Etymology
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hemisphere (n.)

late 14c., hemysperie, in reference to the celestial sphere, from Late Latin hemisphaerium, from Greek hēmisphairion, from hēmi- "half" (see hemi-) + sphaira "sphere" (see sphere). Spelling reformed 16c. Of the Earth, from 1550s; of the brain, 1804.

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antichthon (n.)

c. 1600, antichthones (plural), from Latin antichthontes, from Greek antikhthōntēs "people of the opposite hemisphere," from anti "opposite" (see anti-) + khthōn "land, earth, soil" (from PIE root *dhghem- "earth"). In Pythagorean philosophy, an imagined invisible double of earth.

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merganser (n.)

genus of duck-like water birds of the northern hemisphere, 1752, coined in Modern Latin (1550s), from Latin mergus "waterfowl, diver" (from mergere "to dip, immerse;" see merge (v.)) + anser "goose" (see goose (n.)).

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American (n.)

1570s, originally "one of the aboriginal peoples discovered in the Western Hemisphere by Europeans," from Modern Latin Americanus, from America (q.v.). The original sense is now Native Americans; the sense of "resident of North America of European (originally British) descent" is from 1765.

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toucan (n.)

bright-colored bird of South America, 1560s, from French toucan (1550s) and Spanish tucan; from Tupi (Brazil) tuka, tukana, said to be probably imitative of its call. The constellation Tucana was one of the 11 added to Ptolemy's list in the 1610s by Flemish cartographer Petrus Plancius (1552-1622) after Europeans began to explore the Southern Hemisphere.

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Hydrus 

"fabulous water serpent," 1660s, from Latin Hydrus, from Greek hydros "water-snake" (see hydra). The constellation (attested by 1670s in English) was one of the 11 added to Ptolemy's list in the 1610s by Flemish cartographer Petrus Plancius (1552-1622) after Europeans began to explore the Southern Hemisphere.

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Midi 

"southern France," 1883, from French midi "south," literally "midday" (12c.), from mi "middle" (from Latin medius "middle;" see medial (adj.)) + di "day" (from Latin dies, from PIE root *dyeu- "to shine"). At midday in the northern hemisphere the sun is in the south of the sky. Compare Latin meridianus "of midday, of noon;" also "southerly, to the south" (see meridian), and Middle English mid-dai in its secondary sense "south, to the south" (late 14c.).

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dryas (n.)

evergreen shrub found in cold or Alpine regions in the northern hemisphere, 1798, from Greek dryas (see dryad). As an indicator of tundra climate, the presence of its remains in lake-bed sediments lent its name to the Younger Dryas, the name given to the period of sharp and sudden return to Ice Age conditions in Europe c. 12,000 years ago.

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roundness (n.)

"state of being round or circular," late 14c., from round (adj.) + -ness.

Roundness applies with equal freedom to a circle, a sphere, a cylinder, or a cone, and, by extension, to forms that by approach suggest any one of these : as, roundness of limb or cheek. Rotundity now applies usually to spheres and to forms suggesting a sphere or a hemisphere : as, the rotundity of the earth or of a barrel ; rotundity of abdomen. [Century Dictionary]
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