Actually, I just thought of an even easier way to explain this.
If you want to find a star in the sky, you look up its celestial coordinates, and stick them in a formula together with your position on earth and the time. This gives you azimuth (“heading”) and elevation of the start. You then point your telescope in that direction, and find it right there.
Celestial navigation is the same in reverse. You point your telescope at a star, measure azimuth and elevation, stick this into the formula together with time and the celestial coordinates, and you get your position on earth.
For this to work, you need to know the true horizon/which way is up, and the direction of north. For the horizon, you can use the visible horizon or gyros, and for the azimuth you need a second star sufficiently far away from the other star, unless you look exactly up in which case you don’t need to know it.
I often recommend this one.
The two stars trick is what is used to calibrate telescopes, once that is done then you can use the “go to” function to find any other planets/stars automatically from the database, recognizing what are the names of the two stars you are pointing to is the fun part but new telescopes do that match automatically, I was puzzled there is zero reliance on GPS but it turns out you only need to set clock and date…
Theoretically, you can do without a clock, so long as you can observe the moon and the horizon. The “lunar method”, made “practical” by Nevil Maskelyne, was the main rival to John Harrison’s famous chronometer.
Yes that’s interesting. I am amazed (having watched that video) that anyone can hold the thing steady enough on a boat…
I have a supplementary Q: how do you navigate in deep space, autonomously, and solely by optical observation of celestial bodies?
Taking the hint from Maskelyne’s moon observation principle, clearly you can do something if you can observe significantly moving bodies (e.g. planets orbiting a star) and you know the time. But what if you see no planets?
Holding steady is definitely more about time on board than skill. Its interesting after a week sailing the body becomes naturally attuned to the boats motion, so for the first few days you are exhausted, everything seems an effort, then it just gets easier. Its then just weird when you get back on land, because everything does seem to keep moving for sometimes up to a week!
In low orbit, it is the same principle as for aircraft.
In deep space, I don’t know nowadays, but Apollo’s AGC only used it for attitude reference, not for position reference.
The latter used the data on orbital mechanics and some ref on the Earth and/or Moon (terminator, horizon or ground refs) but I don’t know whether this also needed further data input relayed from the ground, does someone know or have a ref?
Most sailors use one of the Astra sextants if you want something modern and light. Its a lovely piece of equipment infinitely more interesting than the CRP-1, which I also still have but don’t think ever got used!
Apollo was designed to fly to the moon and land and come back even if all ground contact was jammed by the Russians.
They did have a reasonably accurate clock on board, though only quartz crystal controlled, so probably within a few seconds.
I recall a good article (possibly NS) that the independent way to go may well be pulsars. Not only do we know their position by they are also a phenomenally accurate time reference, so in the those two elements you have all you need. Would I be right however that if we could ever travel at anything like sensible speeds for space exploration an additional adjustment would be needed for time dilation?
