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.