Hi Martin - welcome to a new week of dueling postings (in a nice way of course)! I think I was quicker on the draw so to speak this week.

Unfortunately the demonstrator doesn't include the equator,

Unless I'm mistaken the thicker blue line on the simulator is the equator but perhaps I'm wrong.

Ed

astralwanderer wrote: Mon Mar 10, 2025 10:01 am Unless I'm mistaken the thicker blue line on the simulator is the equator but perhaps I'm wrong.

No, you are absolutely right, of course. How odd that I should have missed that! So that means we have all the relevant great circles displayed: horizon, meridian, equator and ecliptic (plus the prime vertical, which isn't essential to understanding these dynamics as such, although used in some systems of house division). You don't get a real sense of the equator actually moving, unfortunately (perhaps that is why I subconsciously filtered it out), but that can be remedied to some extent by setting the date to either of the equinoxes, so that the Sun is right on the equator.

Thanks Martin - good to have your confirmation of this.

Have a good week.

Ed

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The key thing here is to study the movement of the southern pole of the ecliptic which is where the twelve equal divisions of the ecliptic (zodiac signs) meet below horizon. See that this is in a constant state of regular motion about a fixed point. Unfortunately, Walter's graphic doesn't mark the southern pole of the celestial equator but effectively this is what the graphic is showing - the movement of the poles of the ecliptic about the fixed points of the poles of celestial equator over the course of a day.
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I hope that helps.


In a way the southern equatorial pole is marked. Note where the darker Green Meridian line and Purple 0 degree Capricorn line intersect.

Edit to correct as the Prime Meridian is 0 degree Aries (at Greenwich) of course. And we are looking at the temporal N/S meridian.

Great work PhoenixNS - I hadn't spotted this. That is interesting.

Thanks very much. Ed