We may, as well, call it even the “First Law of Cosmology”.
Shall we reckon it as a property of the celestial bodies of the same type as the property of the light rays that get reflected by a mirror by the same angle as may be their angle of incidence?
Have we ever questioned when a light ray gets reflected – why the angle of reflection has to be the same as the angle of incidence?
All we know is that the angle of reflection happens to be the same as may be the angle of incidence? But we have never bothered to also think “Why it ought to have been the same?”
Not to have tried to ever think why it has to be so is what we may call, “Our failure to have looked out – what lies below the belly of the Physics”, the same way as not to have thought of the reason why the Sun oscillates while racing around the centre of the galaxy amounts to “Not having bothered to look out for what lies beneath the belly of the Astronomy”.