Why was 非 chosen for 非洲 (Africa) and 亚 chosen for 亚洲 (Asia)?
I can't find a particular reason to use these characters.
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Sign up to join this community亞洲 (亚洲) is short for 亞細亞洲 (亚细亚洲, Asia) which can be found as early as 坤輿萬國全圖 published in 1602 (萬曆三十年) , mainly by Italian Jesuit priest Matteo Ricci or 利瑪竇 (Lì Mǎdòu), 6 Oct 1552 - 11 May 1610, who is believed to coin this transliteration into Chinese characters.
非洲 is from 阿非利加洲 (Africa). The above work, however, gives a different name for Africa: 利未亞 (Libya) for some historical reason.
利未亞 is also adopted in 海國圖志 (影印本 in National University of Singapore), published in 1843 (道光二十三年), by 魏源, who referenced some British geographical work.
But from 瀛寰志略, published in 1849 (道光二十九年), by 徐繼畬, we can see 阿非利加 there.