Celestial Navigation

This section of the web site is mainly devoted to my research of instruments for celestial navigational in the period 1590 - 1731, which resulted in my PhD thesis in May 2018. By the end of the 16^th century the mariner's astrolabe and the early cross-staff were the main instruments onboard of a vessel for celestial navigation.

This all changed when Master Thomas Hood started to apply a shadow vane to a measuring staff, on which he first published in 1590. The result would become known as Master Hood's Cross-staff. Although this first attempt for a wooden shadow casting instrument was a mediocre one, it was further analysed by Thomas Hariot around 1594 in his ms 'The Doctrine of Nauticall Triangles Compendious'. As a result of that work, Captain John Davis created the first practical backstaffs (or back staves), instruments that used Hood's idea to use a shadow, while allowing the observer to take observations with ones back towards the sun, a method introduced by Hariot. Davis published his instruments in his The Seaman's Secrets in 1595.

One of Davis' instruments was soon improved by the Dutch, resulting in the 1618 Demi-cross. Along the same lines the Dutch developed the hoekboog - or double triangle as it was named in English - around 1623. The hoekboog was possibly influenced by another English instrument, the Davis quadrant. Although named after Captain John Davis, it is doubtful it really was his invention as the name was given to the instrument at the end of the 17^th century.

The Davis quadrant was a seemingly very accurate instrument. By the 1660s it could be read down to a single arc minute. In order to keep pace with that, the hoekboog got finer scales, while the cross-staff was modified to become a back sighting instrument as well. It was not until the early 18^th century that it was realised that the Davis quadrant could not live up its expectations.

It was around this same time that the first reflecting instrument saw the light. Patented by the Dutch Joost (or Joos) van Breen in 1660, the spiegelboog (mirror-staff) was a cross-staff equipped with a mirror attached to the horizon vane and with additional vanes mounted on the cross. It was capable of taking altitudes of the sun, even under hazy conditions, and of the stars in a backward fashion.

In England too experiments were done with mirrors on navigational instruments: Robert Hook created his single reflecting instrument in 1665, while Isaac Newton created the first double reflecting instrument in 1699. Finally the double reflecting octant saw the light in 1731, soon to be followed by the sextant, quintant, semi-circle and full circle.