Latitude is your position on the surface of the Earth in a North/South direction. Unlike longitude, which is pinched together at the poles and spread out at the equator, latitude is a set and constant distance between degrees.
Each degree of latitude is 60 nautical miles, and each minute of latitude is 1 nautical mile. This makes calculations of distance when traveling up and down the coast of the United States very easy to do in your head!
In a previous post, we discussed the simple math to find your latitude using a noon site and none of the reasoning or theory that went behind it. Now, we will do just that.
The easiest way to confirm that your latitude calculations are correct is to check your work at night by sighting Polaris. Polaris, also known as the North Star, will not let you calculate your longitude, but it is a very easy latitude indicator. The angle from the horizon to Polaris is your latitude. This is because Polaris sits practically directly above the North Pole, and your angle to it is equivalent to your latitude. So, if you are unsure if your calculation of your latitude from noon was accurate, double check your work by sighting Polaris at night and it will tell you right away if you are one the right path.
The Sun, on the other hand, is not as obvious to calculate. As you know, the sun climbs higher in the sky during summers and stays lower on the horizon during winters. This is caused by the Earth having a tipped axis, which means that the sun doesn't sit directly over the equator. Instead, the sun is above the equator during the summer and below the equator in the winter. This change in position to the equator is called Declination.
Now, the calculations to find your latitude are rather simple. You measure the angle of the sun to the horizon and then substract it from 90. This number is then added or subtracted by the declination to give you a result which is your latitude. The math is pretty simple, but how do you know if you should add or subtract?
The decision is simple and based on the relative position of you, the sun, and the equator when you took your noon site. If the sun is between you and the equator, then you will add the declination. If you are located between the sun and the equator, then you will subtract.
As stated in the simplified version, if you are unsure, simply do both and figure out which one gives you a more reasonable answer. If you have no idea where you are; as in, you woke up one day floating on a raft with only a sextant, a current Nautical Almanac, and an accurate time piece that is set to UTC time (a very likely scenario) you could easily do both (adding and subtracting) to figure out your posible latitude coordinates. Once the sun sets and the stars come out, you could then site Polaris and see what your actual latitude is. The correct answer could be used to decide which mathematical process to follow and used to calculate your position on the Earth as you slowly drift along this giant ocean!
From experience, I have yet to subtract the declination from my reading, as you would only do this if you are very close to the equator. If you are sailing anything higher than say, 24 degrees North, you will be adding the declination in your calculations.