Celestial Navigation

February 15th, 2008

I started looking at celestial navigation as a result of wanting to recreate a transatlantic flight in FlightSim without Inertial or GPS navigation systems. There is a simulated bubble sextant available for FS2004, and it works well but doesn’t give you that appreciation of the sky that the real process does. So, I thought it would be fun to try for real. The sextant stuff that is, not the transatlantic stuff…

Two problems: 1 – I don’t own a sextant. 2 – It’s really cold outside.

If you’ve not seen the package “Stellarium“, download it and install it now. It gives you a real time view of the sky, and is fantastic. So good in fact I’ve been considering buying a telescope. (In the summer though: Outside. Cold. Really.)

Stellarium also gives you a display of the Elevation (referred to as Height) of stars and planets. As that is essentially all a sextant does, one can do simulated star shots from any location on earth from the comfort of your living room. The skill of operating the sextant device is bypassed by this, but it does let you practice position fixing.

So all one needs now is a Nautical Almanac and Sight Reduction Tables to determine the expected Azimuth and Elevation for a known (assumed) position. Once the excitement of the Sight Reduction Tables has worn off, the US Naval Observatory has a facility to generate an almanac page for a time and assumed position. A map or piece of graph paper to plot the lines of position completes the navigation kit.

More information on the process can be found at celnav.de, including a comprehensive guide. Once you’ve got your position, you can learn how to navigate to your next waypoint thanks to Ed Williams Aviation Formulary.

I used an old 1:500000 VFR chart and set Stellarium to a random location. (Stellarium can’t set a random location, so one needs a glamourous assistant for this part!). Using the USNO table, I was able to get an accuracy of 1NM when over 45NM from the assumed position. That’s a result in my book!

All fascinating stuff, and I’ll probably buy a cheap sextant to give it a try – on the ground though – Distress & Diversion on 121.5 are a better bet if you’re really lost!

However, should you see someone standing outside on the Ben-My-Chree with a cheap sextant, a VFR chart and a Powerbook (and probably a puzzled look) don’t worry, the boat’s not lost!