text: why do planets "spin" on their axis?   planets spin because the stuff that fell together gravitationally to make them up had angular momentum relative to the planet's axis. if a meteor comes in and hits the earth, the chances are nearly zero that it will hit head on. a glancing blow will contribute to the earth's angular momentum. if a planet or star falls together gravitationally from a cloud of matter, even if the cloud has very little rotational motion, the angular speed of spin is increased as the planet or star becomes more compact. this has to do with the conservation of angular momentum, and a typical example is that of a spinning ice skater pulling in his arms to rotate faster. another example is the water draining in a sink. it almost always swirls around as it goes down the drain, even if there was very little apparent motion to begin with in the water in the sink.