Public Lectures for the Session 2001-2002 held in Room E7 of the Renold Building, UMIST.
Delivered to the Manchester Astronomical Society

21st March 2002

''Astronavigation''

Geoff Butt
Marine Associates Ltd, Birkenhead

As a senior instructor of navigation, Geoff had been delegated to represent Marine Associates by Capt. John Percival. He began his talk by saying that the Royal Yachting Association courses, including astronavigation, were taken very seriously by mariners and were obligatory for navigators of smaller boats, in spite of modern electronic navigation aids, on all classes of vessel up to 300 tonnes.

For successful astronavigation it is necessary to measure the altitude of an astronomical object, with respect to the horizon, to an accuracy of 0.1 minutes of arc and the measurement has to be corrected for atmospheric refraction, horizon dip and parallax. The time of the observation has to be known accurately and until the mid 1700’s to evaluate these parameters would have needed an expert mathematician. For the average mariner, it was a simple necessity to enquire; Am I safe? And how do I safely proceed to my destination?

Mr Butts fleshed out his exposition of astronavigation by a detailed account of the history of celestial navigation from the days of Hipparchus, ca 160BC, via Ptolemy, to the more refined observations and study of celestial mechanics of Galileo and Kepler in the 17th century. Following the founding of the Royal Greenwich Observatory in 1675, to investigate ways of improving navigation for seamen, the Longitude Act of 1715 was explained in detail as was the long- laboured construction and eventual approval of a marine chronometer by Harrison. This was to be eventually adopted as a fundamental means of the determination of longitude at sea.

Various methods of determining the altitude of astronomical bodies were described. These included the Astrolabe, Quadrant, Cross-staff and Back-staff, that preceded the Octant and modern Sextant. Geoff Butt then demonstrated the use of the Sextant together with an umbrella to define the celestial sphere, to demonstrate what the instrument measured and how these measurements could be converted to give the mariner’s position on the featureless ocean to within a couple of miles.

Mr. Butt’s talk rounded off the season of lectures on a very unusual and interesting note, being the first time that astronavigation had been included in the syllabus of lectures to the Society. We are most grateful to him.

Synopsis by Kevin J. Kilburn (Secretary)

Maintained by Michael Oates
Page modified 28 October, 2006