How do you explain meridians on a flat earth?  Assuming nothing about the shape of the earth, let's define a meridian as the set of locations that experience the same time of day during the equinox.  We should all agree that on or around March 21 and September 22, people on a given meridian will all observe sunset and sunrise at the same time.

On a globe earth, this is pretty straightforward: a meridian is a half-great-circle joining the two poles.

On a flat earth, what is the shape of meridians?  Are they straight lines, or do they have some curve to them? If they are curved, how can the sun shed daylight on a convex area on one side of a meridian, but then light the opposite side 12 hours later (at equinox) which is concave?