Occur与happen有何区别?

happen
vi.
发生; 出现
(后接不定式)恰好,碰巧,偶然
巧遇; 偶然发现(on,upon)
happen along
[口]碰巧来到,恰好出现
It happened that
偶然...,碰巧...
happen upon sb.in the street
在街上偶遇某人
We happened to be in the neighbourhood.
我们恰好是邻居
occur
vi.
(事件等)发生
想起
出现; 存在; 被发现
if anything should occur,...
如果发生什么事情的话,
...It occurred to me that ...
我刚刚想到...