I was out scouting for a location to hide a new Geocache in “Stora Skuggan” on 1 October 2012. As I was browsing through the area it got dark and I completely ran out of batteries to my GPS. Eventually I managed to find a great location but without my GPS I was unable to get a reading of the coordinates. However, as I always bring along my sextant as a backup, I started looking to the sky to see if I could find some stars that would help other geocachers navigate to the position of the cache. I was able to see Dubhe in Ursa Major, Bellatrix in Orion and Fomalhaut in Piscis Austrinus and made the following observations (in degrees minutes seconds) with my sextant:
At 21:04:55 UT, Dubhe had an observed altitude of 31° 02’ 50.741443’’
At 21:06:44 UT, Bellatrix had an observed altitude of 02° 34’ 47.338464’’
At 21:09:23 UT, Fomalhaut had an observed altitude of 01° 04’ 10.842821’’
You will need to look up relevant data from the Nautical Almanac for the stars. To save you time, I have attached the relevant page to this description. The Greenwich Hour Angle (GHA) for Aries is available in the Nautical Almanac per hour and while you can get an approximation of the GHA for Aries for each observation through linear interpolation, for this exercise to get sufficient precision to find the cache you will need to calculate the GHA for Aries. The formula is available in this guide.
That is all the information that you need. To make things a little easier, there is no atmospheric refraction, precession, nutation or aberration of light so you do not need to adjust any of the values.
There are a number of different methods to calculate the coordinates of the cache from the information provided above and you are free to use your weapon of choice. If you are new to celestial navigation, you can check out this guide that describes the Sight Reduction method. You are welcome to state in your log which method you used to solve the problem.
