Wednesday, 5 July 2017

Celestial Navigation

This weekend, I dusted off my sextant and practised the art of celestial navigation on the shores of Port Phillip Bay.

Shooting the Sun, Port Phillip Bay
Celestial navigation, the practice of fixing position on the earth's surface (at sea and on land), has been rendered technologically obsolete by the Global Positioning System and GPS receivers being miniaturised and incorporated into smartphones, vehicle trackers and integrated navigation systems.

GPS has proven to be remarkable robust and no commercial vessel can effectively navigate without the reliability and precision provided by the constellation of US-developed and maintained satellites.

There are Celestial Navigation blogs and web resources for readers interested in the mathematics of spherical trigonometry. Latitude was the first dimension to be accurately calculated, and for centuries early mariners would sail east or west along the known latitude of a port, island or headland. It was with the development of the chronometer by John Harrison in 1773 that finally enabled time to accurately kept at sea and the celestial position of the Sun, Moon, planets and stars to be calculated by the mariner. Nautical tables by Norie and Bowditch reduced the complex calculations into additive logarithmic tables capable of use by mariners (and not mathematicians) at sea.


With a sextant, a watch and the Nautical Almanac it's possible to fix a Line of Position (LOP) to an accuracy of about two nautical miles anywhere on the Earth's surface. The caveat of the LOP is that multiple LOP's need to be plotted and intersected to fix a position. The Sun shines most days and is by far the most 'useable' celestial body for mariners. On an ocean passage, LOP's calculated from Sun sights taken in the morning are run forward at the vessel's course and speed to the calculated time of Noon (Meridian Passage). The latitude is calculated from the highest altitude that the Sun is observed when it crosses the ship's (and Observer's) meridian.


To 'shoot the sun' from terra firma, I needed a sea horizon and a clear sky. Queenscliff, Victoria sits just inside The Rip, Port Phillip Bay's channel to Bass Strait. The Observation Tower at the marina gave height and a clear horizon past Swan Island to the north east.

Nries Tables, Sight Book and Nautical Almanac
For a morning sight (Sun rising), the sextant measures the angle between the horizon and lower limb (edge) of the sun. Corrections for Height of Eye, refraction and the Sun's diameter are applied to calculate the Observed Altitude - the angle between the centre of the sun and horizontal plane. Using an assumed position (DR, Dead Reckoning) and the Nautical Almanac, the Local Hour Angle and Declination of the Sun provide the data to calculate the True Altitude - the altitude of the Sun if the mariner was at the DR position.

Data for my 'best' sight on Saturday 1st July 2017:

UTC/GMT: 00:24:19
DR Latitude: 38० 15.9' S
DR Longitude: 144० 40.2' E
Height of Eye: 25 metres
Sextant Altitude (corrected for Index Error): 22० 18.8'

If followers of the Blog would like the solution, drop me a line and I'll send you the workings. There are free Android apps on Google Play Store which will calculate the altitude and plot the intersecting longitude, and the Nautical Almanac is available online as a PDF with Correction Tables. With the information needed freely available online, the biggest hurdle is getting yours hands on a sextant. A navigation quality drum micrometer instrument (not one of those faux reproduction antiques) will cost $300 - $500 (Tamiya, Davis, C. Plath, Husun).

The 'impracticality' of celestial navigation is that it takes up to three hours to fix position using the sun, and around two hours (pre-calculation of altitudes and azimuths, sights and plotting) for stars. Pre-GPS, mariners had to allow for wider 'position ambiguity' which is no longer tolerable in the commercial world of shipping. It is however, an art and practice worth preserving.

The Antipodean Mariner

Resources:
Nautical Almanac and Correction Tables
Nautical Astronomy App (Google)

No comments:

Post a Comment