Saturn observation : how to calculate requirements for Parallax method In September, Saturn is 9.9au away from Earth. If we use the parallax method, what should be the base line between the two people?

So I know that $D = B / \theta$, but how do I find out theta? I'm pretty sure the answer is pretty obvious, but my mind really is not working right now, so any help is appreciated

Thank you in advance.
Are you treating $\Theta$ as the only unknown, i.e.  you believe you know the baseline distance exactly?    If so, then  you compare your angular sighting of a distant star vs. Saturn with the same for the other observer.    The devil is in the error analysis.  Your two angles will be absurdly close to each other in magnitude, which means a very small error/uncertainty in value leads to a large error in the calculated distance $D$ to Saturn.    

You should find some first-year-level text on measurement &amp; statistical error analysis to get a better feel for the difficulty here.