Placid, a.
Etymology: [L. placidus, originally, pleasing, mild, from placere to please: cf. F. placide. See Please.]
Pleased; contented; unruffied; undisturbed; serene; peaceful; tranquil; quiet; gentle. "That placid aspect and meek regard." Milton. "Sleeping . . . the placid sleep of infancy." Macaulay.
Shades of meaning
Fernald has no entry of its own for placid; he lists it under calm.
Where he writes about it:
We speak of a calm sea, a placid lake, a serene sky, a still night, a quiet day, a quiet home. — from calm
Tranquil refers to a present state, placid, to a prevailing tendency. — from calm
We speak of a tranquil mind, a placid disposition. — from calm