The economic history of portugal

3.1.3. Cross Staff

A cross staff¹¹

In the days of Columbus, a cross staff was able to measure the angle between the horizon and the North Star and, therefore, determine the latitude of a ship on Earth’s surface. A vertical slide member moved along a horizontal rule so that the tangent of the angle could be determined by sighting along the hypotenuse. The horizontal and vertical sides of the device could rule markings to calculate the angle’s tangent, hence angle calculation. The vertical member of the cross staff was fixed in length which is why the markings on that member could be ignored. The markings on the horizontal member consisted of the angle whose tangent represented the ratio of the two members. When the angle increased, the location of the marker ascended northward. When it decreased, the location of the latitude marker descended southward toward the equator. At the starting point of this particular navigation technique, the mariner had to first locate the Polar Star by finding the Big Dipper. This celestial navigation tool made it possible to calculate latitude without needing a log and the deadreckoning process.