2-The
sextant
The Navigators tool chest Is full of objects and competence that
enable him to determine where the ship is on the maritime globe, whether it is
within the sight of land or in the middle of one of the world's large oceans.
Not only that, he or she could interpret their visible
maritime world whether it be sky, sea or weather to help them make a forecast
and adjust their position, all without external support.
The sextant was one of the most important tools the
navigator had to enable him to fix the ships position anywhere on the ocean.
They're also precious because it was a personal instrument that every budding junior deck officer needed to invest in. In the 1950s or 1960s you either bought a German Plath model or the Kelvin Hughes from the UK.
[1]
The principle of the sextant is relatively simple. It is used
to measure the altitude of celestial objects like stars, planets, and the sun
and moon. Altitude being the angular elevation of the object above the visible
horizon. No visible objects or no visible horizon meant that there was no use for the sextant!
The word sextant is derived from the Latin “sextus”
meaning one sixth[2]. So a sextant can measure an arc of one sixth
of a circle, namely 60 degrees. Because
it has double reflecting mirrors it can measue up to 120 degrees.
[4]The instrument comprises a movable arm over a arc inscribed with angular degrees. The top of the arm has a fixed mirror attached. On the body of the instrument is a second vertically split mirror, the left side of which is clear glass. A telescope is attached to view the contents of the mirror. Each mirror has a set a filters to reduce the sun's glare. Setting the radial arm to zero degrees, you point the instrument at the celestial object, say the sun, and then by releasing
the radial arm you slowly bring the reflected sun down towards the visible horizon in the clear glass of the fixed mirror. Once you can just place the lower edge of the sun on the horizon you have measured its altitude.
Of course the sun rises and sets, but there is one altitude
that is important to the navigator. That is when it has its maximum altitude,
some time in the middle of the day. This is called the meridian passage because
at that precise moment its bearing is south and the sun has reached its maximum
altitude for that day. This altitude can help the navigator calculate latitude.
A simple
diagram shows the process:
·
Measure
the sun’s zenith altitude
·
The
zenith distance is then 90 degrees minus the sun’s altitude
We then
need to take account of the fact that the sun is inclined from the plane of its
orbit around the sun by something called its declination and it varies with the
time. We know this because the sun is
highest in our northern summer and lowest in winter.
[5]So we need the declination for exactly the time we took the altitude reading and this comes from another item in the navigators toolbox, the Nautical almanac where we can find the declination we want.
For
northern latitudes, in the summer period the declination will be northerly so a
calculation is: With an altitude of 47 degrees, the zenith distance is 90
degrees minus sun altitude = 43 degrees.
From the
almanac the declination is 10 degrees north so latitude is 43 + 10 = 53 degrees
north.
I should
explain that this is a simplified example and for other latitudes and
declination the calculation will vary. Also, there are a number of corrections
to be made to the sextant and the observed sun’s altitude that the navigator
must make.
References
‘Brown’s Nautical Almanac - Google Search’. Accessed 21 January 2025.
https://www.google.com/search?sca_esv=68f5f289e15608b2&sxsrf=ADLYWIKGrRTYhttgFFByyiliSP6-e4e2DQ:1737457710086&q=brown%27s+nautical+almanac&udm=2&fbs=AEQNm0Aa4sjWe7Rqy32pFwRj0UkWd8nbOJfsBGGB5IQQO6L3J_TJ4YMS4eRay1mUcjRHkZxQmI-azE4-kvfPRbUUVQX-VdukAcnN3WyeChzMt5JDRBMtwKF_X1ZFdhHy6uNqBY-4IVOFfEGiyJu7GCSojJTrLQS8W8-G9DpcjCnldC9L40SzLN0&sa=X&ved=2ahUKEwikv_ys1oaLAxXnQlUIHaikMTUQtKgLegQIFRAB&biw=1136&bih=480&dpr=1.1#vhid=PK2xbiJKjwohgM&vssid=mosaic.
‘Definition of Sextant - Google Search’.
Accessed 20 January 2025.
https://www.google.com/search?q=definition+of+sextant&oq=definition+of+sextant&gs_lcrp=EgZjaHJvbWUyDggAEEUYORhGGPkBGIAEMggIARAAGBYYHjIICAIQABgWGB4yCAgDEAAYFhgeMggIBBAAGBYYHjIICAUQABgWGB4yDQgGEAAYhgMYgAQYigUyCggHEAAYogQYiQUyCggIEAAYgAQYogTSAQkxMDgxOGowajeoAgCwAgA&sourceid=chrome&ie=UTF-8.
Malm, Bengt. Bengt Demo. of Sextant.
n.d. Photo.
‘NOVA Online | Shackleton’s Voyage of
Endurance | How a Sextant Works | PBS’. Accessed 14 January 2025.
https://www.pbs.org/wgbh/nova/shackleton/navigate/escapeworks.html.
‘Sekstant’. Accessed 20 January 2025.
https://digitaltmuseum.no/011024215631/sekstant.
Acknowledgements
·
Bengt
Malm, Ancient Mariner and volunteer colleague Norwegian Maritime Museum, Oslo
·
Camilla
Nordeng, conservator Norwegin Maritime Museum, Oslo
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