In marine navigation, a pelorus is a reference tool for maintaining bearing of a vessel at sea. It is a "simplified compass" without a directive element, suitably mounted and provided with vanes to permit observation of relative bearings. The instrument was named for one Pelorus, said to have been the pilot for Hannibal, circa 203 BC. Ancient use Harold Gatty described the use of a pelorus by Polynesians before the use of a compass. In equatorial waters the nightly course of stars overhead is nearly uniform during the year. This regularity simplified navigation for the Polynesians using a pelorus, or dummy compass: Reading from North to South, in their rising and setting positions, these stars are: {| class="wikitable" |+ Stars |- valign="top" ! Point ! Star |- valign="top" | N || Polaris |- valign="top" | NbE || "the Guards" (Ursa Minor) |- valign="top" | NNE || Alpha Ursa Major |- valign="top" | NEbN ||Alpha Cassiopeiae |- valign="top" | NE || Capella |- valign="top" | NEbE || Vega |- valign="top" | ENE || Arcturus |- valign="top" | EbN || the Pleiades |- valign="top" | E || Altair |- valign="top" | EbS || Orion's belt |- valign="top" | ESE || Sirius |- valign="top" | SEbE || Beta Scorpionis |- valign="top" | SE || Antares |- valign="top" | SEbS || Alpha Centauri |- valign="top" | SSE || Canopus |- valign="top" | SbE || Achernar |- valign="top" | S || Southern Cross |} The true position of these stars is only approximate to their theoretical equidistant rhumbs on the sidereal compass. Over time, the elaboration of the pelorus points led to the modern compass rose. Modern use In appearance and use, a pelorus resembles a compass or compass repeater, with sighting vanes or a sighting telescope attached, but it has no directive properties. That is, it remains at any relative direction to which it is set. It is generally used by setting 000° at the lubber's line. Relative bearings are then observed. They can be converted to bearings true, magnetic, grid, etc., by adding the appropriate heading. The direct use of relative bearings is sometimes of value. A pelorus is useful, for instance, in determining the moment at which an aid to navigation is broad on the beam. It is also useful in measuring pairs of relative bearings which can be used to determine distance off and distance abeam of a navigational aid. If