Each of these sentences is true, since 'Hesperus' refers to the same object as 'Phosphorus' (the planet Venus which few months apart can be seen as the brightest star of morning or of evening). Nonetheless, (1) and (2) seem to differ in their meaning or what Frege called "cognitive value". (1) is just a truth of logic that can be known a priori, whereas (2) records an empirical truth that was discovered by astronomers. The problem, however, is that proper names are often taken to have no meaning beyond their reference (a view often associated with John Stuart Mill). But this seems to imply that the two statements mean the same thing, or have the same cognitive value.