NavList:

I have a question: can someone define heliacal rising in a quantitative fashion?   The reason I ask is that I'm interested in writing a piece of code to identify heliacal risings, but when I think about the implementation, many questions arise.

Roughly speaking, the heliacal rising is the first time a star is visible just before sunrise.    If I think about what might actually be visible at a given latitude, this seems to me to depend on the magnitude of the star, and even atmospheric effects, and even the height of the observer, if I were to get persnickety.   

On the other hand, if I define some group of stars with a magnitude greater than X, and say it's 'first visible' before local sunrise at, say 6 degrees above the horizon at the start of astronomical twilight, I can get a precise definition that I could implement in a piece of code - although this would probably yield some stars that would be several days past the 'first' observation.

The latter definition is straightforward to implement, while the former is difficult for me to see how it would work.

Any thoughts?