[madphilosopher]

The main principle of celestial navigation is pretty easy to visualize.

Pick a celestial body that's in your sky right now, like the Sun. At any given time, the Sun is directly over a single point on the globe (the GP, or Geographic Position). So if you measure the Sun as being directly over your head, you know where you are exactly on the globe, after consulting your clock and almanac.

But, if you measure the Sun at a non-overhead angle, then you and everyone else with that same measurement must be on a circle whose centre is the Sun's GP. (Visualize the circle as the edge of a flashlight beam being pointed directly downward at the GP.) The rest of celestial navigation is refinements to figure out where you are on that circle.

[Onavo]

You can also do it at night using star charts (or as you call it, an "almanac"). That's how most of the digital celestial navigation solutions work, they use the positions of bright stars to determine a fix based on observatory data.

You can try it yourself here:

https://nova.astrometry.net/

[nonrandomstring]

Might I differ on "easy", from second hand experience of watching my father go through his advanced yachtsmanship RYA Astro Navigation exams and cursing at trig functions? In practice its a lot of paper and compasses.