The 'bright' derivation makes sense if we consider Selene's name (or even that of Selemnus), and if we regard her name as similar to the 'bright' names of water-nymphs such as Electra, Galatea, Glauce and Ianthe (to say nothing of her sister Ino, who became Leucothea and a goddess of the sea).
The 'earth' sense could derive from a Thracian word zemelo, 'earth' (compare modern Russian zemlya, with identical meaning), so that she was an earth goddess with a name something like 'Zemelo'. She was also known as Thyone. The latter does seem to be a Greek name.
