This question is related to the calculation of sunrises and sunsets. There is a JavaScript library https://github.com/Fabiz/MeeusJs that is based on the book Astronomical Algorithms by Jean Meeus.
I guess the code of the library mostly is fine but I found one issue. If I will draw a chart for the time of sunrises then around Equinox chart has a peak.
Turns out that the library do the calculation in a next way calculate approximate value then adjust it based on interpolation Interpolation is based on 3 values of y (y1, y2, y3). For most of the year y3 > y2 > y3. And I guest "linear" interpolation is being used. But around Equinox when Sun change the trajectory y3 can become less than y2. This introduces errors in interpolation.
The book does not provide good information about this situation. It mentions such a situation and I've tried to use it but it seems does not work well. Here are a couple of screenshots from the book.
And here are the links to the code that is based on the book formulas
https://github.com/Fabiz/MeeusJs/blob/master/lib/Astro.Rise.js#L166
https://github.com/Fabiz/MeeusJs/blob/master/lib/Astro.Interp.js#L57
Can you point me to the issue in math formulas. Or maybe suggest a better approach how to calculate it.