Tropic
Collier's New Encyclopedia (1921)
in astronomy, one of the two small circles of the celestial sphere, situated on each side of the equator, at a distance of 23° 28′, and parallel to it, which the sun just reaches at its greatest declination N. or S., and from which it turns again toward the equator, the N. circle being called the Tropic of Cancer, and S. the Tropic of Capricorn, from the names of the two signs at which they touch the ecliptic. The stars are brighter in the tropics than in the temperate zones, and astronomical observation is easier. Cyclones arise within the tropics. The characteristic vegetation of the tropics consists of gigantic endogens, as palms, some of which rise to a height of from 100 to 200 feet. More polypetalous exogens are arborescent than in temperate climes. The Coniferæ exists chiefly on mountains. Ferns abound in tropical islands, and deltas where water is plentiful, so that in some localities from 250 to 300 species may be gathered. The tropical type of vegetation was separated at a remote period into two portions, one in the Old World, the other in the New. Shells are brighter than in lands where the sun is less powerful, the birds more numerous and of gayer plumage, the feline tribe larger and in greater numbers. In geography, one of the two parallels of terrestrial latitude corresponding to the celestial tropics, being at the same distance from the terrestrial equator as the celestial tropics are from the celestial equator. The one N. of the equator is called the Tropic of Cancer and that S. of the equator the Tropic of Capricorn. Over these circles the sun is vertical when his declination is greatest, and they include that portion of the globe called the torrid zone, a zone about 47° wide, having the equator for a central line. The regions lying between the Tropic of Cancer and the Tropic of Capricorn, or near them on either side, are called the tropics.
What Has Changed in the Last Century
The geometry is intact, but the number has moved: the 23° 28′ Collier's gave in 1921 is now about 23° 26′, because the tilt of the Earth's axis is slowly decreasing by roughly half a second of arc a year — which drags both tropic lines about 15 metres closer to the equator annually, a shift local monuments in Mexico and Taiwan have had to be rebuilt or relabelled for. The entry's remark that the tropical flora "was separated at a remote period into two portions, one in the Old World, the other in the New" was a shrewd observation without a mechanism; plate tectonics, accepted only in the 1960s, supplied one in the break-up of Gondwana. The claim that the tropics hold more and gaudier birds, bigger cats and brighter shells is still broadly true and now studied as the latitudinal diversity gradient, though the sun's power is no longer thought to explain it directly. Cyclones do arise within the tropics, but modern meteorology adds that they almost never form within about five degrees of the equator, where the Earth's rotation gives them no spin.
Written for this edition, 2026.