Parallax is a method used to determine a star's distance. As the Earth orbits the Sun, our motion makes nearby stars appear to wobble back and forth as we see the stars first from one point in our orbit, then from another. This motion is always very small, but with good instruments, it can be measured. The amount of the movement is the star's parallax. Once the parallax is known, the distance can be easily derived using trigonometry. This method is also known as trigonometric parallax, to distinguish it from a few non-geometric ways to derive the same information. Only a small number of stars are close enough (out to about 100 light years) for us to measure their parallaxes accurately. For the rest, there are more complex ways to gauge distance, such as by calculating dynamic parallax, which is a method used to measure the distance to binary stars. The first step is to measure the angular separation between the two stars, then to measure how long it takes them to orbit one another. If we can get some idea as to how massive they are (usually from the spectra of the stars), we can then do the math to figure out how far apart they are physically. From that, determining their distance from Earth becomes a straightforward problem.