The Tropic of Cancer is called that because over 2,000 years ago, during the June solstice, the sun appeared in the constellation Cancer. This line of latitude marks the northernmost point where the sun can be seen directly overhead at noon. Although due to the gradual precession of Earth's axis the sun no longer aligns with the Cancer constellation at the solstice (currently it is in Taurus), the original name has remained for historical reasons. The term "tropic" comes from the Greek root meaning "turn," referring to the sun's apparent turning point in the sky during the solstice, and the "Cancer" part refers to the zodiac constellation the sun was in at that time.

This latitude, approximately 23.5 degrees north of the equator, marks the northern boundary of the tropics, the region of Earth that receives the most direct sunlight and generally has warm climates year-round. The Tropic of Cancer plays a key role in defining Earth's climatic zones and seasonal sun positions.

In summary, the Tropic of Cancer is named after the Cancer constellation because historically the sun's northernmost position in its yearly path was in that constellation during the summer solstice, a fact that tied ancient astronomy and geography together in naming this important geographic line.