NavList:

Herbert,

And it's easy to demonstrate: All you need to do is to draw a meridian in your Fig.1 through the point where the sun is cutting through the Small Circle labeled "-12". Done. You will immediately see that this meridian cuts the equator farther than 12 deg away from the terminator. The picture is actually easier to draw in the left diagram for the special case where lat = 0 than for the general case in the right diagram, where you have to draw two meridians (a second one through the terminator)!

If that’s true, then it’s rather neat, and I take my hat of to you.  It would mean that in my Fig1 as the declination line moves north in the Northern Hemisphere the terminator meridian moves west and vice versa, and as it moves south in the Southern Hemisphere the terminator meridian also moves west.  Unfortunately, this isn’t born out by the almanac for latitude = 0.  A glance at the NA, or my Fig2, shows that the duration of twilight hardly changes between solstice and equinox at latitude = 0 whereas using your explanation as described with respect to my Fig1 it should.

It would seem to me that bringing latitude into play only adds confusion to the explanation. The effect of sun declination is one thing and that of observer latitude is another. Both are of a different nature, independent of each other, and have thus their individual explanation, as you can now see from the diagrams in Fig. 1, should you chose to complete them as per the above.

It’s semantics of course, but whilst I agree that both effects are different in nature as I tried to point out, I wouldn’t necessarily agree that they are independent.  The almanac values show that the effect of declination on the duration of twilight increases with latitude.

By the way, nautical twilight starts at the end of civil, i.e. when the sun is 6 deg under the horizon.

Semantics again, I was referring to the time between sunset and -12 degrees.

I deliberately had suggested to look into this phenomenon in order to reduce the problem to a purely geometrical one; in other words, so that we don't get distracted by refraction or sun diameter.  

I agree, ignore refraction, as I said in my text.  However, it would be an interesting new thread to discuss how much refraction affected matters, if at all.

 I am actually a little surprised that we are having this discussion. Does no one besides astrologers use the concept of "equation of time"? I thought  navigators know it, too? What causes the EoT?

During navigator training there’s so much to remember that some things have to be relegated to the interesting but not essential area of the brain.  I’m afraid the EoT comes into that category.  However, I don’t think the EoT comes into it in this case.  If mean time is fast or slow on solar time, it is so by virtually the same amount for both the start and the finish of twilight.  Is this what they call Thesis, Antithesis, Synthesis?  DaveP