This invention is an arcuate wearable device with a circumferential or annular array of spectroscopic sensors which measures a person's body hydration. In an example, this invention can be embodied in a modular smart watch band, a specialized hydration-monitoring band, or a finger ring. A circumferential or annular array of spectroscopic sensors helps to ensure continuous measurement of body hydration, even if a wearable device shifts and/or rotates on a person's wrist, arm, or finger.