With privacy laws put into place, it's not ethical for employers to view personal information through emails but is absolutely possible if need be, provided that the employer has a solid policy in place informing employees that no information sent or received on company equipment is private.
I personally don't believe that a company should fire an employee over a social media post UNLESS what they're doing is illegal or dangerous. However, employers can fire employees for anything, including their social media posts.
the Moon revolves around the earth and while revolving its surface gets illuminated by thevsun. The change in the surface of the moon being illuminated by sun forms a lunar cycle