Brett's video was illustrative as usual but I had to look up the Tropic of Cancer, and had to go beyond wikipedia. The only attribution of its origin I found was 'antiquity'/Aristotle...

The Tropic of Cancer corresponds to the northernmost declination of the ecliptic to the celestial equator. It's a line of latitude approximately 23.4° north of center. Below this point – the zone from this latitude to the Equator - is where the sun is directly overhead at the summer solstice in the Northern Hemisphere (ie, the direct rays of the Sun strike the Tropic of Cancer). Climate is described as temperate above this latitude, and tropical/torrid below. Six months later, the direct rays of the Sun strike the zone above the Tropic of Capricorn, which is 23.4° south of the Equator.

Incidentally, the word "tropic" comes from the Greek meaning of 'turn' (a change of direction or circumstance), referring to the fact that the Sun appears to "turn back" at the solstices. – wikipedia