Die Eule der Minerva in Hegels Rechtsphilosophie

The preface of Hegel's Philosophy of Right ends with a memorable image: "When philosophy paints its gray on gray, one form of life has grown old, and with gray on gray it cannot be rejuvenated, but merely known. The owl of Minerva takes its flight only with twilight closing in." What does this cryptic phrase mean? What is its significance for legal philosophy? While the first question has been pondered for ages, the question of why this particular phrase was placed in the preface to the principles of legal philosophy has hardly been addressed.