The horizon is the limit of your view. As you rise, you see more. On a ball you see more of the ball, but that "more" is further down the ball, and should therefore be at an sharper angle to you. It would appear to descend.

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However, in reality, this is not what happens. This horizon rises to the eye of the observer. This is consistent with a flat plane, where what the horizon is simply the limit of your vision. As you rise, you see more of the plane, but that will always be at eye level.