The Mishna’s three opinions on the earliest time to recite the evening Shema are usually read as a debate about astronomy. They are not. They are a debate about what makes a halachic day begin — and the stars are only the test case.
This essay traces the underlying disagreement from the opening lines of Berachos through its echoes in later sugyas, showing why each Tanna’s position depends on a prior commitment the Mishna leaves unstated.