From: David Pike
Date: 2019 Mar 14, 16:15 -0700
Frank you wrote:
the prime vertical, can be defined in terms of planes, too, right?
Yes, that's what I was thinking. It's as much a plane as a great circle. The only thing I would add to the plane definition is that the plane also includes the centre of the Earth. Including the observer and the observer's zenith in the definition does imply this, but mentioning the Earth's centre would give additional help when imagining it. DaveP