify the mirror of the Three Imperial Regalia.

There are several theories about the location of the Yata no Kagami. The first theory claims that it is still placed in the Ise Kotai-Jingu Shrine, while a second theory argues that the mirror placed in the Naishidokoro (Kashidokoro) (the Three Shrines in the Imperial Court) was sunk offshore in the battle of Dannoura (the final battle of the Heike-Genji wars) with Emperor Antoku at the defeat of the Taira clan but later collected along with the Yasakani no magatama (comma-shape jewel [another of the Three Imperial Regalia]) by MINAMOTO no Yoshitsune, and then another theory describes that the mirror was moved to another place before the defeat of the Taira clan.

According to documents such as the "Shito Gobusho" (a five-volume apologia of Shinto), the word 'Hachiyo' is written on the mirror that was enshrined in the Inner Shrine of Ise. Dairoku HARADA estimates that the mirror had the same shape of the Naiko Kamonkyo Mirror, which was excavated from the Hirabaru ruins in Maebara City, Fukuoka Prefecture. The mirror in the Inner Shrine of Ise is said to have been sealed after being viewed by the Meiji Emperor.

Amaterasu Omikami is also referred to as 'Naishidokoro' since the mirror was placed in the Naishidokoro in the Imperial Palace.