The significance of the Tropic of Cancer in geography lies in its designation as a latitude line at approximately 23.5 degrees North. It marks the northernmost point where the sun can be directly overhead at noon during the summer solstice. This line influences climate, ecosystems, and agricultural zones in the regions it passes through, including parts of North Africa, India, and Mexico. The Tropic of Cancer is a crucial reference for understanding global solar patterns and ecological diversity.