Go Back   IceInSpace > General Astronomy > General Chat

Reply
 
Thread Tools Rate Thread
  #1  
Old 17-02-2019, 05:59 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 919
Anyone ever actually use a sextant?

I find it incredible that accurate measurements could be taken from onboard a ship that is pitching and rolling.

Although, of course, their bigger problem was longitude as described in that famous book by Dava Sobel.

But has anyone ever tried to actually get measurements from one (on land or on a boat) and can attest to the ease or difficulty of this?

Markus
Reply With Quote
  #2  
Old 17-02-2019, 06:14 PM
LewisM's Avatar
LewisM
Novichok test rabbit

LewisM is offline
 
Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 8,901
Try doing it in the air... apparently that was fun! Getting a star shot during turbulent flight must have been as much fun as on the water.
Reply With Quote
  #3  
Old 17-02-2019, 06:23 PM
Wavytone (Nick)
BigBanger

Wavytone is offline
 
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 3,700
Quote:
Originally Posted by Stonius View Post
has anyone ever tried to actually get measurements from one
Yes, I had to pass navigation exams way back in the 1980's in my sailing days - I used to be a navigator for a couple of crack crews at MHYC.
Within sight of land we usually took bearings of landmarks and triangulated a position from that - which was the point of lighthouses.

But out of sight of land yes you had to be able to calculate long and lat from a sextant and marine chronometer and a slide rule. and you're right, it's not easy on a small yacht. The other gadget was a ships log trailing out the back, a kind of little propeller which integrated the revolutions over time to give an estimate of how far you'd sailed in a line; this had to be adjusted with various corrections too (currents).

Even when GPS arrived, the navigator still had to be able to do it without GPS - in case the GPS unit failed. Electronics don't tolerate salty humid environments well.
Reply With Quote
  #4  
Old 17-02-2019, 09:30 PM
glend (Glen)
Registered User

glend is offline
 
Join Date: Jun 2013
Posts: 5,179
I have heard that the US Navy has recently started requiring officers in training to be able to use a sextant again, after this had been phased out after GPS became available. Seems they are spooked by the threats to the GPS system satellites, and recent jamming activities by the Russians over Norway. Scandinavian airlines operating in the affected area have issues advisories suggesting the use of dead reckoning skills as a backup.

https://www.npr.org/2016/02/22/46721...s-for-officers
Reply With Quote
  #5  
Old 17-02-2019, 10:38 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 919
Quote:
Originally Posted by glend View Post
I have heard that the US Navy has recently started requiring officers in training to be able to use a sextant again, after this had been phased out after GPS became available. Seems they are spooked by the threats to the GPS system satellites, and recent jamming activities by the Russians over Norway. Scandinavian airlines operating in the affected area have issues advisories suggesting the use of dead reckoning skills as a backup.

https://www.npr.org/2016/02/22/46721...s-for-officers
I remember a story a while back (Maybe 5 years?) about an enemy that stole an unmanned drone by broadcasting spoof GPS so it thought it was somewhere other than where it was. Then all they needed was an airfield at a similar altitude to its home base and they got it to land at their airfield, nice as you please and thank you for the donation of millions of dollars of military hardware.
Reply With Quote
  #6  
Old 17-02-2019, 10:43 PM
alocky's Avatar
alocky (Andrew lockwood)
PI popular people's front

alocky is offline
 
Join Date: Aug 2010
Location: perth australia
Posts: 1,275
I used a sextant out of a tinny as an undergraduate doing hydrographic surveys on lake Moogerah (SW of Brisbane). The state of the art back then (late 80's) was accurate to 100s of metres and with a bit of patience most of us were able to achieve this from a dinghy. I don't know if I'd want to try it on an ocean going yacht in 10m seas though.
Reply With Quote
  #7  
Old 17-02-2019, 11:04 PM
Stonius's Avatar
Stonius (Markus)
Registered User

