I do not recall any Hebrew word for sphere. I will check my Hebrew dictionary at home, though if there is a word, it may well be a twentieth-century coinage. (Modern Israeli Hebrew includes many words that did not exist in the Bible, but which were usually formed from Biblical roots. The word for "computer", for example, comes from the root meaning "to think"; the word "railroad" comes from a root meaning "to ride").
The verse in Isaiah speaking of "the circle of the earth" uses the word chug, which usually means a disc or ring.