Parallels of Latitude form one of the components for fixing a position without ambiguity on the surface of earth. Equator forms the basis for defining the parallels of latitude. We are ready to understand about the parallels of latitude since we had learnt about the Equator and Meridians in our previous lesson.
Small Circles are circles on surface of earth whose centre and radius are not the same as that of earth. To be more exact the radius of a Small Circle would be smaller than the circumference of earth. That is the reason they are named as Small Circles.
If we draw circles on the earth with their planes parallel to the that of the Equator, their diameters would obviously be smaller than that of the Equator. These Small Circles are called Latitudes. In addition, these Small Circles which are parallel to the Equator would have their planes would also be perpendicular to the Axis of Rotation of earth.
NORTH AND SOUTH LATITUDE
Equator is taken as the datum reference for Measurement of Latitudes. Latitudes can be either North or South depending upon its position relative to that of the Equator. When we say Northern Latitude, we mean that the place is in the Northern Hemisphere or North of the Equator.
When we say Southern Latitude, we mean to say that the place is in the Southern Hemisphere or South of the Equator. Obviously, Latitudes can only be North or South and cannot be East or West.
The Equator and the Parallels of Latitudes run in an East to West direction. That is to say, if you fly along any Parallel of Latitude, you would be flying in the Easterly direction on one side and in the Westerly direction on the other side.
MEASUREMENT OF LATITUDE
Draw a line from any point on the earth’s surface to the centre of earth. Now measure the angle this line subtends with the plane of the Equator. This measured angle is the value of Latitude at that point. In short, Latitudes are measured as an angle from the plane of the Equator.
Since Latitude is an angle, it is measured in degrees. For more accuracy, degrees can be subdivided into 60 minutes. The convention of expressing Latitude is to use degrees and minutes. For example, 45 Degrees would be referred to as 45 Degrees, 0 Minutes and would be written as N4500.
Can you have a Latitude of 100 degrees? No, the maximum value of Latitude can be only 90 degrees which would be at the poles. Therefore, Latitudes are normally expressed in four figures prefixed by either N or S to signify Northern or Southern Latitude.
In degrees and minutes, for example, 35 1/2 degrees North, will be written as N3530. This means that the place is at 35 degrees and 30 minutes North Latitude.
When we indicate just the value of latitude, our aircraft could be anywhere on the Small Circle which defines that Latitude. In fact, all the places on this circle would have the same Latitude. Therefore, Latitudes by itself cannot be used to define a position uniquely. We need a combination of Latitudes and Longitudes to define the position.
Having understood about the Parallels of Latitude lets move on to the two different types of latitude measurement. In the next lesson, we shall understand about Geocentric and Geodetic Latitude.