Some flat-Earthers like to observe celestial phenomena and the positions of celestial objects. Sometimes, they insist such events cannot possibly happen if the Earth is a sphere orbiting the Sun, and took the wrong conclusion that the Earth must be flat.
We can try asking them the time of the occurrence and the position of the observer, then use Stellarium or similar applications to simulate it. If the result matches the observation, then they have nothing to complain about. Their confusion was only the result of their ignorance.
Stellarium is a planetarium software. It can render a realistic projection of the sky in real time, as well as any time in the past or the future, with relatively good accuracy. Stellarium is free software and costs nothing to download. It is freely available for all the major desktop operating system. There is a version for mobile phones/tablets, albeit a paid one; and a simpler web-based version of it.
Stellarium uses the spherical Earth model internally. If the result from Stellarium matches observation, then there’s nothing wrong with the spherical Earth model. If any flat-Earthers claim it impossible to occur, then it actually means they don’t understand how it can happen.
Some flat-Earthers claim that Stellarium is made using the flat Earth model. We can easily prove this wrong by using Stellarium to simulate how the Earth appears from the surface of the Moon:
- Find the Moon: Ctrl-F, type “moon”, Enter
- Move observer to the selected object (Moon): Ctrl-G
- Find the Earth: Ctrl-F, type “earth”, Enter
- Turn off horizon if the Earth is below the horizon: G
- Zoom on to the selected object (Earth): /
If done correctly, Stellarium will show the spherical Earth in all its glory, as seen from the Moon.