A seaside illusion

The snap left being the view from Yaverland beach, looking south towards Dunnose, a little beyond Shanklin. With Dunnose being a little to the left of Shanklin in this view. With Culver Cliff, about eight kilometres from Dunnose, being out of snap to the left.

Looking in any one direction, I can take in about half the horizon between these two points, about four kilometres.

But although I know that the horizon is flat, that a theodolite will show near zero all the way round, I have a strong sense that the horizon is a portion of a circle, so slightly humped in the middle, a sense which does not survive in this snap.

It is true that if I were able to rise above the ground, to rise high into the sky, eventually the earth would start to look like a ball. Think of those snaps of the earth you get from spacemen. But this does not apply in this case with my nose being about six feet above sea level.

It is also true that in about eight kilometres, measured on a geodesic along the surface of the earth, rather than taking the straight line through the earth, there is a certain amount of rise, a visible amount of hump. A rise which my early morning trigonometry does not rise to computing. Perhaps a little VB later will do the trick. But, in the meantime, the horizon I see from the beach is not a geodesic.

So I don't think that this is the answer either. So where is the illusion coming from?

PS: later: first effort at VB suggests a hump of the order of 5 metres on 20 kilometres, with the hump per kilometre rising quite quickly with distance.

Reference 1: some useful horizon numbers to be found at https://en.wikipedia.org/wiki/Horizon.