The altitude of celestial objects, such as Jupiter, is typically measured relative to the astronomical horizon in astronomy software like Starry Night. The astronomical horizon is an idealized horizon that does not take into account atmospheric refraction, which is the bending of light as it passes through Earth's atmosphere.
In astronomy, altitudes are often reported relative to this astronomical horizon because it provides a consistent reference point for observations and calculations. True and apparent horizons take into account the effects of atmospheric refraction and are used in other contexts, such as navigation and meteorology.
So, when Starry Night or similar astronomy software reports the "altitude" of celestial objects, it's usually with respect to the astronomical horizon. This allows for more precise astronomical observations and calculations.