Set the true home direction and how far a pigeon's internal clock has been shifted — see the predicted heading error.
Green = true home direction
Red = predicted (clock-shifted) heading
Predicted initial heading: 90°
Heading error: 90°
Rule: the sun moves ~15°/hour, so a clock shifted by H hours produces a predicted-heading error of H × 15°, in a direction set by the sign of the shift.