Migratory birds need various compass systems for successfully migrating in their season-specific and species-specific directions. The first compass systems to be revealed were the sun and the star compasses, but after the discovery of a magnetic compass, emphasis has very much shifted towards that latter system. The sun compass and the star compass sometimes are lumped under the umbrella of a celestial system, which is opposed to the magnetic compass; however, this viewpoint remains debatable. To use the sun compass, the birds have to be able to compensate for the uneven movement of the sun during the day, i.e., to use their inner clocks. Because of this unevenness, as well as both seasonal and regional unevenness of the movement of the sun, migrating birds are believed not to use a sun compass during their large-scale movements. Birds also might use the polarized light pattern of the sun from the sunrise and the sunset to calibrate other compass systems. Unlike a sun compass, an avian star compass is time-independent. Neither a sun nor a star compass is innate; they both need to be learned. Birds are assumed to learn the form of the sun arch during the first weeks of their life and the rotation of the stellar sky around Polaris before their first migration. The usage of the moon as a celestial compass cue is unlikely.