20091014
Outline
To get the number of airports in the world, I start by estimate land area. Then guess the density of airports
US has 4 timezones
(360 degrees)/(24 hours) = 15 degrees per timezone
top-to-bottom size of US is about 1000 miles
circumfrence of the earth = 2*pi*radius = 24,000 miles
Area of earth = 4*pi*radius^2
Thus the radius of Earth is about 4000 miles
Thus area of earth is 192,000,000 square miles
1/3 of the earth is land, so that's 6E7 square miles
From my memory of airport sectional maps, in a 100x100 square there are about 10 airports
1 airport per 1000 square miles
Estimate: 60,000 airports in the world
Actual answer: According to
https://www.cia.gov/library/publications/the-world-factbook/fields/2053.html
total airports - 41,821 (2013)
validating contributory estimates:
Radius of Earth is actually 3,959 miles