Etymology
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terra incognita (n.)
"unknown or unexplored region," 1610s, Latin, literally "unknown land," from terra "earth" (from PIE root *ters- "to dry") + fem. of incognito.
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terra firma (n.)

c. 1600, "part of the Italian mainland ruled by Venice," from Modern Latin terra firma, literally "firm land," from Latin terra "earth, land" (from PIE root *ters- "to dry") + firma, fem. of firmus "strong, steadfast" (from suffixed form of PIE root *dher- "to hold firmly, support"). Meaning "the land" (as distinct from "the sea") is first attested 1690s. Hakluyt and Sandys also used English firm (n.) to mean "the firm land, the mainland, terra firma."

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dog days (n.)

"period of dry, hot weather at the height of summer," 1530s, from Latin dies caniculares, the idea, though not the phrase, from Greek; so called because they occur around the time of the heliacal rising of Sirius, the Dog Star (kyōn seirios). Noted as the hottest and most unwholesome time of the year; often reckoned as July 3 to August 11, but variously calculated, depending on latitude and on whether the greater Dog-star (Sirius) or the lesser one (Procyon) is reckoned.

The heliacal rising of Sirius has shifted down the calendar with the precession of the equinoxes; in ancient Egypt c. 3000 B.C.E. it coincided with the summer solstice, which also was the new year and the beginning of the inundation of the Nile. The "dog" association apparently began here (the star's hieroglyph was a dog), but the reasons for it are now obscure.

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