As described to me, a sextant is used to determine the angle of a celestial object to the horizon. Huh? Obviously I am wrong, but as you are looking at any star, planet, etc. isn’t the angle always 90 degrees?
It’s not the angle the the light makes with your eye (which is pretty much always 90°).
The Sextant uses partially-silvered mirrors so that the horizon can be seen at the same time as the star. The angle is read off of a scale.
Basically, the sextant is an optical protractor.
Look straight up. That’s 90 degrees off the horizon (the altitude angle). Now look straight ahead: that’s 0 degrees. As you can tell, most objects are between straight up and straight ahead.
You can make a simple sextant yourself. Just take a protractor and attach a string with a little weight on it (to make it hang straight) to the center of the straight part of the protractor. Aim along the straight edge of the protractor to the object that you want to measure, then look and see what angle the string is at on the protractor’s angle measurement.
That’s a very crude sextant, and if you are off by a few degrees that can translate to hundreds of miles in location. The stringy bit also doesn’t work very well on a rocking ship. So real sextants tend to be a bit more complex.
You have a tube that you look through. One half of the tube goes straight through, and you can line that part up with the horizon. The other half is blocked by a mirror, which is lined up to point to a second mirror, and this second mirror moves along a protractor-like scale. You move the slide part until the star you want to measure is perfectly in line with the horizon in your viewing tube. Then you look at the angle on the protractor-type scale on the side of the sextant and you know the angle between the star and the horizon.
You can determine your latitude using pretty much any star in the sky doing this, but you have to know where in the sky that star should be at any given time. Conveniently, if you are in the northern hemisphere, there is one particular star (Polaris) that sits pretty close to right on top of the north pole. Since it is pretty much always at 90 degrees to the equator, if you measure the angle between Polaris and the horizon this will give you your latitude.
Longitude is harder. To know your longitude you need to know precisely what time it is. If you set your watch to show the correct time in a particular place (say, Greenwich, England), and you have a book with tables of what time the sun comes up on each day of the year, then all you do is look and see what time the sun comes up in your particular location, according to the time on your Greenwich watch. The time difference between sunrise where you are and sunrise where Greenwich is will tell you your longitude. You can do this with star positions as well, but again you have to have a table of what position that star should be in at any given time on any given day.
Since the folks that first started doing this were at the Royal Observatory in Greenwich England, that’s how Greenwich came to be the “zero” standard for longitude.
This site has a pretty good explanation (with pictures!) that shows you how to use a sextant and how it works.
http://www.clipperlight.com/howusesextant.html
You’re not kidding. If you’re off by one minute of angle, basically you will be off by one nautical mile of distance (at the equator with respect to longitude, or north-south anywhere). Plenty enough to put you on the rocks.
And one minute of angle is a teeny tiny amount. Quite close to one inch at 100 yards.
And single digit fractions of a minute are about as good as you can get out of a sextant, even under ideal conditions, and in the hands of a very skilled operator. The instrument is only intended to give you results accurate enough to let you know what you ought to be looking for in visual range. The wiki article has this to say about accuracy:
There are a couple things that have to be factored in. If you’re looking at the sun, it’s easiest to align the bottom of the sun with the horizon, but since you’re really interested in the angle to the center of the sun, you add a correction factor to account for the sun’s radius. You also have to estimate the height of your eye above the water (assuming you’re doing this on a ship). The line from your eye to the horizon is not perfectly level (relative to your present location); it actually angles downward slightly. The higher you are, the greater the angle. If you want to be accurate, you correct for that. And you have to do this all at the right time; the number you’re looking for is the angle to the sun at its highest point during the day.
As for using a sextant at night, it’s tricky. You can see the stars, but once it’s dark you can’t see the horizon. There are a few stars that are bright enough to be visible while there’s still a horizon, and there are tables and methods to use them, but I don’t know if Polaris is one of them.
For a start, you also need to know your latitude. On a given day, sunrise at Greenwich might be at 6:00, but at your latitude it could be 5:00 or 7:00. It’s the difference between when it should come up and when it does that helps figure out longitude.
