Before Global Positioning Satellite systems, how was precise location determined? I understand that two devices are able to accomplish this: sextant; and theodolite.

What are these things? Rather than request explanation of how each works, I would appreciate links to instructions/explanations.

What are the basic differences between theodolite and sextant? I have been told that sextants are to be used at sea while theodolites are to be used on land. But this is not particularly helpful in the absence of how and why.

I understand that air navigation was accomplished, perhaps, differently, that identifying precise location was difficult-to-impossible (Amelia Earhart's aircraft being an example??). Why?

Try this link for instructions on how to use a sextant. Davis Mark 25 Sextant.

A Google search for sextant instructions or training aids will give lots of info including some animated examples that I've used to teach Boy Scouts.


Can't help with a theodolite.

Thanks for that link! Now they'll all go out and by a GPS :-)

The sextant shot the stars on a north-south arrangement. To find out how far you were from the prime meridian, you used a very accurate clock. When you determined noon, you shot the stars, got your north-south position, looked at your ship's clock, figured out how many hours you were ahead of or behind Greenwich Time and with that info, you knew where you were east-west. At least this is how I saw it explained on Pawn Stars when a man brought in a stopped ship's clock. An expert was called to determine if the clock was broken. It wasn't; it worked perfectly. The expert then proceeded to explain how early seafarers kept track of their location using such a clock...

quote:

Originally posted by Naphtali:
Before Global Positioning Satellite systems, how was precise location determined? I understand that two devices are able to accomplish this: sextant; and theodolite.

What are these things? Rather than request explanation of how each works, I would appreciate links to instructions/explanations.

What are the basic differences between theodolite and sextant? I have been told that sextants are to be used at sea while theodolites are to be used on land. But this is not particularly helpful in the absence of how and why.

I understand that air navigation was accomplished, perhaps, differently, that identifying precise location was difficult-to-impossible (Amelia Earhart's aircraft being an example??). Why?

In short, a sextant measures angles- usually between a celestial body and the horizon but not always, you can turn it sideways and measure the angle between 2 points on the horizon and triangulate your position that way.

A theodolite measures angles but between the sighted object and level, as the instrument is leveled with a bubble.It also measures in two dimensions, heading (most are set up using a compass too) and elevation.

I know you want links, but you can google just as well as I so forgive me here but I'll try to explain some things in my own words here.

The simplest thing to do with a sextant in the Northern Hemisphere is to go out at night and shoot the north star (polaris) to find out your latitude. Look at it this way, if you stand at the North Pole the north star will be directly overhead, so if you measured the angle between the star and the horizon through where you are standing it would measure 90 degrees, correct? Now look at where you are at on a globe, 90 degrees North! If you are looking at polaris while standing at the equator, it will be right on the horizon so basically at zero degrees North. Anywhere between the 2 extremes, if you measure the angle between polaris and the horizon that number will be your latitude in degrees north of the equator.

I am speaking in general terms, as there is a small correction factor you need to work in to get super-accurate but what is written is good enough to get you unlost when you are on the sea in a lifeboat with a Ginger and Mary Ann.

Second installment, I'm typing this on a netbook at the inlaws and the little backstabber posted it on its own. So instead of editing the previous one I'll just keep going here....

Anyway, where was I? Oh yeah, see the earth is tilted a little bit off the axis to polaris, but the distance from it to us is so huge that the star doesn't really wobble around that visibly but it does so if say, you are using polaris to determine the north fence line between you and your neighbors property then you'd have to look it up in a table and figure out the correction factor to use.

Figuring out longitude is done basically using the same principles, but you have to shift your thinking a bit. Here is an example of the way of looking at it-

Suppose you are in a ship at sea and you KNOW your lattitude, you could just set your transit to this predicted angle and go up on deck and point the transit north and if everything is right you'd have polaris centered in the viewfinder, correct?

