A compass rose, sometimes called a wind rose, rose of the winds or compass star, is a figure on a compass, map, nautical chart, or monument used to display the orientation of the cardinal directions (north, east, south, and west) and their intermediate points. It is also the term for the graduated markings found on the traditional magnetic compass. Today, a form of compass rose is found on, or featured in, almost all navigation systems, including nautical charts, non-directional beacons (NDB), VHF omnidirectional range (VOR) systems, global-positioning systems (GPS), and similar equipment.
Types
Linguistic anthropological studies have shown that most human communities have four points of cardinal direction. The names given to these directions are usually derived from either locally-specific geographic features (e.g. "towards the hills", "towards the sea") or from celestial bodies (especially the sun) or from atmospheric features (winds, temperature). Most mobile populations tend to adopt sunrise and sunset for East and West and the direction from where different winds blow to denote North and South.
Classical
The ancient Greeks originally maintained distinct and separate systems of points and winds. The four Greek cardinal points (, , and ) were based on celestial bodies and used for orientation. The four Greek winds (, , , ) were confined to meteorology. Nonetheless, both systems were gradually conflated, and wind names came eventually to denote cardinal directions as well.
In his meteorological studies, Aristotle identified ten distinct winds: two north-south winds (, ) and four sets of east-west winds blowing from different latitudes—the Arctic circle (, ), the summer solstice horizon (, ), the equinox (, ) and the winter solstice (, ). Aristotle's system was asymmetric. To restore balance, Timosthenes of Rhodes added two more winds to produce the classical 12-wind rose, and began using the winds to denote geographical direction in navigation. Eratosthenes deducted two winds from Aristotle's system, to produce the classical 8-wind rose.
The Romans (e.g. Seneca, Pliny) adopted the Greek 12-wind system, and replaced its names with Latin equivalents, e.g. , , , , etc. The De architectura of the Roman architect Vitruvius describes 24 winds.
According to the chronicler Einhard (), the Frankish king Charlemagne himself came up with his own names for the classical 12 winds. During the Migration Period, the Germanic names for the cardinal directions entered the Romance languages, where they replaced the Latin names borealis with north, australis with south, occidentalis with west and orientalis with east.
The following table gives a rough equivalence of the classical 12-wind rose with the modern compass directions (Note: the directions are imprecise since it is not clear at what angles the classical winds are supposed to be with each other; some have argued that they should be equally spaced at 30 degrees each; for more details, see the article on Classical compass winds).
Sidereal
The sidereal compass rose demarcates the compass points by the position of stars ("steering stars"; not to be confused with zenith stars) in the night sky, rather than winds. Arab navigators in the Red Sea and the Indian Ocean, who depended on celestial navigation, were using a 32-point sidereal compass rose before the end of the 10th century. In the northern hemisphere, the steady Pole Star (Polaris) was used for the N-S axis; the less-steady Southern Cross had to do for the southern hemisphere, as the southern pole star, Sigma Octantis, is too dim to be easily seen from Earth with the naked eye. The other thirty points on the sidereal rose were determined by the rising and setting positions of fifteen bright stars. Reading from North to South, in their rising and setting positions, these are:
The western half of the rose would be the same stars in their setting position. The true position of these stars is only approximate to their theoretical equidistant rhumbs on the sidereal compass. Stars with the same declination formed a "linear constellation" or to provide direction as the night progressed.
A similar sidereal compass was used by Polynesian and Micronesian navigators in the Pacific Ocean, although different stars were used in a number of cases, clustering around the East-West axis.