It can be helpful to fit an ellipse to the galaxy image, and then measure the amount of light found within the ellipse (the total galaxy luminosity). We can orient the ellipse at a particular angle (the position angle), and by varying its ellipticity from 1 (round) to 0 (elongated along one axis into a long oval shape) determine the galaxy's axial ratio.
The picture below provides you with the definition of the semi-major axis a for an ellipse (the largest radial length you can fit inside it) and the semi-minor axis b (the perpendicular radial length). The axial ratio is b/a. If the semi-major axis were pointing straight up, we would say that the galaxy had a position of 0◦; but in our picture, as it is tilted over by 90◦, it has a position angle of 90◦.
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