Well, instead of looking north we take a guess at our longitude (east or west) and look up in the "Nautical Almanac" which is a book that has the positions of stars all over the planet worked out to the hour and you figure out a star that will be directly overhead of your longitude at a certain time. So you go out and use your sextant to measure the angle at the time that it is supposed to be at 90 degrees. If it is 90 degrees then you of course know exactly where you are on the E/W line, and if it is not directly overhead you can measure the angle with your trusty sextant and then using simple geometry figure out how far east or west of the line you are and combining your latitude that you got from shooting polaris you know know your exact location, or so the theory goes!

Air navigation is done the same way, except an aeronautical sextant will use a bubble level instead of the horizon. The main issues with air navigation is that you are moving at 200 mph so if it takes you 10 minutes to work out the math of figuring out where you are based on the info you got from the sightings, well you're already 35 miles away from that spot!

If you want to read more, rather than websites I'd like to suggest 2 books.

The best book describing all this to a layperson who may or may not be interested in navigation but likes a good read is called "Longitude" and is written by Dava Sobel. It deals with the person who created the first ship-going clock and how he got screwed over by the british establishment, and in the process of the book it explains the principles behind navigation really really well. Before you ready ANY OTHER book on celestial navigation I'd suggest reading this one first as it will do a good job of explaining basics to anyone and not just someone with an engineering degree.

The other book I highly recommend is one called "Bushcraft" by Richard Graves. This is one of the best books on bushcraft/camping/survival skills I have ever read and true to form the chapters on navigation are really really good.

It is also online somewhere, I'll see if I can find it but if you ever come across a copy on ebay or in a used book store I'd tell you to buy it without hesitation.

Here's another interesting piece of information-

Seafarers were not the only people to use celestial navigation, if you think about it the desert is also flat.

So the Arabs also used these same principles for the last couple thousand of years plying the trade routes over the desert, and one of the most obvious artifacts of this is that of the 57 most commonly used Navigational Stars, 52 of them have Arabic names.

Here's a link to the Bushcraft books, goldmines aplenty here!

http://tions.net/CA256EA900408...vwWWW/outdoor~03~000


Here is the book on navigation:

http://tions.net/CA256EA900408...vwWWW/outdoor~03~101

at sea where there are no obstructions and you already know your latitude (north-south) you can get a close idea of your longitude by sunrise-sinset sightings, but a "noon" sighting to calculate your latitude first is important to being accurate

Distortion near the horizon is affected by other factors such as temperature... so...

with a clock and knowing when sun reaches it's zenith at actual local "noon" makes the longitude calculation rather precise...

Navigations accuracy boils down to eventually drawing a triangle on your map and knowing you are somewhere inside it, and the challenge is to create as small a triangle as possible.

the smallest theoretical accurace is 200yards
but even with a skilled navigator better than 1/3 of a nautical mile is suspect...

Aerial celestial navigation was based far more on wishful thinking than reality, because yo could move a rather large distance in the time it takes to take your sightings and do your calculations even with instruments that would automatically take multiple signtings ina short period of time to negate the effect of movement..

Ths the saying with aerial navigators that "You always know where you've been, and generally know where you are going, you never know where you are because by the time you figuire out you are you are somewhere else."

Aerial Navigation "way back when" (excepting dead reckoning) was often based not on the stars, but on a directional radio antenna and trying to determine a bearing to a known radio source, which could either be a radio beacon set up and operated for the purpose, OR any convenient broadcast station.

Like the B17's heading to Hawaii on Dec 7th where "riding" the Honolulu broadcast station with their directional radio antennae

The presumption being that if the radio station was directly ahead you were going the correct direction to find the Hawaiian Islands instead of empty ocean....


