Apr 16, 2009
Sometimes called a Jacob’s staff, the cross-staff is a cruciform instrument that determines latitude by measuring the altitude of the sun or the polestar. Invented by the Chinese and inspired by the crossbow, it replaced the astrolabe as a mariner’s tool in the 16th century. It consisted of a three-foot-long staff marked with degrees and a sliding vertical crosspiece. Mariners held the main staff against their cheek and moved the crosspiece until the top edge aligned with the sun or Polaris, while the bottom end fitted with the horizon. The corresponding degree on the main staff gave them the elevation, which they then converted into latitude. Since it required staring directly at the sun, the cross-staff could be very difficult to use. In 1594, Englishman John Davis created an improved version, the backstaff, which allowed observers to face away from the sun.

Sometimes called a Jacob’s staff, the cross-staff is a cruciform instrument that determines latitude by measuring the altitude of the sun or the polestar. Invented by the Chinese and inspired by the crossbow, it replaced the astrolabe as a mariner’s tool in the 16th century. It consisted of a three-foot-long staff marked with degrees and a sliding vertical crosspiece. Mariners held the main staff against their cheek and moved the crosspiece until the top edge aligned with the sun or Polaris, while the bottom end fitted with the horizon. The corresponding degree on the main staff gave them the elevation, which they then converted into latitude. Since it required staring directly at the sun, the cross-staff could be very difficult to use. In 1594, Englishman John Davis created an improved version, the backstaff, which allowed observers to face away from the sun.

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