Category Archives: Navigation

Sextant and Octant

Many of us have heard of a sextant – but maybe not an octant.

So called because it traverses an arc covering 60 degrees or one sixth of a circle.

It would be used on a ship to measure the elevation of celestial objects – sun, moon, planets or stars – and then by reference to tables, clock or calendar determine how far north or south the vessel was – to an accuracy of around a thousand feet.

The sextant was preceded by the octant which worked through an arc of 45 degrees or an eighth of a circle. As a navel cadet, engineer Marc Brunel made his own octant.

The sextant found favour in the late 1700s when new navigational techniques required a device which would compare the elevations of the sun and moon. The octants 45 degree ‘sweep’ was often insufficient as larger angle were needed.

Port of London Authority

To the west of Trinity Gardens by the Tower of London you will see a tall white building called the Four Seasons hotel. It was built in the Beaux-Arts style by Edward Cooper and is 100 years old this year.

Near the top, a statue of a bearded figure holding a trident. One might think of the god Neptune but this is our very own watery deity, Old Father Thames, and this gives a clue to the first occupants of the building: the POLA

In the early 1900s things were not well on the river. The Thames was unregulated, many docks and wharfs were in need of improvement, and businesses serving the river were in chaotic competition. In short, something of a shambles and it was felt that ships would start favouring rival ports, especially in Europe.

In 1908 the Port Of London Act was launched in parliament and successfully navigated through the House of Commons by a Liberal party member called Winston Churchill.

The POLA was formed in 1909. So what did it do?

  • Some overdue dredging of shallow parts of the rivers.
  • Effectively nationalised the various dock companies.
  • Provided investment and modernisation.
  • Co-ordinated the port’s operations.

Ax a result London stayed as a successful port.

Today the Authority still looks after the river.

  • It looks after the Thames Barrier which guards against flooding by large tides.
  • Patrols the tidal river 
  • Surveys the bed of the river to find obstructions and where necessary, organises dredging.
  • Monitors river users for compliance with the various regulations.]

One service it provides is Pilotage. What is this?

Let us say that a container ship enters the Thames. The captain may not be familiar with the river’s features – its currents, hidden hazards such as sandbanks and perhaps not the exact location of where they are meant to dock.

To deal with this, they are met by a small boat, or even a helicopter, carrying an official called the Pilot who will board the ship and join the captain on the bridge. The Pilot is a skilled navigator, knows the river, and ensures that the vessel makes safe passage.