Why would they have "big stresses on their orbits"? Unless, of course, you accept universal gravitation. Neville Not accepting universal gravitation even if the formula applies in local systems, and I prefer to say everything , all the planets included rotate around the earth every 24 hours. For 'stress' let me clarify. Lets suppose the sun has Venus in orbit around it at a nominal distance of 108 million k or 64million miles . Let us stop its movement around the sun for the relatively short movement of one earth day to lessen the complexity of what I am about to say. If the sun moves around the earth in 24 hours, this means it has prescribed a circle of 180 million miles dia. This means that Venus to keep its relative orbital position to the sun and all that implies, must if the venus system does not rotate, also prescribe the same 180 million miles every 24 hours. We know that Venus does maintain the same relative position to the earth and the sun over the 24 hours, therefore The sun venus position to each other must rotate one revolution in 24 hours. (this is the same mechanism required of the moon to keep its same face .. . The same must apply to any and every object that is considered to be orbiting the sun. And such a scenario creates many more problems than just the one I am demonstrating. However it does mean that on the canvass of space, As venus orbits the sun It must over a year make a spiraling petal pattern relative to the earth. This is the stress I am referring to Philip.