The stars appear to rotate around the celestial North Pole in 23 hours and 56 minutes. Because the cycle is four minutes shorter than 24 hours, the next day, at the same time, the stars will appear to have shifted by about one degree. The shifts accumulated over one year result in a further complete rotation of the stars around the celestial North Pole. Starting in the fifteenth century, the annual and daily rotations were used to determine the time of night with the nocturnal. The rim of the instrument is engraved with the months and days of the year. Holding the handle perpendicular to the horizon, the mariner would aim through the hole in the central rivet at the celestial North Pole—identified, for simplicity's sake, with the North Star in the Small Dipper (Ursa Minor). The mariner would then turn the hour dial to align the index arm on the dial with the mark of the current day on the calendar. The hour dial would thus be oriented relative to the stars. The mariner then rotated a second index arm to align it with the last two stars of the Big Dipper (Ursa Major). Like the hands of a clock, the second index arm gave the time of night on the hour dial.