Short answer... no one knows... Online Etymolgy Dictionary says, concerning the origin of the name "... O.E. eor�e "ground, soil, dry land," also used (along with middangeard) for "the (material) world" (as opposed to the heavens or the underworld), from P.Gmc. *ertho (cf. O.N. j�r�, M.Du. eerde, O.H.G. erda, Goth. air�a), from PIE base *er-. The earth considered as a planet was so called from c.1400. Earthy in the fig. sense of "coarse, unrefined" is from 1594. Earthworm first attested 1591. Earthwork is from 1633. Earthlight apparently coined 1833 by British astronomer John Herschel..."