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The cross staff, historically used for navigation and surveying, measures angles between two objects. It consists of a long pole with markings and a sliding crossbar. Users line up two objects to calculate latitude. It has been replaced by modern navigational devices.
A staff cross is a mechanical device used to measure the angle between two objects. Historically, the stick of the cross was famously used in navigation to help sailors orient themselves, and was also used by astronomers who studied the sky and surveyors who wanted to be able to take accurate measurements. This device appears to have been first invented by the Chinese, with Europeans starting to use it around the 14th century; like many devices, it was likely independently invented in several places, meaning many people and nations probably deserve the credit.
Also known as Jacob’s staff, the cross staff consists of a long pole with a series of markings and a sliding bar known as a crossbar or crossbar that is mounted at a perpendicular angle. Many rods have a set of crosspieces, so people can select the length that would be most appropriate; this allows the staff itself to be relatively short, making it easier and more comfortable to use.
To use a cross stick, the navigator places the end of the pole on the cheek just below the eye and selects two objects to be seen, such as the horizon and the sun. He or she slides the crossbar along the staff until one end lines up with one object and the opposite end lines up with the other object. While this sounds simple, it’s actually quite tricky, as the user has to look in two directions at once without allowing the staff to slip or wobble, and this can be even more difficult aboard a rolling ship.
Once the transom is in place, the marking covered by the transom indicates the angle between the two objects, which can be used to calculate latitude and gather other information. One of the big disadvantages of using a cross pole is the need to look directly at the sun for sightings during the day; later models of the device featured filters that allowed sailors to watch more comfortably. Also, cloud cover could obscure sightings at night using various stars or sightings during the day under very heavy cloud cover, which could mean people could be forced to go several days without taking a navigation site, making it difficult stay on course.
Crossed staves are sometimes used in classroom exercises to introduce students to the mathematics behind the staff or the history of navigation. These devices have since been supplanted by global positioning satellites and other navigational devices which are highly accurate and very easy to use.
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