On a flat earth, the horizon is relative, depending upon the device used to view it. This is because, as the vanishing point moves farther away, resolving it requires more resolution.
An optical device with circular entrance pupil, such as the eye, or a telescope, has an angular resolution, theta, given by the formula,
theta = 1.22 x lambda / D
where
lambda is the wavelength of the light and D is the diameter of the entrance pupil.
On a bright sunny day, the pupil of the human eye has a diameter of about 3 mm. Let's take the wavelength of the light to be around 510 nm (the mid-point for green, off the top of my head, I haven't looked it up in a while).
theta (eye) = 1.22 x 510 x 10^(-9) / (3 x 10^(-3)) = 0.0021 rad (~ 7 arcminutes)
Take a telescope of primary diameter 10 cm,
theta (telescope) = 1.22 x 510 x 10^(-9) / 10^(-1) = 0.00006 rad (~ 13 arcseconds)
The 10-cm telescope is thus 35x better at resolving the image being viewed than is the eye on a bright sunny day.