nonagesimal in Kepler's parallax computation I'm starting to learn Kepler's Astronomia Nova using William Donahue's translation . In the translator's introduction, it is demonstrated how Kepler did his parallax computation. I failed to understand the following paragraph:

 

specifically: 

1) nonagesimal's definition: there are 2 points of intersection between the ecliptic and the horizon: one in the East, the other in West. another ambiguity is the direction of the $90^\circ$

2) I can't see why the altitude of the nonagesimal is "the same as the angle between the ecliptic and the horizon". It does make sense to me, but I'm missing a kind of "proof" for that. 

3) Actually I also failed to understand what follows in that paragraph - but maybe your explanation for the points above, will clarify that for me.

your help is very much appreciated
It looks like the assumption regarding the 90° point is that the amount of the ecliptic visible is 180°. So the 90° direction is the halfway point.

The nonagesimal is on the ecliptic, so the altitude approximates the arc. The wording in the initial part has that correct, but then it goes on to say "the same as the angle" - this seems to be a mistake. The arc is directly related to the angle, yes, but it is not the angle.