The angular momentum of the earth is constant?

More or less. There is a law of conservation of angular momentum, according to which Earth can't gain or lose angular momentum on its own - if for example it loses angular momentum, it has to go somewhere. A meteor who falls into the Earth, or a rocket leaving the Earth can change Earth's angular momentum - but the total angular momentum (e.g., of the system meteor + Earth) is the same, before and after the impact.