The origin of Earendil and the Silmaril in the sky was Tolkien's fascination by a line of Old English (Anglo-Saxon) which runs (here my spelling is very approximate!) "Eala Earendel! Engla beorhtest ofer Middangeard monnum sended." Which means "Hail Earendel! Brightest of Angels over Middle Earth, sent unto men." Tolkien took this to be a reference to the "Evening Star" which I think is Venus. I'm not too sure on the astronomy but the derivation is kosher!