Sun and moon side by side: the brightest objects humans know, fused into one character for brightness. Bright light became clear understanding, and the coming daylight became 明天 tomorrow.
聪明 clever, 明白 to understand ('bright-clear'), 明星 star/celebrity. The Ming dynasty (1368-1644) chose this character for its name: the Bright Dynasty. Few characters are this optimistic.
SUN + MOON together — maximum BRIGHTNESS, everything CLEAR.
明白 'understand' literally reads 'bright-white' — comprehension as light switching on.