When measuring latitude we have a wonderful starting or datum point at the equator. However, we do not have a naturally occurring datum to start our lines of longitude and needed an arbitrary ‘prime’ position. The Greenwich meridian was chosen and internationally agreed as the primary reference or ‘prime meridian’ for longitude.
The Greenwich ‘prime meridian’ starts at north pole and runs through the site of the original Greenwich observatory in London before continuing on to the south pole. If we were to follow this line, in a complete circumference around the earth, it would form a great circle. Positions of longitude are named West or East of the prime meridian at Greenwich, which is 000º longitude. The great circle formed by the Greenwich Meridian divides the earth into the western and eastern hemispheres (Fig 2.49). If you are standing on the Greenwich meridian and looking northwards, the western hemisphere would be towards your left hand the eastern hemisphere towards your right hand. The two meridians, Greenwich and the 180th, which make up the same great circle are not termed east and west but the 000° or prime meridian and 180th meridian respectively. Every meridian of longitude is a ‘semi’ great circle whereas the only great circle of latitude is the equator.
So the longitude of a place is the angular measurement between the Greenwich meridian and the meridian of the position you want to find. The angle is measured at the centre of the earth in the plane of the equator and is expressed in degrees, minutes and seconds from 000º to 180º east or west of the Greenwich meridian.
A degree of longitude on the earths surface is a relatively large distance and we need to subdivide it to gain more accuracy. 1 degree is subdivided into 60’ minutes of arc. When we work with latitude and longitude we use sexagesimal notation instead of decimal notation. This may be a little confusing at first but becomes easier to work with. For even greater accuracy each minute or arc is divided further into seconds but can be a little cumbersome to use and depending on the scale we are using it is common to use tenths of a minute. Confused?, Don’t be, as should become clear when we work-up some examples. The basic points you should remember are:
We measure our position west and east of the Greenwich meridian as longitude which is an angle which can be anything from 000° to 180° West or East
1° (one degree) = 60’ (sixty minutes)
1’ (one minute) = 60’’ (sixty seconds)
We use the word ‘arc’ to express an angular measurement, and when we use minutes and seconds it has nothing to do with time, instead its the nomenclature we use to break degrees into smaller units.
If you now look at the longitude scale on RYA Training Chart 3 (Fig 2.50) you will see that the longitude scale runs from 005°35’.00W at the right side of the chart (Arrow ‘A’) to 006°25’.00E on the left side of the chart (Arrow ‘B’’). From East to West this chart covers an area of 50’ (50 minutes or arc) which is just under 1°. The scale bar is marked at 5’ increments and we have the 006°00’ meridian of longitude near the middle of the chart (Arrow ‘C’). We are informed on the longitude scale that we are ‘west’ of the Greenwich meridian and should always read the scale of this chart from the right to the left (Arrow ‘D’)
Remember that the RYA Training Charts are fictitious charts! If we were in the USA we would also read longitude scale from right to left (east to west) but if we were in Russia and east of the Prime Meridian we would read the scale from left to right (west to east).
Plotting a position on the longitude scale
We will now plot the longitude of Tidal Diamond ‘N’. This is where tidal information was gathered in this area. The diamond is located on RYA Chart 3 between Slade Island and the Southern Peninsula (Fig 2.51). We will use the dividers to transfer the position of Tidal Diamond ‘N’ to the latitude scale to determine its position.
We position the dividers with one point in the middle of the tidal diamond and the other on the 005°40‘W meridian of longitude, which is the first solid black line to the right of the tidal diamond (Fig 2.52). We could use the 005°50’W meridian of longitude, which is closer, but best-practice when we are in the western hemisphere to measure in a westerly direction from Greenwich, the prime meridian.
Without further adjustment to the dividers we can then transfer this distance to the longitude scale, keeping one point on the 005°40’W meridian, and mark the the position on the scale with the pencil (Fig 2.53).
Reading the longitude
Now we have transferred a pencil mark from the position of the dividers to the longitude scale , we can read it off.
When we read a longitude scale in the western hemisphere is is always good practice to start from the right side of the chart and work to the left. As you may notice, the number of whole degrees is not always written on the scale and we may have to scan the whole longitude scale to get an idea of how many whole Degrees west of the equator we are. In this case we are working around 5° and 6° west of Greenwich.
The longitude scale on this chart is marked in 5’ increments and the pencil mark is between the 005°45’.00W and 005°50’.00W meridians (FIg 2.54). The black and white bars on the scale help us identify individual minutes of longitude. Each minute of longitude is further divided into 5 smaller subdivisions representing 0.2‘ minutes (2/10th s of a minute.)
We can now read off the position of our pencil mark and see that it is 2’ west of the 005°45‘W meridian scale mark and one further subdivision representing 0.2’ minutes
The longitude of Tidal Diamond ‘N’ is therefore in a position 005°47’.20W of the Greenwich Meridian. (pronounced on a VHF radio as, “zero zero five degrees four seven point two zero minutes west).
So the latitude and longitude of Tidal Diamond ‘N’ is 45°48’.2N 005°47’.2W
If you remember the tidal stream section discussed earlier, and look at the Tidal Diamond table at the top of RYA Training Chart 3 you can see that this position is the same that is given for this tidal diamond. It is worthwhile exercise practicing your latitude and longitude position fixing with the other tidal diamonds on the chart, and can be checked against the information on the tidal diamond table.
Secondary longitude and latitude scales
Sometimes if you are working in the middle of the chart area it may be difficult to reach and work on the latitude and longitude scales at the side of the chart. Yacht chart tables are notoriously small and can be tricky trying to plot and take positions when the chart is folded up. It is quite common for charts to have secondary graduated scales running along parallels of latitude (Fig 2.55 Arrow B) of meridian of longitude (Fig 2.55 Arrow A). They often lack any written numerical information on them and care must be taken when using them. However, they can be handy and useful as secondary scales but we should always try to use and reference the longitude scales at the top and bottom and latitude scales at the sides of the chart