Chiaroscuro: now there's a fancy word to impress your friends. Coming from the Italian chiaro (clear, light) and oscuro (obscure, dark), it means the interplay between light and dark. That makes it the perfect description of when you lie down under a big old oak tree draped with Spanish Moss and let the sun come down in dapples through the leaves In fact, nearly everything in this poem aligns around this border. Are you on the side of day and light, or is it dark and night? Without light, you would be unable to see anything. But without the help of shadow, you wouldn't be able to see dimensions. In this poem, it's the contrast that lends things (and feelings).
Lines 2-3: The scene is described early on as divided in realms of light and darkness, a light that is layered through the leaves and moss that filter it.
Line 6: This day is in no great hurry to be over. The light here makes its way "languorously," that is slowly, even lethargically, and the light-dark contrast seems to linger.
Line 9: Joined with "time" in this line, the mention of light here reminds of that first light in time, the very beginning of creation.
Lines 11-12: Here light is subject to the tides, like the whole marine scene. When it recedes, the lovers are revealed as land masses made of coral, surrounded by—you guessed it—darkness.
Lines 17-18, 20: The contrast in the chiaroscuro is most pronounced in these lines when day time light is described as rolling and furious (think churning water) and the dark is settling into a great depth of stillness. Dark is the force of night and death, as light is the force of day and life. Yeah, everyone knows that, right?
Lines 35-36: Light gives the spark of life and makes possible all the visible world. But wait, and darkness will take it all back, bit by bit. Yikes.