The centres of circles forming a coaxal system are collinear 2.
A and a are called the "pointers," since they are collinear with, or point to, the pole-star.
The collinear centres are the three sets of one external and two internal centres, and the three external centres.
- the sum of the moments of the forces about each of the three points which are not collinear must be zero.
The origins of these four plane co-ordinate systems may be collinear with the axis of the optical system; and the corresponding axes may be parallel.
Three collinear stars ?', c and 3 Orionis constitute the "belt of Orion"; of these E, the central star, is of the ist magnitude, 3 of the 2nd, while Orionis is a fine double star, its components having magnitudes 2 and 6; there is also a faint companion of magnitude io.