long day, clear
昶 centers on a day that stretches out long and clear, combining the ideas of extended daylight and unobstructed brightness.
昶 combines 永 (long, perpetual) with 日 (sun, day), suggesting a long or clear day. The exact historical development is uncertain, but the component combination fits the meaning.
The sun 日 sits inside the idea of something long and flowing 永, like a day that stretches on endlessly under a clear sky.
For チョウ, imagine a long, clear day stretching out like a 蝶々 (ちょうちょう, butterfly) gliding through the sunlight: チョウ from ちょうちょう cues the reading.
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