NavList:

First of all, Declination is just the latitude on the globe of the subStar point. So a point with Dec = 0.000° is directly above a latitude of 0.000°. So that's easy.

Next, for longitude, what you're looking for is really "GHA Aries" which is identical to "sidereal time" in astronomy. By definition (an "original" definition), Local Sidereal Time is the Right Ascension of the local meridian. We can skip over the fact that R.A. is/was traditionally given in time units. If you calculate the SidT of your meridian and get 45°, for an example, that means that "Aries", in the coordinate system sense, is 45° from your meridian so that means that Aries is directly above a longitude equal to your longitude + 45°. Making life simpler, if we set the longitude of the observer equal to 0°, then we can calculate Greenwich Sidereal Time, and that is immediately equal to the longitude directly under the "First Point of Aries". You can find formulae with various levels of minute detail online by searching on 'calculate Greenwich Sidereal Time' or something similar [here is just one example]. Or try an astronomical computing manual... an actual book! I suggest a nice old book from forty years ago: Astronomical Formulae for Calculators by Jean Meeus. I do not recommend his later Astronomical Algorithms for beginners (or anyone really).

Frank Reed