The zenith distance of any point or celestial body is its angular distance from the zenith of the observer.
Z0 is the zenith distance of the moon's center midway through the hour of observation, that is, at the half hour.
The latitudes were determined by observations with a sector (made by George Graham) of the zenith distance of α and δ Draconis.
The zenith distance of a body is measured on a vertical circle passing through that body.
So far as illustrating the zenith distance minus the Declination, however, the diagram is correct.
He certainly found an annual variation of zenith distance, but not at the times of year required by the parallax.
Then the zenith distance is the distance from your zenith to the center of the sun.
zenith distance is the distance in degrees, minutes and seconds from your zenith to the center of the observed body.
Or it may be used to measure the altitude of a body, or its zenith distance, at the time of its meridian passage.
The last case is where you are, say, 10 N of the sun (your zenith distance is 10) and the sun is in 20 S declination.