Bru declares that he left the observatory at half past nine. That night he left behind a half-exposed plate with no time written on it. The sky turns fifteen degrees every hour around Polaris, and it does not know how to lie.
Turn the star chart until the Big Dipper falls the way it does on the plate.
The chart sky does not match the plate.
A plate from that night was left in the tray, half exposed and unlabelled. Bru swears he was already gone by half past nine. But the sky on the plate is where it is, and the vault turns fifteen degrees every hour around Polaris: the photograph itself says what time it was taken.
—