I was discussing the relative smallness of human history compared to the evolution and size of the universe when a student asked me this question: "Who named the Earth 'Earth'? When? Why? Does this answer apply to the origins of the French word Terre?"
Unfortunately, I think it's pretty impossible to say exactly who first named the planet 'Earth'. Actually, I really doubt one person really named it intentionally; rather it developed over time as part of the English language. Earth is Old English and German in origin, related to the Old Saxon 'ertha', the Dutch 'aerde', and the German 'erda'. Terra is a French and Latin word, and so isn't part of the 'Earth' etymology. I'm not really an expert on words and word origins, but it seems likely that people used Earth to mean 'land' and then it was the natural thing to refer to all the land and the planet. I tried to look up more specific details about the specific usage of the word over time, but even the Oxford English Dictionary (online) admits:
"Men's notions of the shape and position of the earth have so greatly changed since Old Teutonic times, while the language of the older notions has long outlived them, that it is very difficult to arrange the senses and applications of the word in any historical order."
So, as with the names of the other planets that have been known throughout human history (Mercury, Venus, Mars, Jupiter and Saturn), it's difficult to say who first thought of the planet as Earth. The names were part of culture even before we really understood the significance of what planets are and where they are in space. For more information on astronomical names, please see the previously asked questions listed below.
Note (KLM) The name of the Sun has a similar origin to that of the Earth.
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