AD

Now that is an interesting thread perhaps raising some interest for a nearly lost art. When I started sailing some 45 years ago there were electronical aids like Decca, LoranC, radio beacons, most of them now dismantled since GPS rules, and of course astronomical navigation. We used the HO249 tables still available and had good results with them. What did not work to satisfaction in an aircraft proved to be ideal for sailing boats. It is possible to train astronomical navigation in your garden,but you will have to use an artificial horizon, since you need a perfect flat surface to report your angle and that is only available at sea. The easiest way to bypass the problem is to report your measured angle to the reflection of sun or moon in a mirror. Dirty water or motor oil will do. You than only have to divide the measured angle by 2 and off you go.
GPS is fine but you entirely depend on the governments supply of satellite data that may be withheld or distorted as needed in case of war or crisis. That is the reason, western Europe an Russia are implementing alternative systems. Last but not least, the first thing to pack up in a cold moist environment like a small sailing boat are electricity and electronical devices. Never rely on one GPS receiver, always carry a handheld as backup and give the old methods a try. Celestial bodies cannot be switched out as desired, their use does not cost a cent and it is a rewarding hobby.
For beginners a plastic sextant and a radio controlled timer clock will do.

Loran C is still active.

It's schedueled demise has been postponed several times.

AD

I just wandered aimlessly. Even a blind squirrel finds a nut!

quote:

Originally posted by homebrewer:
The sextant shot the stars on a north-south arrangement. To find out how far you were from the prime meridian, you used a very accurate clock. When you determined noon, you shot the stars, got your north-south position, looked at your ship's clock, figured out how many hours you were ahead of or behind Greenwich Time and with that info, you knew where you were east-west. At least this is how I saw it explained on Pawn Stars when a man brought in a stopped ship's clock. An expert was called to determine if the clock was broken. It wasn't; it worked perfectly. The expert then proceeded to explain how early seafarers kept track of their location using such a clock...

You measure the altitude of the Sun for a noon position

quote:

Originally posted by brainshot:
Now that is an interesting thread perhaps raising some interest for a nearly lost art. When I started sailing some 45 years ago there were electronical aids like Decca, LoranC, radio beacons, most of them now dismantled since GPS rules, and of course astronomical navigation. We used the HO249 tables still available and had good results with them. What did not work to satisfaction in an aircraft proved to be ideal for sailing boats. It is possible to train astronomical navigation in your garden,but you will have to use an artificial horizon, since you need a perfect flat surface to report your angle and that is only available at sea. The easiest way to bypass the problem is to report your measured angle to the reflection of sun or moon in a mirror. Dirty water or motor oil will do. You than only have to divide the measured angle by 2 and off you go.
GPS is fine but you entirely depend on the governments supply of satellite data that may be withheld or distorted as needed in case of war or crisis. That is the reason, western Europe an Russia are implementing alternative systems. Last but not least, the first thing to pack up in a cold moist environment like a small sailing boat are electricity and electronical devices. Never rely on one GPS receiver, always carry a handheld as backup and give the old methods a try. Celestial bodies cannot be switched out as desired, their use does not cost a cent and it is a rewarding hobby.
For beginners a plastic sextant and a radio controlled timer clock will do.

I too have sailed for quite a while and remember the excitement of getting 6 fixes a day on the Sat-Nav that agreed with my own. I still have my C Plath sextant but admit that I am pretty rusty in Celestial Navigation, having relied on GPS for so many years.

I think a lot of folks just relied on their inborne senses so consequently got their a$$ hopelessly lost??

Larry Sellers
SCI Life Member

quote:

theodolite

en.wikipedia.org/wiki/Theodolite

Theodolite - Wikipedia, the free encyclopedia
A theodolite (pronounced /θiːˈɒdəlаɪt/) is an instrument for measuring both horizontal and vertical angles, as used in triangulation networks.

From the looks of the photos, and vague recollections, these are more like a transit and used for surveying.

360 degrees in a circle relate to days in a year, orbit around the sun (back before we figured it out to 365 1/4 days).

Minute of Angle, hours, minutes, seconds relate to navigational positions (don't ask exactly how) and the use of chronographs (clocks) in navigation. A nautical mile also correlates to minutes, seconds in a circle.

If you know your position in minutes and seconds, you can compute distance in nautical miles. When you put it all together the "big picture" starts to reflect time/space relationships. And then Einstein takes over and everything is relative.