short, span
咫 centers on a short distance or length, specifically a traditional span measurement. The idea of shortness extends naturally to describing something brief or limited in extent.
咫 combines 尺 (a traditional unit of length) with 只, which likely contributes a phonetic element. The character originally referred to a specific short span, roughly the distance from the thumb to the little finger.
The outer enclosure 尺 is a traditional foot measure, and inside is 只, which looks like a mouth 口 with legs ハ. Imagine a tiny person with a big mouth trying to measure a short span with a ruler: the whole scene is about something short.
For シ, picture a short span of silk thread: the word 'silk' starts with 'si', which cues シ. The short thread is measured with the ruler-like 尺.
Yatagarasu; raven who aided Emperor Jimmu on his eastern expedition
eight ata (approx. 144 cm)
Yatagarasu; raven who aided Emperor Jimmu on his eastern expedition
Yata no Kagami (the eight-span mirror; one of the Imperial regalia)
distance between outstretched thumb and middle finger (approx. 18 cm)
very short distance