NavList:

I don't know if this was mentioned this time around, but it's worth repeating. Local horizontal coordinates, as already discussed, are defined by the plumb-line. The zenith is opposite the direction of local gravity and therefore the horizon of the coordinate system is essentially parallel with the sea horizon. How much difference would it make if we did it differently? If we defined the zenith to be opposite the direction to the Earth's exact center, where would the horizon be? If you work it out, the difference is just about (12')*sin(2*Lat). At 45 degrees latitude, the direction to the zenith would be fully 12 minutes of arc shifted if we defined the coordinates in terms of the Earth's center. That's a substantial difference from the point of view of sextant observations.

 

Another way to think about this: when you look straight down at the ground, you're not quite looking in the direction of the Earth's center. The local plumb-line points slightly south of there in the northern hemisphere. If you want to look "straight at" the Earth's geometric center, place an ordinary aspirin tablet on the floor. That's about 12' minutes of arc in apparent diameter (use half an aspirin if you're at 15 or 60 degrees latitude<g>). Aim your gaze about that much to the north from the plumb-line direction and your line of sight will be passing through the very center of the Earth.