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Back-Staff, Peter Ifland Collection, The Mariners Museum, (1998)

Cross-Staff

The Cross-Staff, in one very early version, goes back as early as 400 BCE. That’s a very long time ago. The Chaldeans in the Mid-East used it, but sailors did not use it until the early 1500s; the first recorded date was 1514. As with other early navigation instruments, the first use for the Cross-Staff was in astrology, in measuring the altitude of stars to help forecast the future. Sailors only became interested in it as a navigational tool when their voyages took them to places unfamiliar to them, such as Africa, India and the New World.

The first European to learn of a device similar to the Cross-Staff, the Ka-Mal, was Vasco da Gama, who learned of the Ka-Mal from the Arabs when he visited India in 1498. The Cross-Staff, as used by sailors, was developed from the Ka-Mal.

The observer of the sun or Polaris would place the end of the long part of the Cross-Staff below his eye and observe the sun/Polaris across the upper part of the cross-bar while also observing the horizon at the bottom of the cross-bar. This can be done by moving the crossbar closer or farther from the observer's eye along the long central bar. The crossbar was called the transom and the long central part was the staff. When both the heavenly body (the sun or Polaris) and the horizon were lined up with the transom, the observer could read the angular altitude (degrees) on a scale on the staff.

This angular altitude could then be mathematically converted to the latitude of the observer.

The transom (cross-bar) came in different lengths depending on the altitude of the body to be measured. The smallest transom was for altitudes of about 15 °, the next size was for altitudes of about 30° and the last size for altitudes of about 60°. The best range for the use of the Cross-Staff was for bodies between 20 ° and 60 °. Smaller and larger angles could be read, but they were not as accurate. The staff (long part of the instrument) had a different scale for each transom marked on the staff. Therefore, there were usually three scales inscribed on the staff.

When a person was holding the Cross-Staff, it looked like that person was shooting a bow and arrow; the transom looked like the bow and the staff like an arrow. This gave rise to the term "shooting the sun" whenever someone measured the angle of the sun above the horizon, even when the instrument did not look like a Cross-Staff.