The web browsing that I had done during this process did not lead me to any explicit solutions (such as finding someone offering a complete suite of Excel spreadsheets for celestial navigation computations…).  I did find, however, a discussion list of like-minded people that I promptly joined.  This "NavList" has been and continues to be a great source of wisdom.  Recently a very good overview of NavList was published here:

http://columbianewsservice.com/2010/03/they-sail-their-ship-alone/

What is especially valuable is that there are number of members who are accomplished sailors, scientists, and sometimes both, who are a treasure trove of knowledge and experience.  On a number of occasions they presented real-life data they themselves took while sailing, which enabled me to successfully verify the performance of the suite.  In fact, recently one gentleman serving on a merchant ship was in the Indian Ocean taking sights with his sextant and has put my noon spreadsheets to work.

Through NavList I came to learn about John Karl, a physicist, artist, and sailor whose excellent book "Celestial Navigation in the GPS Age" I subsequently obtained.  Finally I had a reference whose style was more in sync with my own thinking; in fact, some of the things I had already done with Excel were even suggested in the book as exercises for the reader!   But the most valuable piece of information in this book for me was a reference to another book titled "Astronomical Algorithms" by the Belgian astronomer Jean Meeus.

The spreadsheets that I had developed up to that point had to do with sight-reduction, that is calculating one's position by solving the relevant geometry on the surface of the Earth.  This however was not a self-contained suite yet, as it required astronomical data as input.  This information, most notably the positions of the observed celestial bodies at the moment of measurement, is precomputed in advance and published in almanacs.  Meeus's book clearly explained how this can be done and even provided analytic formulas that describe our solar system.  The word analytic (as opposed to numeric) is important because this way it is possible to describe the planetary orbits with formulae of very broad validity rather than tables whose size scales with the chosen time period and which would require search and interpolation procedures.  Furthermore, the accuracy of these equations was way better than what is realistically required (and achievable) for celestial navigation purposes.

And thus the idea came whether it would be possible to complete the suite by encoding these formulae into Excel as well.  The formulae were quite large, with lots of terms, and there were a number of steps to keep track of.  Therefore, I decided to first write a program in Fortran, a computer programming language with which I am well familiar.  After I ran the program and convinced myself that the results agree with the official Nautical Almanac, I encoded the same calculations into Excel using the now verified Fortran program as a guide.  This took a fair amount of time, several weeks in fact.  Some of the spreadsheets exceeded 1 MB in size due to the number of terms included.  But it worked and by the end of June 2009 we rolled out a major extension to the suite, now including these new "almanac spreadsheets."  I was pleasantly surprised to see that Excel was able to handle this kind of stuff.  Think about it, a software tool designed for accounting, inventory and whatnot, can be actually used to predict the positions of planets and stars with accuracy better than one minute of arc!  I just think that's awesome, in a freaky-geeky kind of way.

Excel also has the advantage that it (or its sufficiently compatible alternatives and competitors…) are available on pretty much any computing platform.  The spreadsheet app then serves as a kind of "virtual machine" in the spirit of Java.  Thus there is only one version of Navigation Spreadsheets, and it can be run on Windows, Mac, Linux, even on a smartphone, without modification.  On the website there are links to YouTube demonstration videos in which I used an iPod Touch.

So that's the summary of how this project came to be and what it is about.  I want to come back to give occasional updates, maybe give an example or two, and provide additional historical details about the suite from time to time.  Stay tuned.