Direct opposites:
* Zenith: This refers to the highest point in the sky, directly overhead. It's a direct opposite in terms of vertical position.
* Nadir: This is the opposite of zenith, the lowest point in the sky, directly below.
Related but not direct opposites:
* Interior: This can be an opposite in the sense of "inside" compared to "horizon" as the edge of what you can see.
* Center: This is the opposite of the "edge" implied by horizon, but only works in certain contexts.
* Past: This is the opposite of the "future" that the horizon might represent, particularly when used metaphorically.
Ultimately, the best antonym for "horizon" depends on the specific meaning you're trying to convey.