Forget about GPS, Sights and all complex methods to fix your position. Today I am going to show you how you can find your position just with a meter stick. And it can be of great help out at sea especially, suppose you are in a lifeboat without GPS and you need your position.
Kindly, note that position finding techniques we are going to discuss today are very rudimentary and hence the accuracy of position obtained would be within few degrees of actual fix i.e very low. But it will serve you in an emergency. So let’s begin
- Meter Stick
- Bright Sunny day
- A rough idea of present declination of the sun.
- A calculator.
Finding latitude is the easy part of finding your position. In this method, you will require a rough idea of present sun’s declination. Sun’s declination follows a definite pattern and as OOW must be calculating the compass error using the sun. So it is fair to assume that OOW has a rough idea of sun’s present declination.
Step 1: Sometimes after sunrise, put your meter stick in an open area such that sunlight fall directly on meter-stick.
Step 2: Mark the height of shadow formed on the ground. The length of the shadow will change(decrease in this case) as time passes, so keep marking the length frequently for a long period of time say until well after noon.
Step 3: Observe your markings and measure the length of shortest shadow with the meter stick.
Step 4: Do a little mathematics I am going to show you in a while and determine your latitude.
You may not realize it right now but you have already determined your latitude I will just prove you how.
- At the time of shortest shadow, the sun was on your meridian. i.e mid-day.
- If the shortest shadow points towards the north it means the sun is south of you and vice versa.
And we know both h and s, so we can calculate the value of alpha. And this is the moment of truth guys. This alpha is nothing but “OBSERVED ALTITUDE ” of the sun. And very cleverly we have measured this at the time of meridian passage of the sun. So it became our observed meridian altitude. All the error applied to observed altitude to obtain true altitude are ignored because they will sum up to half a degree and we are only calculating approximate latitude.
Now if we would have the value of declination of the sun, we can calculate our latitude using latitude by meridian altitude method. To use above method we also need to know the azimuth of the sun which will be opposite to the direction of the shadow. The accuracy of latitude obtained is directly proportional to the value of declination we use.
A rough diagram of sun’s declination at different time of year is shown below.
ERRORS IN METHOD
- Error in measuring “h” or “s” will directly result an error in latitude.
- The observer may not be able to determine the length of the shortest shadow, constant observation, and very high concentration is required.
- Assuming observed altitude equal to true altitude will induce error in latitude.
- Your approximate value of declination of the sun.
- Remember an 1 degre error in latitude will produce an error of 60 miles in position! so be careful.
Well, this is not so practical method. We have to assume that you know what is GMT.
- Meter Stick
- Bright Sunny day
- A clock showing GMT
I know guys our assumption about perception of GMT is not practical. But just for information sake just look at it.
Step 1: Find the instant of when shortest shadow occurs and mark it 1200 LAT.
Step 2: Assume 1200 LAT as 1200 LMT because error is minor(equation of time)
Step 3: Using GMT and LMT calculate your longitude.
- Error of equation of time i.e error due to assuming 1200 LMT as 1200 LAT
- Error in GMT used.
MILLION DOLLAR QUESTION
Question – Suppose I sent you a picture of myself standing and my shadow on the floor, and you know the date and GMT and LMT of the photo taken? Can you determine my location on earth from data provided?
Ans – NO, YOU CAN NOT. You will get a position circle. This is my doubt since pre-sea after watching the movie “GI-JOE RISE OF COBRA”. I will show you that clip because of which I got slight inspiration for celestial navigation. If you know my height and height of my shadow you know the true altitude of the sun. The form that you can calculate zenith distance. Declination can be obtained from Almanac. Since it is a photograph and you don’t know which side is north so you don’t know which in which direction shadow is pointing. If I would have captured the photograph standing on a compass then the answer would have been a resounding “YES”. How? Let’s see mathematically, but first, watch the movie. Also, you need to know both GMT and LMT of photo taken otherwise we won’t be able to solve the PZX triangle
THANX FOR READING GUYS
Fun Facts: I am very inspired by Greek philosopher named Eratosthenesis, who in 2nd century B.C calculated the diameter of earth assuming the earth is a perfect sphere using celestial navigation. He is called Father of Celestial navigation.