Declinometer, an instrument to measure magnetic declination. It consists essentially of a telescope capable of rotation in a vertical plane about a horizontal spindle carefully levelled, and a horizontal magnetic needle. The telescope is adjusted in the geographical meridian by reference to a star either in the meridian or at a known angle to it. Then the angle between the vertical planes of the telescope and of the needle is observed on a graduated arc round the instrument. This angle is the required declination.