On either the flat Earth or globe Earth models, the horizon wouldn't be a "straight line." The horizon argument is often one of the first cited by flat Earthers in my experience debating with them. "The horizon is a straight line," they say, "so the Earth must be flat." But the horizon is circular, on a flat Earth or a spherical Earth.

Say you're on a massive ocean that covers the Earth, an Earth which may or may not be spherical. You can see an equal distance in every angular direction. You may be far from the edge on a flat ocean Earth or at any point on a spherical ocean Earth. The horizon is both ahead of you and behind you, an equal distance away in every direction. That's a circle. It curves around you, irrespective to which Earth you've found yourself in. If this Earth is flat, your horizon is a circle. Its diameter is determined entirely by your height and how much the atmosphere scatters light. If this Earth is spherical, your horizon is still a circle. Its diameter is this time determined by your height, the atmosphere as mentioned previously, and the diameter of the sphere. The only discernible difference, unless you come to a visible edge, is that the spherical Earth's horizon appears lower than the flat Earth's horizon. How much lower is dependent on the atmospheric scattering of light once more and the diameter of the Earth.

Any measurements of how "low" the horizon is, taking into account instrument height, atmospheric scattering of light, and a proposed diameter of the Earth could never disprove a spherical Earth. It could certainly disprove our current understanding of gravity and even the proposed circumference of the potentially spherical Earth, but you couldn't use that measurement to disprove a globular Earth completely. I've never seen anybody do that regardless.

I'd like to see this "straight horizon" argument put to rest, unless I'm missing something critical here, in which case, I'd be delighted if you attempted to educate me