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  Double Star: Astronomy: Double Star - POSITION ANGLE
Posted on Thursday, December 02 @ 13:59:46 CST by astronomy
  Double Star


The position of the secondary star with respect to the primary is called the position angle, or PA. The primary star is designated component A, and the secondary, component B. Other components, if any, are listed as C, D, and so forth.

You can estimate the position angle by eye, although a filar micrometer allows you to make precise measurements.

Position angles help astronomers keep track of the orbital motion of the secondary over the years. When its position angle is increasing, the motion is said to be direct. When it is decreasing, it is said to be in retrograde.




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