I've noticed that Stellarium uses the center of the sun for entering a constellation but pyephem seems to use the edge and so there are a few hours of difference.
What's the correct calculation (center or edge) ??
If it is stellarium... how to calculate entering the center of the sun in pyephem ??
Thanks
The constellations as defined by the IAU are simply areas in the sky delimited by boundaries. The center of the Sun can only be in one constellation area at a given time, but the Sun's disk is actually is overlapping potentially more than one constellation, when it's crossing a boundary.
In Stellarium we decided to say that the Sun (or any other sky objects) cannot be in more than one constellations areas at the same time simply because we want to display only one constellation name in the object information window.
Please note that if your question is related to astrology or ancient sky depictions then you should not use the IAU boundaries. The zodiacal constellations exists since the antiquity wile the IAU constellations precise boundaries were defined much later (in 1930).