sun beginning decline, reflect
暎 centers on the interplay of light and reflection, especially as the sun begins to decline and its rays cast a mirrored glow. The core idea is a luminous reflection or projection of light.
暎 combines 日 (sun) with 英, which likely contributes the sound and the idea of brilliance or a flower-like radiance. The exact historical development is uncertain, but the character is associated with the sun's declining light and reflection.
The sun 日 on the left and 英 (brilliant, flower) on the right suggest the sun's brilliant light being reflected or cast outward as it begins to set.
For エイ, imagine the sun reflecting off a calm lake, creating a long 'A'-shaped ray of light: 'A' -> エイ, as the sun declines and its image mirrors on the water.