The sun shines by day; the moon shines by night. (classical phrasing)
Tips
memory
昱 = 日 (sun) on top of 立 (stand) - a sun standing tall in the sky, hence 'bright, shining.' Mostly used in given names today (suggesting brilliance and brightness) rather than in everyday vocabulary.
日 (the sun radical) supplies the meaning directly - 昱 is the bright shining of the sun, a poetic and personal-name char. The sun-on-top placement gives literal high light, fitting the sense of luminance from above.
立 supplies the sound, drifted from lì to yù through an old initial change. Standing imagery also adds a faint flavour of upright brightness - sun standing tall in the sky. Same phonetic-shift family includes 翌 (yì, next day).
