The longitude of a point on the Earth is the angle between the meridian on which it lies and the meridian passing through Greenwich, London (known as the ‘prime meridian’). Longitudes are usually specified as something like \(30^\circ\,\text{E}\) or \(30^\circ\,\text{W}\) for locations east and west of Greenwich respectively. Alternatively, they can be specified as \(30^\circ\) and \(-30^\circ\) respectively.