The brighter star of a visual binary is considered the "primary" star, and the dimmer is considered the "secondary." The position angle[?] of the secondary with respect to the primary is measured, together with the angular distance between the two stars. The time of observation is also recorded. After a sufficient number of observations are recorded over a period of time, they are plotted in polar coordinates with the primary star at the origin, and the most probable ellipse is drawn through these points such that the Keplerian law of areas is satisfied. This ellipse is known as the "apparent ellipse," and is the projection of the actual elliptical orbit of the secondary with reference to the primary on a plane perpendicular to the line of sight of the observer. From this projected ellipse the complete elements of the orbit may be computed, with the semimajor axis being expressed in angular units unless the stellar parallax of the system is known.
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