>Can we see the eastern horizon from the standing stone ?
Nope. Modern earthworks obscure St Mary's Island, which is roughly due east of the orthostat.
Maximum northen moonrise would probably be similarly obscured, but bearing in mind how much the coastline has changed even in the last couple of centuries, who knows what to make of anything around there. There's also the Starlight Castle earthworks, which have disappeared from recent maps, but are still there on the ground, other side of Holywell Dene, no-one knows wtf they are either. I blame the Delevals.

ps: SL, if you wanna really go out on a limb at Hartley, Wor Kid reckons there's a long mound a couple of fields away from the stone, but one must be wary of disturbing Lord Hastings' lunch.