For equinox go to Equator.
There's place in Brazil caled Sao Gabriel de Cachoeira.
In the morning Sun is above Macapa, Barsil. From Sao Gabriel de Cachoeira you see Sun at azimuth of 90 degrees, which is directly to the East.
In the afternoon Sun is above Quito, Ecuador. From Sao Gabriel de Cachoeira you see Sun at azimuth of 270 degrees, which is directly to the West.
Macapa is at Equator, Qito is at Equator, and line the Sun travels during the equinox day is straight.
So, if you fly between Macapa, Brasil and Quito, Ecuador via Sao Gabriel de Cachoeira, you will fly in straight line.
Same goes for flight between Kampala, Uganda and Libreville, Gabon via Bosio, Congo, and Mbandaka, Congo.
Sun path above those places for equinox is straight line above heads of all those people, and they know it.
You see, the problem is if people can fly in straight lines (really great circles), FE theory cannot work. If you can fly in a straight line, then you'll see that you can track a line of latitude; with FE this should be impossible. If you can fly in a straight line, you can measure distances which would disprove the FE theory (as has already been attempted). If you can fly in straight lines, you can measure the angles of a triangle, which disproves FE theory.
So FE theory will always point to a "distortion" or "not perfectly straight" argument to throw out these proofs.
So I propose something else: why not use a statistical argument to prove something? We should note that by symmetry the deviations will be about 50% to the "left" and 50% to the "right". I'm sure the compass experiment could then be repeated many times and we could figure out exactly what's happening.
You don't need flight.
Globe Earth, equinox (March 21st):
Sunrise is directly to the east at azimuth of 90 degrees.
Sunset is directly to the west at azimuth of 270 degrees.
It is like that anywhere on Earth.
Flat Earth, equinox (March 21st):
Sunrise and sunset depend on observer's latitude.
If you are on Equator your sunrise is at azimuth of 45 degrees (NorthEast) and sunset at azimuth of 315 degrees (NorthWest).
If you are more to the north, sunrise azimuth is closer to East and sunset azimuths is closer to west.
If you are more to the south, sunrise and sunset azimuths are more to the north.
Draw north pole at center of paper, Ice Wall as big as possible around it, and Equator with half ot Ice Wall radius.
Then draw line of solar noon from north pole outward, and sunrise and sunset will be on equator 90 degrees left and right.
Then measure angles where sunrise and sunset will be visible as you move observer position along solar noon line.
Finally, compare with angles measured in real life on March 21st.
EDIT: Like this, and it doesn't change however you draw continents.
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