In the thirteenth century celestial navigation was already known, guided by the sun position. For celestial navigation the Portuguese, like other Europeans, used Arab navigation tools, like the astrolabe and quadrant, which they made easier and simpler. They also created the cross-staff, or "cane of Jacob", for measuring at sea the height of the sun and other stars. The Southern Cross become a reference upon arrival at the Southern hemisphere by João de Santarém and Pedro Escobar in 1471, starting the celestial navigation on this constellation. But the results varied throughout the year, which required corrections.