It seems that flat-earth believers deny the existence of satellites, which means all global navigation system satellites (GNSS) including the American GPS satellites (at 20,000 km altitude), all geostationary satellites (at 35,000 km altitude) and all low earth orbit TV transmitting satellites (around 300 km altitude). The elephant not in the kitchen but in space is the largest of satellites, the International Space Station (ISS), the largest man-made structure in space. It orbits at an altitude of about 400 km, which is well outside of the atmosphere.
The ISS can be seen from the earth’s surface with the naked eye, when it passes overhead on a 90 minute circumnavigation of the planet, travelling much faster than stars move. It looks like a fast-moving plane only much higher and traveling thousands of kilometers an hour faster. When it comes into view in your area it is best viewed within two hours of sunset or sunrise. Because of the very large solar panels, used to power the spacecraft, sunlight can reflect off those in your direction.
Here is a live video feed from the ISS. But flat-earthers say that all such video is faked. They say that all space agencies fake everything they show. So you can’t win.
NASA has it own TV showing live feeds from the ISS when available. Below is a YouTube recording of a past live feed.
Why does the existence of the ISS refute the flat earth nonsense? Because the ISS can be easily seen from the earth’s surface without any aids and the space vehicle exactly follows the published orbits, which means it is in space around a spherical planet. I know it seem crazy to even have to say this.
Personally I have been involved in the Atomic Clock Ensemble in Space mission, which was first orogrammed to be placed in the the ISS but due to delays has not yet been launched. A cold-atom atomic clock was built in France, financed to the tune of hundreds of millions of dollars, by the European Space Agency (ESA). Why would they fund such a mission if the ISS does not exist?
Also I have personally used a geostationary satellites to send microwave signals back and forth between the Physics department at the University of Western Australia (UWA), Perth, and the National Measurement Institute in Sydney. Using this methods we compared out atomic clock at UWA (a Hydrogen maser locked to my Cryogenic Sapphire Oscillator (or Sapphire ‘clock’)) with the Cesium atomic clocks at NMI.
I pointed the satellite antenna at the geostationary satellite at an altitude of 35,000 km (about 6 earth radii) and the folks at NMI pointed their antenna at the same satellite. This allowed us to essentially bounce our signal off the satellite to get the signal across the Australia continent. It is about 4000 km from Perth to Sydney. No direct line of sight is possible because of the curvature of the earth.
The geostationary satellite stays over the same location of the planet because it is orbiting the planet, synchronised with the planet’s 24 hour rotation. Physics requires that it must be at the specified altitude and moving around 40,000 km in 24 hours or out 1,700 km/hour. To stay in obit at lower altitudes it would have to move much faster and it cannot remain over the same location. Hence the ISS at 400 km altitude moves at about 28,000 km/hour and does one orbit every 90 minutes. So in that time it covers 25,000 km which is approximately the circumference of the planet. It is a little more due to the height above the surface.