Tropic of Capricorn & Cancer

[SteveS]

The Tropic of Capricorn is one of the five major circles of latitude that mark maps of Earth. As of 19 February 2018, its latitude is 23°26′13.0″ (or 23.43693°)[1] south of the Equator, but it is very gradually moving northward, currently at the rate of 0.47 arcseconds, or 15 metres, per year.

Why was these two points (Capricorn/Cancer) used to mark maps of Earth? Possibly navigation purposes? Where/Who originated these two map coordinates?

[TheScales_BothWays]

Digging back what I've learnt in Geography classes a few years ago...

The Tropics of Cancer and Capricorn denote the latitudes where the Sun is directly overhead on local noon, on the Summer (June 21/22) and Winter (December 21/22) solstices respectively. Their names are such due to the fact that Tropically, the Sun enters Cancer right on the moment of the Summer solstice and the Sun enters Capricorn right on the moment of the Winter solstice.

Geographically, (and arguably, astronomically) the Tropics of Cancer and Capricorn are important. The area between the latitudes of the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S) are called the tropics. (That also means the Sun never reaches overhead on noon in latitudes higher and lower the tropics.) Monsoon winds also shift direction by about 90° when they reach the circles of the Tropics and the Equator. (Plus many other significances...)

Please correct me if I'm wrong anywhere.

Possibly navigation purposes? Where/Who originated these two map coordinates?

For these questions, I believe our fellow Virgo wizard would have the answers.

[Jim Eshelman]

Scales, you got it exactly right. Let me say it a different way that might intrigue Steve.

The Tropic of Cancer (named in ancient times when it really was Cancer at the Tropics) is the maximum declination of the northern edge of the ecliptic. This means important things to navigators - probably far more than I know since I'm not a navigator. But, for example, here's something simple: If you are north of the Tropic of Cancer, the entire ecliptic (zodiacal stars at night, Sun at day) is always south of you. Similarly, if you are south of the Tropic of Capricorn, the entire ecliptic is north of you. This kind of instant information surely is of immediate use to navigators making their way across oceans by line of sight, with no technical instruments.

If you are between the two Tropics, you are... "in the tropics." (That's where the term originated.) Here, besides information about climate and sea behavior (based simply by being on the equator), parts of the ecliptic overhead will be north of you and part south of you. (And "overhead" has a definite meaning, since you are always directly under part of the ecliptic.

Of the five circles mentioned, one is the equator. With the Tropics, this leaves only two, the Arctic and Antarctic circles. These are directly tied into the Tropics, because they are the co-latitudes of the Tropics. Subtract the current geographic latitude of the Tropics (23°26'13") from 90° and you get 66°33'47". This as north latitude is the Arctic circle; as south latitude, it is the Antarctic circle.

This is also an important astronomical line. As a young kid, I thought the Arctic and Antarctic circles were something simpler, like where people noticed it was super-cold. (Ah, youthful naiveté.) It's cooler than that: The co-declination of any planet, star, or other point in the sky is that geographic latitude beyond which it cannot cross the horizon. For example, if a planet had declination 10°00' North, then it can't be above the horizon north of 80N, can't be below the horizon north of 80S. Therefore, it cannot rise or set beyond these extremes.

The star Vega currently has declination 38N48. Subtract 38°48' from 90° and you get 51°12'. So, above 51N12, Vega is circumpolar - that is, it just keeps circling the poles, which is another way of saying it never sets. Below 51S12, it never rises.

So... coming back to the ecliptic and these "five circles" of ancient maps... what does this tell us? It tells us that the entire ecliptic rises and sets as long as you are south of the Arctic circle. (Explanation: No part of the ecliptic can be farther north than 23N26. The upper edge of the ecliptic (23N26) is circumpolar exactly at the Arctic circle (66N34). All other parts of the ecliptic, having smaller declination, are circumpolar at some point closer to the North Pole than the Arctic circle.)

Does that help? These are quite natural locations determined by astronomy based on the obliquity of the ecliptic, and of great value to ancient navigators who, of course, were told that there were giant sea monsters in the oceans of the tropics

[SteveS]

Thanks guys for allowing me a better understanding with my question.