what is known to everybody is that the moon travels around t

what is known to everybody这是由what引导的句子作为整个句子的主语,即主语从句.is为联系动词,that the moon travels around the earth once every month这是由that引导的表语从句,与动词be is一起构成谓语.
在该句中,哪部分都不缺少主语.what is known to everybody中,what作主语； that the moon travels around the earth once every month
中,the moon作主语.

What is known to everybody这个由what引导的句子作为整句的主语，这叫主语从句。第二个is是全句的谓语动次。后面的that及其所引导的句子作为宾语，叫宾语从句。整个句子（不论主句还是从句）不缺主语，是个完全正确的佳句。谢谢，希望能帮到你。