I am having some issues with understanding what my adviser means by galactic roll, but I have not been able to find anything online regarding this.

My project involves generating an array of az/el coordinates, converting them to ra/dec, then converting them to galactic lon/lat. I then use these coordinates to scan a Healpy map (all in Python).

She did mention starting off by setting horizontal roll = 0, and then converting horizontal coordinates to galactic coordinates. The galactic roll should then take on nonzero values, which is important to the data I am analyzing.

I've read about Euler angles and quaternions and I think I'm on the right path. She provided some code for me to reference, and that uses quaternions and Euler angles. Does galactic roll refer to one of the Euler angles, perhaps $\theta$? Another reference code she provided mentions parallactic angle; could that be it?

  • $\begingroup$ You should primarily ask your supervisor. Nobody expects students to know everything from the start. The learning process is part of science and nobody should feel embarassed to ask. $\endgroup$ Mar 23, 2018 at 22:05
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    $\begingroup$ @AtmosphericPrisonEscape I'm not embarrassed to ask her at all (I've already asked dozens of questions, what's one more?) This is more of a personal challenge than anything. $\endgroup$
    – Rose R
    Mar 23, 2018 at 22:10
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    $\begingroup$ I keep reading this question and thinking "Well, I would look for it in my local supermarket, next to the arctic roll." $\endgroup$
    – Mick
    Mar 24, 2018 at 15:14

1 Answer 1


For an aircraft there are three principal axis: Pitch, roll, and yaw. I never heard the terms used in astronomy, but it does not seem to be too far-fetched in your setup, so I am confident:

A roll-angle would correspond to $\theta$ of the Euler angles.

Pitch - roll - yawn of an aircraft.

PS: The drawing can be found on both wiki pages.

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    $\begingroup$ I've also never heard this term wrt. galaxies. What puzzles me is that the normal (aviation) roll is in a 3D coordinate system, whereas the OP's problem seems to be a 2D one. I'm wondering if the OP misheard the adviser, but I guess we'll never know, since they seem to have left StackExchange two years ago. Anyway, +1 for this attempt. $\endgroup$
    – pela
    Jun 23, 2021 at 6:57

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