Short-cut URL for this page: https://chrismolloy.com/sunclock.
I was fascinated with the idea of a Sun Compass/Clock presented here, here and here in the 10 Bushcraft Books texts. The basic principle is that it can help you to accurately calculate either the current time, or a true north bearing (or any other bearing by extension) or your current latitude. All that is required is that you know any two of the three and that you have an accurate Sun Compass/Clock diagram to work from. Thus I set out to build a small application that anyone can use to create a Sun Compass/Clock that can be printed out and used 'in the field'. Technical wizards out there should even be able to figure out how to get it onto their PDA for 'live' use when they're out-and-about (you'll need to figure out your own way of attaching a gnomon to your screen though).
My application is a Java Web Start application. All the files you require will be downloaded automatically from Sun Microsystems and/or this web site. The application was built using Java version 1.5.0 (jdk1.5.0_01), so if your JVM is out-of-date it will be updated from Sun the first time you use this application. All this downloading only happens the first time you run the application - after that the only downloading that happens is the retrieval of any future updates to the 'Sun Compass/Clock' application that I might periodically release. Once you have downloaded the application it can be used whilst you are off-line - see the Java Web Start documentation for guidance.
The Java Web Start security model is extremely strict and it is nigh impossible for this application to do anything untoward once it is on your computer. Ultimately, however, you have to be satisfied that content from this web site is not likely to be malicious. I have posted the source code for this application (see below) for those who wish to verify that it only does what it says on the box (or for those that wish to play with the code).
To get the application click on the 'Launch' button, below. You will be prompted to allow the application access to your printer (so it can print you out a Sun Compass/Clock). I'm not sure what will happen if you decline this request, but assuming you can still run the application, you could print a screen-shot instead of using the in-built Print function.
The 10 Bushcraft Books links here, here and here explain how to use a Sun Compass/Clock to determine the current time/latitude/bearing to true north. What follows is a brief explanation of my version of the Sun Compass/Clock. It is intended that you print out a Sun Compass/Clock for your latitude for use in the field - you might like to grab the text that follows and/or the text from the links above to stick on the back of your hard-copy.Key:
- Gnomon locator for specified date
- "Gnomon date shown"
- "Time of Meridian Transit" for specified date
- Sun line for specified latitude
- "Latitude shown"
- Time lines - in hour intervals
- Time lines - in 15-minute intervals
- Compass lines - in 5° intervals
Set up: first you need to print out a copy of the Sun Compass/Clock for your latitude and date. Use the 'Display' menu options to set the latitude and date displayed. A Sun Compass/Clock can of course be used for any date (see the sections linked to above), but by specifying a date you get an accurate gnomon locator plus a time correction estimate (see below) for free. Next you need to add a gnomon (a 'stick' that will cast a shadow across your Sun Compass/Clock). Your gnomon should be perfectly vertical and long enough to cast a shadow that will cross the sun line (4) at all times of the day. Next you need to hold your Sun Compass/Clock perfectly level and in a place where the sun will fall on it for the duration of what you are using it for. To use it as a clock, orientate it to true north. To use it as a compass, orientate it so that the current sun time matches the time on the Sun Compass/Clock.
Time correction: several time correction factors are mentioned in the sections linked to above. These corrections help to increase the accuracy of what the Sun Compass/Clock tells you. I have built an Equation of Time calculator into the Sun Compass/Clock application that will display the Time of Meridian Transit for the specified date so you don't have to work it out. To work out the longitude-dependant time correction factor, that is based on your position relative to your local time zone, see here. The final 'correction' to be aware of is whether or not Daylight Savings Time is being observed and to make allowance accordingly.
Equation Of Time Graph
Instead of reading the Equation of Time data from a table, how about a graphical version instead? This might also be a useful item to print out and stick on the back of your Sun Compass/Clock.
The following source code is for v2.0.1 of this application and is supplied under the Creative Commons Attribution-ShareAlike 3.0 Unported License:
Enhancements coming soon: latitude in ° ' " format as well as decimal format; and some 'help with pictures' to show the Sun Compass/Clock in action.
In The Field...
A bloke in Western Australia by the name of Pete sent in this photo of a Sun Compass/Clock in action out at "a place called Paynes Find about 400km NE of Perth". Pete writes "We had a few gnomon issues, getting a bit of braising rod perfectly perpendicular in 2 planes wasn't easy, and also we realised that the car was parked on a bit of a slope as well. After corrections, accuracy was amazing from about 0800 to 1600. Before and after that we aren't too sure coz of the gnomon... we shall overcome!". Cheers, Pete!
Java Web Start 60 Second Guide
"Java Web Start provides a platform-independent, secure, and robust deployment technology. It enables developers to deploy full-featured applications to end-users by making the applications available on a standard Web server. By using any Web browser, end-users can launch the applications and be confident they always have the most-recent version." - from Java Web Start FAQ
Java Web Start is now included in the Java Runtime Environment (JRE), so if you've got Java, you've almost certainly got Java Web Start. If you're not sure, try launching any Java Web Start application - it will set you up if you're not already able to run the application. Note that this 'set up' stage will only happen the first time you run the application - after that you'll be good to go with any future Java Web Start launch.
A few useful links: Java Web Start Home; Java Web Start FAQ; Java Web Start ReadMe.
The easiest way to manage Java Web Start applications (including launching in 'offline' mode) is to use the Java Web Start Viewer. Under Windows, run (or create a batch file to run) the following command: "javaws -viewer". The 'javaws.exe' lives in your 'C:\Windows\System32\' folder.
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