It was only by blizzard-blind luck that the astronomer passed through the enemy fortifications, but pass he did, scarf freezing to his face.

At first he thought it was a cannon resuming fire, madly, in the snow-whipped darkness.  The sound rolled through his shivering body with a note so low as to be percussive.  Then the other bells followed, building an arpeggio, and when the deepest began again he understood:  it was midnight, and the great clock was turning Eve into Day–

The clock.

Of course.

The astronomer ran, following the bells to the heart of the Chestnut City.