We are aware of the fact that each house has its location and address, for example, the city, the house number, the street’s name etc. In the same way, every single point on the earth’s surface has its respective coordinates defined by the terms ‘latitude and longitude‘. Hence by the coordinates of latitude and longitude, one can virtually locate any point on the surface of the earth. A prime meridian is arbitrary as opposed to the equator.

WHAT ARE LATITUDE AND LONGITUDE?

The symbolic representation of latitude is denoted by ‘phi’. Latitude indicates the angle made between the equatorial plane and the straight line at a specific point. We use degrees to specify the latitude that begins from 0° (at the point of the equator) and ends up at 90° ranging to either side of the equator (North or south zone at the poles). This marks the distribution of Northern and Southern latitudes. The symbolic representation of longitude is denoted by lambda. As latitude, it is another coordinate defined by angles measuring the location of a point on the Earth’s surface.

By definition, the longitude is an angle made directing toward the east or west concerning the Greenwich Meridian. The Greenwich Meridian is considered the Prime Meridian.

Furthermore, the longitude can range from a maximum i.e. 180° east taken from the Prime Meridian and 180° west taken from the Prime Meridian.

The longitude is a coordinate specifying the east-west site of the position of a point on the surface of either the Earth or the surface of any other celestial body. It gives an angular measurement 

The symbolic representation of longitude is expressed usually in degrees. It is denoted by the Greek letter called lambda (λ). The Meridians are the lines running from one pole to another pole. They link all the points having the same measure of the longitude with each other 

Near the Royal Observatory in Greenwich, England, According to the convention, the prime meridian is described as having a  zero degree longitude. The east of the prime meridian is marked as the positive longitude, and the west of the prime meridian is marked as the negative longitude.

PRIME MERIDIAN – FACTS AND DATA

• In a geographical coordinate system, the prime meridian is described as a line of longitude at which the coordinate of longitude is zero degrees (0°). The great circle comprises both the prime meridian and anti-Meridian. 
• The anti meridian is the 180th number of meridians in a 360-degree system. The great circle segments a spheroid into two fragments namely the two hemispheres. If from a defined and a specified prime meridian, a user takes the directions of East and West, it can be referred to as the Eastern Hemisphere and the Western Hemisphere respectively. There are numerous conventions proposed for the prime meridian of the Earth.
• The current set of Earth’s international standard prime meridian is known as the IERS Reference Meridian.  
• From the prime meridian at 180° west and 0°-180° east, we can measure the coordinates of longitudes for the moon and the earth respectively. 
• The other prevailing bodies in the solar system, the coordinates of longitude, are measured from their respective prime meridian (0°) to 360 degrees.
• For retrograde rotations, east longitudes are been used 
• For the direct or the prograde rotations of the bodies (referring to the rotation of the body same as that of the orbit it revolves around), west longitudes are used.

GREENWICH MERIDIAN

• The lines of longitude like that of the known Greenwich Meridian travel from the direction of north to south. They then converge at the site of the poles.

• The range between -180° and +180° is written for the longitude lines.
• The special line (north-south line) is a unique longitudinal line. All of the coordinates of the x-axis begin at 0 degrees. This line is referred to as the Prime Meridian where we measure the east and west direction.
• The name Greenwich Meridian is due to its location running through the City of Greenwich located in the country of England.

CONCLUSION

A meridian can be defined as a line running from north to south. For various astronomical observations reasons, it is chosen as the zero reference line. In the year 1884, the definition of the Prime Meridian was based on the location of the  ‘Transit Circle’ telescope present in the Observatory’s Meridian Observatory. In 1850, the 7th Astronomer Royal, Sir George Biddell Airy, built the transit circle. The accurate mapping of the sky can be done by performing comparative studies from the observations done through the reference of the same meridian