text: why does the earth and other planets rotate, but not our moon? i understand that without rotation we would not be here but just why does the earth rotate anyway? >>>>>why does the earth and other planets rotate, but not our moon?  the moon does rotate, it has day length of just over 28 days. if the moon didn’t rotate then it would show different faces to the earth as it orbits us, and there would be no ‘dark side’.  the moon doesn’t appear to rotate to an observer on earth because gravitational forces have forced the rotation of the moon to synchronise with the earth, the two bodies are tidally locked. as a result the moon rotates at exactly the same rate as it orbits the earth. it orbits once a month and it rotates once a month.  >>>>> but just why does the earth rotate anyway?   it’s called conservation of angular momentum. the earth condensed out of a massive cloud of dust and rock billions of years ago.  that cloud was all spread out over billions of miles, it covered the entire circular orbit path we take around the sun as well as stretching half way to mars and halfway to venus. it was big cloud. and it was spinning very slowly because of the way it was formed.  the cloud gradually condensed under the influence of gravity and became a small earth. but when a spinning object gets smaller it can’t just stop keep spinning at the same speed,  it has to keep spinning faster. you can see this principle applied by ice skaters working up speed. they start spinning with their arms out to make themselves bug, but when they pull their arms in the get smaller and spin faster. the same thing happened with the earth.  when all that spinning dust got pulled in from billions of miles away the earth got smaller, and as a result it started spinning a lot faster. so fast in fact that it became noticeable to observers on earth ( if there had been any) that the earth spun around totally once a day.  of course it took another 2 or 3 billion years for life to arise, and another 4 billion years for life to get clever enough to figure out that the erath did rotate and that the sun wasn’t whizzing around us.