How can the longitude be a negative number?

Answer 1

Longitude is either 'East' or 'West' of the [Greenwich] meridian. Adding letters to a number makes calculations with it difficult, along with being awkward to store in computers, so using the normal number line with negative numbers to the left:

• longitudes West are Left of the Greenwich meridian and so are represented by negative numbers;
• longitudes East are Right of the Greenwich meridian and so are represented by positive number

makes the calculations and storage easy.

For example, Land's End at 5o44'W becomes -5o44'

and Great Yarmouth at 1o44'E becomes 1o44'

Answer 2

Longitude can be expressed as negative when measured west of the prime meridian, which is at 0 degrees longitude. This convention helps to establish a consistent global coordinate system where east longitudes are positive and west longitudes are negative.