# Meridians of Longitude

In this post, let us understand about Longitudes or Meridians of Longitudes. A combination of latitudes and Longitude is required to fix a position without ambiguity. In our earlier lesson, we had learnt about Parallels of Latitude and the differences between Geocentric and Geodetic Latitudes.

### PRIME MERIDIAN

In our earlier lessons, we had learnt that lines connecting one pole to another are called Meridians. To be more specific, Meridians are Semi-Great Circles joining the poles. A combination of Meridian and its Anti-Meridian would constitute a Great Circle.

The Meridian which connects the poles and pass through a place called Greenwich near London is called as the Prime Meridian. Therefore, the Prime Meridian is also referred to as the Greenwich Meridian.

### LONGITUDES

Longitude of any point is the shorter angular distance of the arc along the Equator between the Prime Meridian and the Meridian passing through that point. The angle is measured in terms of whether it is East or West of the Prime Meridian.

For example, if the angle at the centre of the earth between these two Meridians is 50 degrees, then the Longitude would be called as W 050 00. This means that the Longitude is 50 Degrees 0 Minutes, West of the Prime Meridian.

As you can observe, the maximum value of Longitudes can be up to 180 degrees East or West. Longitude can be a maximum of either West 180 or East 180 degrees. Therefore, Longitudes are expressed in five figures, the first three are degrees and the last two are minutes.

The Meridian which is exactly on the opposite side of the earth to the Prime Meridian is called Anti-Meridian of Greenwich. In other words, any Meridian which is displaced 180 degrees from a Meridian is called its Anti-Meridian.

### ORIENTATION OF LONGITUDE

When you look at the portion of earth with Prime Meridian in view, Easterly Longitudes are to your right and Westerly Longitudes are to your left. Conversely, when you look at the portion of earth with Anti Meridian of Greenwich in view, Easterly Longitudes are to your left and Westerly Longitudes are to your right.

Some confusion can arise when calculating Longitudes near to the Anti-Meridian of Greenwich. When looking at the earth from the Greenwich Meridian, the sense of East and West is readily apparent. In the sense, East is to your Right and West is to your Left.

However viewing the earth from the Anti-Meridian of Greenwich, the West and East Longitudes appears to reverse. In the sense, East appears to your Left and West appears to your Right. In reality, the orientation of East and West Longitudes remains the same.

In summary, Longitude of any point is the shorter angular distance of the arc along the Equator between the Prime Meridian and the Meridian through that point. It is measured in relation as an angular distance West or East of the Prime Meridian.

Until now, we have understood about both Latitudes as well as Longitudes independent of each other. In the next lesson we shall understand about the Position Reference System using Latitude and Longitude.