The world's first transit instrument, 1684.
© Science Museum / Science & Society Picture Library
Engraving labelled 'Machina Domestica' (Tab III) taken from the book 'Basis Astronomiae' by Peder Horrebow (1679-1764) published in 1735. This transit instrument was built in 1684 by the Danish Astronomer, Ole Romer and erected in his home in Copenhagen. A transit instrument is used to measure the right ascension of a star, which is the celestial equivalent of longitude.