__is known to everybody,the moon travels around the earth once erver month,

as is known to everyboby 意思是：众所周知的；=it is well-know

如果用it 做形式主语的话，句子应该改为：
it is known to the moon travels around the earth once erver month.

不可以 如果用it 这就是两句话了。不能加逗号
as is known to everyboby,是一个短语

如果用it做形式主语,后面的句子做真正的主语,那么逗号要去掉,添上一个that.
It is known to everybody that the moon travels around the earth once a month.因为此句有逗号隔开,所以前面用as短语.