The three main instruments used for celestial observation in the Spanish and Portuguese navies of the fifteenth century were the astrolabe, the quadrant, and the nocturnal astrolabe. Amerigo Vespucci (1454-1512) specifically mentions the first two in his letters, explaining that, together with the sea chart, they formed the basis for scientific navigation. The astrolabe had been introduced into the Christian West by Arab astronomers. It was essentially a map of the sky that reproduced the circumpolar movement of the stars on a plane surface. Using a set of interchangeable map plates (called tympani), the observer could view the northern constellations at different latitudes, so that the astrolabe map would always provide a perfect match with the changing appearance of the sky in different regions of the Earth. On ships, the standard instrument was the nautical astrolabe, whose applications were confined to measuring the declinations of heavenly bodies, in particular the North Star, during its transit over the meridian. Mariners could thus determine the latitude of the ship's position. The nautical astrolabe lacked the tympani, that is, the map plates characteristic of the planispheric astrolabe; its only parts were the body or mater, the graduated limb, the alidade, and the suspension ring, called the throne. To facilitate observation in adverse weather conditions, the mater of the nautical astrolabe was thick, heavy, and abundantly perforated. The instrument's weight kept it perpendicular to the ground despite the ship's pitch and roll; the perforations ensured that in a strong wind the instrument would not behave like a sail flapping in the hands of the observer, making it impossible to take stable aim at the chosen celestial body. The quadrant served a similar purpose and differed only by the presence of a sinical quadrant, which mariners referred to as a quadrant of reduction. This consisted of grid of orthogonal lines intersected by a 90° arc. As its name indicates, it served to reduce the calculation of trigonometric functions from an angle greater than 90° to an angle contained in the quadrant. The nocturnal astrolabe or simply "nocturnal" was in fact a night clock that, when pointed toward the North Star and the first two stars of the Big Dipper, gave the hour at any time of night.