Binary stars are often resolved as separate stars, in which case they are called "visual binaries". Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known. They may also be detected by indirect techniques, such as spectroscopy ("spectroscopic binaries") or astrometry ("astrometric binaries"). If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other; these pairs are called "eclipsing binaries", or, together with other binaries that change brightness as they orbit, "photometric binaries".