noun
sextant
A navigational instrument used to measure the angle between two visible objects, especially for determining latitude and longitude at sea.
船乗りは六分儀を使って現在位置を確認した。
The sailor used a sextant to confirm his current position.
An octant, a similar but older instrument with a 45° arc, whereas a sextant has a 60° arc.
From 六 (six) + 分 (part) + 儀 (instrument), referring to the instrument's 60° arc, which is one-sixth of a circle.