A Brass Sundial Shadow

I rolled from the patio border across the gravel path, mounting the sloped base of an old stone garden bench. An antique cast brass pocket compass lay open on the weathered surface, its hinged lid propped upright against the sun. The lid's dark shadow cast a sharp boundary across the cold stone. I halted at the line. Instead of circling the perimeter, I tested the temperature difference, nosing my forward runner from the warm sunlit patch into the cool silhouette. A sudden convection pulse stirred between the thermal boundary. It met my port propeller, engaging the central gear with two crisp clicks. I aligned my chassis precisely with the shadow edge, holding position.