What is the source for the time of Isha and Maghrib in degrees of the sun?

Ive heard people say that the time of isha is when the red glow dissapears from the western sky, Ive heard some people say this is when the sun is at 13 degrees however Ive heard other people say this is when the sun is at 12 degrees. I am trying to find the source for this? Where is the source that gave these amount of degrees? I am also wondering about the Shia tradition for the Maghrib prayer which is when the redness disappears from the Eatern sky, do we know how many degrees the sun is at this point?