Next message: Bill: "Re: Plumb-line horizon vs. geocentric horizon"
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.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
