The Observation Watch
Below you: nothing. Cloud cover has swallowed every city, every river bend, every landmark. Blackout regulations have done the rest. France is invisible.
Your crew is seven men at 15,000 feet, crossing 400 miles of hostile territory to reach a target they will see only when they are directly above it.
GPS is fifty years away. Ground radar belongs to the enemy. The only navigation system on board is the navigator himself — a pencil, a chart, and arithmetic.
Take your heading from the compass. Take your airspeed from the gauge. Multiply speed by time to get distance. Plot that distance on the chart. That point is your position.
Then do it again. Every few minutes, for hours.
Every calculation rests on the one before it. Your position at 01:15 depends on 01:08, which depends on 01:00. Error is cumulative. Drift compounds. A small mistake at the French coast becomes a large mistake over the Ruhr.
Heading can be checked at any moment. Airspeed holds steady. Time is the one variable that must be measured fresh at every leg — and measured precisely.
One minute of error puts you four miles from where you think you are.
At 240 mph cruising speed
The navigator's watch is the most consequential instrument in the aircraft.
In 1940, the German Air Ministry issued specification FL 23883 — an observation watch precise enough to keep twelve aircraft flying as one.