The First Explorers- Arab Traders in the Indian Ocean

The kamal was invented by Arab navigators centuries ago to measure the altitude of the sun and stars above the horizon to determine latitude. The Kamal was a great advance as previously navigators had used the span of their hand to measure the elevation of Polaris, the north star.  

The height of Polaris alters depending upon where you are observing it from Earth. Thus, a sailor could tell their latitude by measuring the height of Polaris above the horizon. The kamal is believed to have been devised around the 9th century. As trade and travel between Europe and the Middle East increased in the 16th century, the kamal began to be utilised by Europeans which, in turn, led the to the invention of the cross staff. 

The kamal relies on knowing that the sum of all angles within a triangle equals 180 degrees and that the sides and angles of a triangle are connected through sine, cosine and tangents. Before navigators left port they would hold the end of a length of cord between their teeth while holding the wooden staff out vertically in front. They would tie a knot in the cord in line with Polaris. From there you could sail either north or south and when Polaris and the horizon line up along each end of the kamal, you know that your course is on the same degree of latitude as your port of departure.

The method of using a kamal.
Credit: The Madras Journal of Literature and Science, vol. XVI, 1850