Subject: What is the propagation speed (if any) of gravitational influence?

suppose a second moon instantaneously appeared in the sky... How long would it take for the mass of 
that moon to influence the earth?  Is it absolute?  If a massive object is moving at a very high speed 
does it's gravitational influence on other objects cause a type of doppler effect?  Or, if an 
electromagnet is instananeously switched on (coil fully charged) then how long does it take for the 
magnetic fields to appear or be felt by another object.  I think that all these questions illustrate my 
question... Is there a speed of gravitational influence?

Re: What is the propagation speed (if any) of gravitational influence?