Question

Two stars of declinations 56◦ 30` 10`` and 56◦ 30` 50`` are used for observation with a
zenith tube. After observation of the first star, the instrument is rotated through 180◦ and in order to
observe the transit of the second star, the micrometer needs to be turned by 0·4 of a thread in a direction moving the wire away from the zenith. If the plate scale of the telescope is 30 seconds of arc per mm and the pitch of the thread 1 turn per mm, what is the latitude of the observing station?