Jacob's staff
/ˈdʒeɪkəbz
/noun
A rod with a sliding cursor formerly used for measuring distances and heights.In January 1472 he made observations of a comet, using his Jacob's staff, which were accurate enough to allow it to be identified with Halley's comet 210 years (and three returns of the 70 year period comet) later.
Origin
Mid 16th century (denoting a pilgrim's staff): alluding to St James (Jacobus in ecclesiastical Latin), whose symbols are a pilgrim's staff and a scallop shell.