Why the Tropic of Cancer is drifting south
The Tropic of Cancer marks the northernmost latitude where the Sun can appear directly overhead at noon. That happens at the June solstice, when Earth’s Northern Hemisphere is tilted most toward the Sun. The line is not fixed to a particular strip of land: its position depends on the angle of Earth’s tilt.
Earth’s axis is tilted about 23.4 degrees, but that angle changes slowly. The Moon and Sun tug on Earth’s slightly bulging equator, gradually altering the tilt over long periods. Right now the tilt is decreasing, so the Sun’s farthest-north overhead position is edging closer to the equator. The tropic therefore shifts south by roughly 50 feet a year.
What that means for markers on the ground
A sign or monument marking the tropic records where the line was when it was surveyed, not a permanent boundary in the landscape. As the astronomical line creeps south, an old marker can end up north of the current Tropic of Cancer. The change is tiny from year to year—far too small to notice during a visit—but it accumulates over time.
The tropics are thus moving reference lines, not fixed geographic features. A marker can still tell a story about where the line once passed, even after the solstice Sun’s overhead point has moved on.
