Why Does a Football Field Look Flat?

Why does a football field look flat if the Earth is round? Stretch a straight line between the two goal lines, 100 yards (91.44 m) apart, and the ground in the middle rises above it by only about 0.0065 inches (0.164 mm): less than the thickness of two sheets of paper. This video works that bulge out from the Earth's radius, shows that two upright poles on the goal lines are less than 1/1,000 of a degree from parallel, explains why the horizon looks straight from 5.5 feet up, and looks at how high you need to be, about 35,000 feet (10.7 km), to make out the curve. Try it yourself: ▶ https://www.metric-conversions.org/length/millimeters-to-inches.htm Chapters: 0:00 The question 0:29 The bulge 1:18 Too gentle for your eyes 1:47 Why the horizon looks straight 2:15 When does the curve show? 2:41 Takeaway Sources: • American football field – Wikipedia: https://en.wikipedia.org/wiki/American_football_field • NASA Earth Fact Sheet: https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html • metric-conversions.org millimeters to inches: https://www.metric-conversions.org/length/millimeters-to-inches.htm • Just How Small Is "Nano"? (nano.gov): https://www.nano.gov/about-nanotechnology/just-how-small-is-nano • AAO EyeWiki: Visual Acuity Testing in Adults: https://eyewiki.aao.org/Visual_Acuity_Testing_in_Adults • Lynch, "Visually discerning the curvature of the Earth", Applied Optics 47 (2008): https://opg.optica.org/ao/abstract.cfm?uri=ao-47-34-h39 • Cruise (aeronautics) – Wikipedia: https://en.wikipedia.org/wiki/Cruise_(aeronautics) Is there anything else you'd like us to cover? Let us know in the comments below.