An article in Forbes written last November :
5 Impossible Facts that would be Necessary for A Flat Earth/#139f95047c4f
To summarize those facts :
1. Lunar Eclipses DON'T only occur at midnight. As a flat disk, there is only one orientation of the Sun-Earth-Moon alignment that can take place to create a perpendicular circular (as opposed to elliptical) shadow (i.e. Sun at "midnight" location) . Since lunar eclipses occur around the earth at all sorts of different times - depending on location - a FE is impossible.
2. Seasons. The angle of the suns rays creates seasons. A flat earth would require (since the sun's rays are almost parallel) the same season all over the earth at the same time.
3. Different stars visible from different parts of the earth. There are some stars that are just never visible from the North pole that are visible from the South pole - indeed even stars visible to the northern hemisphere that are never visible to the southern hemisphere. Not everyone sees the same night sky as would be (essentially) necessitated by a Flat Earth.
4. Mauna Kea - highest peak in Hawaii - is nearly 13800' above sea level. One of its sister islands (Kauai) has the 7th highest point at over 5200' AMSL (Kawaikini). As the crow flies, they are 303 miles apart. On a flat earth, they would be visible to each other. However, they are never visible from each other's location. {My note here is that you can't use environmental factors blurring or obscuring vision as an excuse. We are capable of seeing further than 300 miles with the naked eye into the sky.}.
EDIT :
This would be a hard thing to overlook if true. I only take it as true since it has been asserted as such and I don't have evidence of this negative.
5. Sunsets and Sunrises. If FE is true, then it couldn't be daytime in one part of the earth and nighttime in another. The article says it would have to be daytime everywhere all at once. I can see that being not necessarily true, but the extended periods of daylight and night at the poles during winter and summer (while the rest of the earth sees more or less regular daylight transitions) can't be explained using FE.
