Photograph showing a zenith telescope used at the Royal Observatory of the Campidoglio, Rome in 1876. The German maker Ertel constructed this instrument while Merz of Munich supplied the optics. The zenith telescope was used to measure and check the changes in the Earth's axis of rotation called nutation and Chandler's wobble. This is achieved by measuring the position of stars overhead to detect any changes with respect the zenith. This is detected as periodic changes in the apparent attitude of the observer and their instrument.
© Science Museum / Science & Society Picture Library