It is the apparent position of an astronomical object referred to another and measured in degrees from the North point of the main object to the East.
The position angle is indispensable to know the relative position of the two components of a double star, or the direction of the tail of a comet with respect to the head.
The position angle of Alcor with respect to Mizar, for example, is currently 150 (Mizar and Alcor are the two main components of a famous double star in the constellation of the Big Dipper).
To calculate the position angle, a micrometer is usually used, which is applied to the eyepiece of a telescope of the refractor type.
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