Given that $\displaystyle \sin \theta = \frac{1}{3}$, how does one find the exact value of $\displaystyle \cos \theta$ without using a calculator?

The answer is supposed to be $\displaystyle \cos \theta = \frac{2\sqrt{2}}{3}$, but I don't see how one reaches it. I know the standard sine/cosine angles ($\displaystyle \frac{\pi}{2}, \frac{\pi}{3}, \frac{\pi}{4}$...) and their corresponding Cartesian coordinates on the unit circle; is the result supposed to be derived from these? If so, how?