How many civilian airliners were over the Moscow region yesterday? (Fermi estimation question)

To estimate the number of civilian airliners over the Moscow region yesterday, you can use Fermi's method — break the problem into several approximate parameters and multiply them.

1. The area of the Moscow region is approximately 44,000 km².
2. The average speed of a civilian airliner is about 800 km/h.
3. The flight time of an aircraft over the region (assuming the aircraft crosses the area along a straight line about 200 km long): 200 km / 800 km/h = 0.25 hours (15 minutes).
4. Estimate the number of flights passing through the airspace of the region during a day. The Moscow air hub is one of the largest in the world; assume that on average, about 50 civilian aircraft pass over the region per hour.

Then for a day: 50 flights/hour * 24 hours = 1200 flights.

Since an aircraft is over the region for about 15 minutes, at any given moment, there are approximately 50 aircraft over the region.

Result: During yesterday, about 1200 civilian airliners flew over the Moscow region, and at any given moment, there were about 50.