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Why do we say Tropic of Cancer, Tropic of Capricorn?

They are respectively the northernmost and southernmost lines of latitude where, at some points in the year, it's possible to see the sun directly overhead. The intertropical zone (also called the tropics) is delimited by them.

Tropic of Capricorn, Tropic of Cancer; the only place I've heard the word "tropic" used as a singular noun.

We can holiday to the tropics, not the tropic (unless you were making a play on words and went to literally a point on the Tropic of Capricorn).

This (tropics vs Tropic of Capricorn/Cancer) raised my suspicion that perhaps "tropic" is a mathematical term, and all lines of latitude within this region are called tropics. And the boundaries, as points of interest (as well as the equator), are the only ones to have names. This doesn't seem to be the case though (after some research).

Etymology of the word "tropic" from Wiktionary:

From Late Latin tropicus (“of or pertaining to the solstice, as a noun, one of the tropics”), from Ancient Greek τροπικός (tropikós, “of or pertaining to a turn or change; or the solstice; or a trope or figure; tropic; tropical; etc.”), from τροπή (tropḗ, “turn; solstice; trope”).

So it seems to be to do with the solstice, which makes sense, because only during a solstice is the sun directly overhead at one of the two tropics.

But now I wonder why the entire region in-between them can be called "the tropics" too? Or tropical.... doesn't seem to fit with this greek/latin origin.

And why was this word chosen to name the two lines?

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  • Here's the first quarter of the full OED's "Etymology" for tropic : As noun Middle French tropic, tropicque, tropike, tropique (French tropique ) each of two circles of the celestial sphere that are parallel to the celestial equator and touch the ecliptic at the solstitial points (a1324), equinox (a1374), each of two parallels of latitude on the earth's surface corresponding to the celestial circles (1532; the use as plural noun in sense ‘the torrid zone’ (see sense A. 2b) is apparently not paralleled in French until later than in English: 1765), – FumbleFingers Jun 30 '20 at 12:41
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    The Online Etymological Dictionary is a good resource: etymonline.com/search?q=tropic late 14c. [..] from Greek tropikos "...of or pertaining to the solstice" (as a noun, "the solstice," short for tropikos kyklos), from trope "a turning" (from PIE root *trep- "to turn"). __ The notion is of the point at which the sun "turns back" after reaching its northernmost or southernmost point in the sky. Extended 1520s to the corresponding latitudes on the earth's surface (23 degrees 28 minutes north and south); meaning "region between the tropics of Cancer and Capricorn" is from 1837. – Greybeard Jun 30 '20 at 13:24
  • Wiktionary got it from etymonline, which got it from the OED, which got it from Agnes. – Mitch Jun 30 '20 at 13:26
  • This is a very complicated question to answer—because there is so much information about "tropics" in, for example, Early English Books Online. I spent most of a day a month ago, trying to sort through the data there and finally had to give up because there was simply too much material to cover. For one thing, I think it is very likely that Etymology Online has greatly underestimated the age of "tropics" in the sense of "region between the tropics of Cancer and Capricorn." It's a good question, though. – Sven Yargs Sep 13 '20 at 7:44
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The question is was answerered in the comments:

The Online Etymological Dictionary is a good resource: etymonline.com/search?q=tropic late 14c. [..] from Greek tropikos "...of or pertaining to the solstice" (as a noun, "the solstice," short for tropikos kyklos), from trope "a turning" (from PIE root *trep- "to turn"). __ The notion is of the point at which the sun "turns back" after reaching its northernmost or southernmost point in the sky. Extended 1520s to the corresponding latitudes on the earth's surface (23 degrees 28 minutes north and south); meaning "region between the tropics of Cancer and Capricorn" is from 1837. – Greybeard Jun 30 at 13:24

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