They say a picture says a thousand words. This one needs some.
Prior to GPS and the like, sailors entering the river Tyne, in the north east of England were at great risk due to the rock formations known as the black middens lurking below the calm surface... to combat this, two lighthouses were constructed, one on the fish quay, and one on the bank top, known as the high and low lights.
When the light from these two beacons was aligned, the skipper was sailing the safe course.

Nowadays they are redundant, but remain as silent guardians of a busier time...
Prior to GPS and the like, sailors entering the river Tyne, in the north east of England were at great risk due to the rock formations known as the black middens lurking below the calm surface... to combat this, two lighthouses were constructed, one on the fish quay, and one on the bank top, known as the high and low lights.
When the light from these two beacons was aligned, the skipper was sailing the safe course.

Nowadays they are redundant, but remain as silent guardians of a busier time...