That's probably it, SC. Since every floating-point number in Blitz is single precision, numbers start to jump internally to exponents around 16 million.
Have you considered dropping your solar system to 1/1000 scale? (though if your scale gets too small you start having the opposite problem, that you can't move items in small enough increments because the numbers are smaller than the precision.) Alternately, you could create a mathematical model of the solar system, only render planets that are within view, and actually move the solar system around the camera instead of the other way around. I mean, who cares if Neptune is placed EXACTLY where it should be when you're flying around Mercury?
Have you considered dropping your solar system to 1/1000 scale? (though if your scale gets too small you start having the opposite problem, that you can't move items in small enough increments because the numbers are smaller than the precision.) Alternately, you could create a mathematical model of the solar system, only render planets that are within view, and actually move the solar system around the camera instead of the other way around. I mean, who cares if Neptune is placed EXACTLY where it should be when you're flying around Mercury?