Eyepieces from the Cook Equatorial No 2 telescope, used on one of the Transit of Venus expeditions of 1874. Eight equatorial telescopes were bought by the Royal Observatory Greenwich for these expeditions - two new telescopes by Simms, two new telescopes by Cooke and four second-hand instruments. After the expeditions, this telescope was brought back and used at the Observatory. An equatorial telescope is one that is mounted so that it can rotate about a polar axis parallel to the axis of the Earth. This allows the telescope to be used to follow the apparent motion of heavenly bodies across the sky by simply rotating it about this polar axis.