Relative Positions in Visual Double Stars

The separation is one of two quantities needed to fully describe the relative position of double stars, the other being the position angle. With the brighter of the two stars being taken as the origin, the separation is defined as the angular distance in arcseconds between the two stars and the position angle is the bearing of the fainter star from the brighter in degrees with north being taken as 0 degrees, east is 90 degrees, and so on. Figure 1.1a shows the situation for the naked eye and binoculars. When a telescope is used the view is inverted so Figure 1.1b applies to telescopic views.

It is usual to represent separation by the Greek letter rho (p) and position angle by the Greek letter theta (0). These terms will be used throughout this book. Another common term is Am which is shorthand for the difference of magnitude between the primary and secondary stars. Unless otherwise stated the magnitudes in this book will be visual. The fainter of the two stars is sometimes called the comes, a Latin word meaning companion.

Figure 1.1.

a Telescopic view; b naked eye and binoculars.

Figure 1.1.

a Telescopic view; b naked eye and binoculars.

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