The equatorial sector was a complicated instrument both to make and to use. Ideally, the equatorial sector was able to swing around and directly measure two coordinates from any position, thus freeing it from the meridian (the line through the poles that most astronomical instruments needed to be aligned in).

In practice, the instrument was frequently unstable. Furthermore, instrument makers had to prepare lengthy instruction manuals for the owners.

The lower circle on the instrument measured right ascension, the angle in the plane parallel to the celestial equator. The upper circle, from which a piece is currently on display, measured declination, the angle above or below this celestial equator.

The use of this instrument varied between makers and designs, and it's hard to say exactly how Graham's equatorial sector was used in practice.

Click here to see a better illustration of this complicated instrument.