Stonius is offline
 
Join Date: Mar 2015
Location: Melbourne
Posts: 919
Quote:
Originally Posted by alocky View Post
I used a sextant out of a tinny as an undergraduate doing hydrographic surveys on lake Moogerah (SW of Brisbane). The state of the art back then (late 80's) was accurate to 100s of metres and with a bit of patience most of us were able to achieve this from a dinghy. I don't know if I'd want to try it on an ocean going yacht in 10m seas though.
Wow! That's impressive!
Reply With Quote
  #8  
Old 18-02-2019, 06:59 AM
theodog's Avatar
theodog (Jeff)
Every photon is sacred !

theodog is offline
 
Join Date: Oct 2007
Location: Coonabarabran
Posts: 1,030
Quote:
Originally Posted by alocky View Post
was accurate to 100s of metres and with a bit of patience
I'd have thought the bearings of land points and a good HEMA map would have been more accurate/useful on Lake Moogerah.
Reply With Quote
  #9  
Old 18-02-2019, 07:57 AM
Wavytone (Nick)
BigBanger

Wavytone is offline
 
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 3,700
Jeff the point of it is - when out of sight of land - make sure you won’t bang into reefs or gnarly things like Balls Pyramid in the middle of the night.

Even within sight of land a huge number of vessels were sunk in the the 18th and 19th centuries along to southern coastline between Albany and Bass Strait, and along the NSW coast.
Reply With Quote
  #10  
Old 18-02-2019, 08:32 AM
Outcast's Avatar
Outcast (Carlton)
Still a Noob....

Outcast is offline
 
Join Date: Oct 2008
Location: Cairns, Qld
Posts: 414
Hi there,

I'm ex- Navy, Seaman Officer (Now known as a Maritime Warfare Officer)

Trained as a midshipman to use a sextant & conduct navigation out of sight of land to obtain open ocean navigation qualification & then required to do a full days run every 6 months to retain the qual (morning stars, morni g sun sight, meridian passage, afternoon sun & evening stars). Accuracy required, within 1nm of gps position.

It's not easy but, with correct technique you can get very good at accurate measurements. First of all there is getting yourself well balanced & then once you bring a star down to the horizon you need to swing the sextant (sweeping motion) across the horizon to make sure you have brought it down vertically.

Takes practice but, very doable.

You can also use one for extremely precise coastal navigation using horizontal sextant angles & for the measuremement of heights using vertical sextant angles together with some calculations for height of eye. We commonly used a book called Norries maritime tables (or something like that) for VSA calculations.

I take my hat off to mariners such as Magellan & the like who used quadrants as well as the later, sextant to accurately plot their positions without the use of accurate time pieces (required for calculating longitude).

I could ramble for hours about 'old school's navigation... it's a fascinating topic...

Cheers

Carlton

Last edited by Outcast; 18-02-2019 at 12:01 PM.
Reply With Quote
  #11  
Old 18-02-2019, 10:14 AM
speach's Avatar
speach (Simon)
Registered User

speach is offline
 
Join Date: Oct 2013
Location: Wonthaggi Vic
Posts: 620
They generally had a few deck officers taking a shot at the same time and averaged the results. Of course this was on larger ships.
Reply With Quote
  #12  
Old 18-02-2019, 11:09 AM
alocky's Avatar
alocky (Andrew lockwood)
PI popular people's front

alocky is offline
 
Join Date: Aug 2010
Location: perth australia
Posts: 1,275
Quote:
Originally Posted by theodog View Post
I'd have thought the bearings of land points and a good HEMA map would have been more accurate/useful on Lake Moogerah.
True - but we were learning to use a sextant! Our final bathy map was made using a sounding lead and trig sightings off a few landmarks.
Cheers
Andrew.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 05:44 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Advertisement
Meade Australia
Advertisement
NexDome Observatories
Advertisement
SkyWatcher Australia
Advertisement
Lunatico Astronomical
Advertisement
Bintel
Advertisement
Nitecore
Advertisement
OzScopes Authorised Dealer
Advertisement
Astronomy and Electronics Centre
Advertisement