Need help with navigation presentation.

[Stan]

Hey all, I have been asked by the Canadian Coast Guard College and Parks Canada to give a talk on how ancient sailors used the stars to navigate with a sextant. I have no background in sailing and I am looking for any suggestions you may provide. The date for this is the weekend of the Perseid Meteor Shower, August 11 and they want an hour presentation along with viewing after the presentation. They want this presented at the first observatory in North America, inside the Fortress of Louisbourg National Historic Site. Any thoughts would be greatly appreciated.

[iwatkins]

Look for someone who is an expert in the subject to present something?

[dav1d]

Pretty sure ancient sailors didn't use sextants. They may have navigated by the stars - using the pole star and knowing what comes up at sunset as a rough compass, but navigational sextants didn't make an appearance until the 18th century AFAIK. Most navigation was done by dead reckoning

Sorry.

[knobby]

If no other ideas crop up, maybe copy the post and try a sailing forum?

Ideally you need a sailor that's into astronomy or an astronomer that's into sailing

[mike.brightmore]

Hi Stan

The story of maritime navigation is a fascinating subject but does take a bit of getting your head round. I have given talks on the subject and also on the life and work of John Harrison (1663 - 1776) whose maritime chronometer finally solved the problem of determining longitude at sea. I do have quite a bit of information (too much to relate here) but would be happy to help with any specific queries.

Determining latitude was never a problem and can be calculated by measuring the altitude of the Sun at midday (using a sextant or before that an octant or quadrant). In the Northern Hemisphere the altitude of the Pole Star (Polaris) is a direct measure of the observer's latitude. In the Soutern Hemisphere the position of the South Celestial Pole in the Southern Cross can similarly be used.

The difficulty was the determination of longitude. There is no direct way to measure longitude using celestial observation and it was realised long ago that the only way was to know the time difference between Greenwich, England (0 degrees longitude) and where the ship actually was. As the Earth turns 360 degrees every 24 hours so each hour represents 15 degrees of longitude. So Eastern Canada has a longitude of around 75 degrees Weso so its time difference with Greenwich is 5 hours. (See world map below). Consequently mariners had to establish their time difference with Greenwich then convert this into degrees longitude (1 degree for every 4 minutes). So all that was needed was a very accurate clock set and maintained at GMT and establishing local time using the Sun, which is at its highest point at midday.

Prior to this ancient mariners (when not shooting albatross) would navigate using a 'log line'. This was a long piece of rope tied to a log which was thrown over the stern of the ship. The rope had knots tied at regular intervals and as the ship pulled away from the log the number of knots reeling out in a given time were counted and through this the speed of the ship calculated. The direction the ship was sailing was given by a compass and the resulting distance and direction of travel were noted in a 'log book' and plotted on a chart. Hence the speed opf a ship is still given in 'knots'.

Clocks were very unreliable up to 1759 when John Harrison (after 40 years trial and error) perfected his H4 marine chronometer. Until then the only accurate clocks were operated by pendulums and these were useless at sea due to the movement of the ship. The astronomers of Harrison's day (including a succession of Astronomers Royal) were convinced that a man made clock would never work at sea and that the answer to the longitude problem could only be found in God's Celestial clock. Poor navigation was causing a calamitous loss of ships, seafarers and cargos at a time when most European nations were seeking world trade dominance.

Great observatories were built in European capitals (including Greenwich) for the sole purpose of accurately measuring the relative positions of the Moon and stars so that these could be compiled into a nautical almanac from which Greenwich time and so longitude could be calculated. These measurements were known as 'Lunars' as in the illustration below. Another way was to accurately measure the positions of Jupiter's Moons hour by hour at Greenwich, compile this data into an almanac which could be used at sea to compare the observed position of the Jovian Moons with those in the almanac hence determine the time at Greenwich. This method proved difficult on board a moving ship as a telescope was required.

In 1714 The British Government enacted the 'Longitude Act' which offered large prizes to anyone who solved the longitude problem. John Harrison was eventually granted the first prize (worth about £2 million in today's money).

Hope this gives you a few ideas. Let me know if you need any more explanation as I have had to rush this a bit. There are some photos I took of John Harrison's chronometers in my profile pictures.

[steppenwolf]

Mike, that was a fascinating précis - I'd love to attend one of your talks on the subject but you are miles away!

[LukeSuppalot]

Mike, that was a fascinating précis - I'd love to attend one of your talks on the subject but you are miles away!

Have to agree with Steppenwolf. Great stuff.

Thats what I love about SGL, every days a school day.

Luke.

[Stan]

Hi Stan

The story of maritime navigation is a fascinating subject but does take a bit of getting your head round. I have given talks on the subject and also on the life and work of John Harrison (1663 - 1776) whose maritime chronometer finally solved the problem of determining longitude at sea. I do have quite a bit of information (too much to relate here) but would be happy to help with any specific queries.

Hey Mike, great info, i love learning new things. I need more on how the stars and constellations were used as navigational aids, before anything else was invented.