I often see Flat Earth folks trying to "measure how far away the sun is" by using altitude (angle of the sun in the sky from that location) values from two far away cities.
The first obvious Folly here is that they invariably IGNORE that on a curved Earth you cannot simply compare two altitude values -- each location has 0° on THEIR horizon, so they are rotated about the Earth's center. So before we can USE this altitude information we have to already know the shape of the Earth (which we do, it's a Spheroid).
Imagine a viewer at the North Pole viewing the Sun which is in Equinox over the Equator. The person at the Equator sees the Sun at 90° altitude, directly overhead. But the person at the North Pole would see the Sun on the Horizon at 0° -- that is an IMPOSSIBILITY on a Flat Earth.
We also know that a nearby Sun is IMPOSSIBLE because of the actual parallax that would be visible!
Imagine a Sun that is only 4000 miles up and directly overhead observer A, an observer B a mere 1000km away would see the Sun at an angle of 8.8° lower! That is HUGE. If this were true ANYONE could trivially measure this enormous shift just by having two people 1000 km away from each other. THIS NEVER HAPPENS. This conclusively shows that the Sun *cannot* be 4000 miles up. You can test this out using this calculator. But don't just test only ONE distance -- if you are going to try to validate your model you need to test at multiple distances between actual observers and see if you observe the amount of parallax expected.
The actual parallax for two viewers on the Earth 1000km distance from each other is just 0.0001981409° - this is WELL below the ability to measure without fairly sophisticated equipment. This is why you cannot simply use altitude reports which only have a few decimal points of precision, you need about 9 decimal points to get in the right ballpark.