Tropic of Cancer
What is the Tropic of Cancer?
It is the circle at 23.4° north. The Sun is straight overhead there once a year, on the June solstice, and never north of it. That number is Earth’s tilt, not a climate zone someone drew.
Where the number comes from
Earth does not sit upright in its orbit. Its spin axis leans. The size of that lean is the obliquity of the ecliptic, which this page computes from the same solar series that draws the map: about 23.4° in our century, and slowly shrinking.
Because the axis keeps pointing the same way all year, the lean facing the Sun changes as Earth goes round. In June the north end leans into the light. The centre-to-centre line — Sun centre to Earth centre — then hits as far north as the lean itself. That hit is the Tropic of Cancer.
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Why 23.4° — and why it is not a climate stripe
A climate map that colours a band “tropical” is a later idea. The tropic itself is a measurement: the maximum solar declination. If the tilt were 10°, the tropic would sit at 10° N. If the tilt were zero, both tropics would collapse onto the equator, and there would be no seasons.
The two circles are drawn on the map as dashed parallels at plus and minus the tilt. They do not move with the day. What moves is the yellow subsolar point, which runs a lap of longitude every 24 hours and a lap of latitude once a year, bouncing between those two dashed lines.
Why it is named Cancer
Two thousand years ago the Sun stood in the constellation Cancer at the June solstice. It no longer does. Earth’s axis slowly circles (precession), so the solstice Sun has drifted about a whole constellation since. The name stayed. The latitude is still exactly the tilt; only the star charts moved.
What the polar circles have to do with it
The Arctic and Antarctic Circles sit at 90° minus the same tilt — about 66.6° N and S. Inside them the Sun can fail to set in summer or fail to rise in winter. One number, four circles: two tropics and two polar circles. Change the tilt and all four move together.
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