as is known to..为什么这么用啊?怎么没有了it啊,

是非限制性定语从句,在句首用as如 As is known to us,the Great Wall is very famous。如在句中就用which
如The Great Wall is very famous ,which is known to us。
至于it如放在句首该用It is known to us that the Great Wall is very famous。