[1] The angle of position measures the inclination to the meridian of a line drawn between the principal star and its companion; in other words, it shows in what direction from the primary we must look for the companion. It is reckoned from 0° up to 360°, beginning at the north point and passing around by east through south and west to north again. Thus, if the angle of position is 0° or 360°, the companion is on the north side of the primary; if the angle is 90°, the companion is to the east; if 180°, to the south; if 270°, to the west, and so for intermediate angles. It must be remembered, however, that in the field of the telescope the top is south and the bottom north, unless a prism is used, when directions become complicated. East and west can be readily identified by noticing the motion of a star through the field; it moves toward the west and from the east.

[2] The term "binary" is used to describe double stars which are in motion about their common center of gravity.

[3] Is the slight green tint perceptible in Sirius variable? I am sometimes disposed to think it is.

[4] For further details on this subject see Astronomy with an Opera-glass.

[5] Their names, in the same order as their numbers, are Io, Europa, Ganymede, and Callisto.