A navagator will shoot the star, sun, or moon. At the time he takes the sighting he will start a stop watch. Then he heads back into the chart room. He (or she) will note the time on the cronometer (an very accurate clock). He will subtract the time on his stop watch to get the correct time of his shooting. Then he digs out the book (Blue?) and begins his calculations. If the shooting is doneat sun set or sunrise he then can calculate where he is. If it is mid day or durring the night it will take a second shooting. Then using the course and speed of the ship he will calculate where he is. I had to take one course with the Naval Science classes so this what I remember about it. Sorry but I am an engineer so I did not need it more than to fill my NS requirement.
And if you’re inland, you can’t use the real horizon, since that might have mountains or the like on it. You have to use an “artificial horizon”, which is produced using a glass container with water or other liquid in it, and measure the angle between the target object and its reflection, then divide by two.
We’re going into a lot more detail here than is probably useful to the OP, but it’d really help to get some feedback from him, so we can narrow down the misconception that led to the question in the OP.
I’ll hazard a guess that he’s talking about the angle the vertical line between the star and horizon makes with the horizon. Which is 90 degrees, of course, but not useful, and not what a sextant measures.
Their prices for reproductions of sextants are also (while widely varying!) often within reason. I notice, looking up sextants at Amazon.com, some that are under $20 (although, it would appear, more useful as decoration than as an actual navigational device.)
I bought a “training” sextant, and began by calibrating my skills on measuring the heights of trees, telephone poles, buildings, and so on. After a while, I was able to get decent estimates of distances between hilltops or distances from one tall building to another. (Hold the instrument sideways!)
You don’t have to know a lot of trig to use one…but, of course, the more trig you know, the better, as it can allow you to make slightly more difficult calculations. I’ve also had the joy of doing some amateur outdoors surveying using a transit (also useful to have a younger sibling to carry around the sighting pole!)
Of course, nowadays, GPS devices make it all easy as pie…but the U.S. Navy, I’m told, still teaches old-fashioned celestial navigation, the way Wm. Bligh did it!
Many years ago (more than thirty years ago), Bergstrom AFB had an open house called AeroFest. Many aircraft were on display. I once toured an AWACS aircraft. The airmen and officers on board couldn’t (as in wouldn’t) give us much information on what the equipment on the aircraft actually did.
However, amongst all of the high-tech equipment, was a sextant. It was fastened into a special compartment in the ceiling above one of the doors. How could a sextant be used on a moving airplane? An airplane is moving pretty fast. If it is stopped, it is clearly at a known location. Why would they need a sextant?
Shoot the star note the time. Keep track of the course and speed. Take second shooting note new time and record everything. Then do the math same as on a ship. The only major differance would be a shorter time between each shooting.
It can be done with a total loss of electric power. Just a pencil, paper and the table books.
It was also used for airlines crossing oceans. Read Ernie Gahn stories of the 1930-1950 aviation. ( Fate is the Hunter, etc. )
They would shoot the stars or sun, do the math & know where they were when they took the shot. Plot DR for the 15 - 20 minutes it took a slow newbie to work the problem.
I would probably buy one if I could get it cheap enough. I looked on Amazon though and the cheap ones look decent but then you look in the description and they are all of about 3 inches in size. I don’t want a micro-sextant.
I, too, have been interested in acquiring a sextant and looked at the ones on Amazon. Reading the reviews pretty much indicated to me that they were for decorative purposes.
I did, however, see them for sale at Harbor Freight for, I believe, around twenty dollars. I haven’t picked one up yet but I did take it out of of the package and check it out and it seemed functional.
If you don’t have a Harbor Freight local to you, they are online and some of the items will ship free.
One interesting tidbit:
A sextant is called that because the arm swings through one sixth of a circle. Note that in use, it is required to measure angles up to 90 degrees… but 360/6 = 30! what gives? A sextant uses two mirrors, each of which doubles any angle changes, so the scale which covers only 30 physical degrees covers 120 optical degrees, and is so calibrated.
If you are interested in celestial navigation, check Alibris for an old copy of Dutton’s. I got mine for a couple of bucks at a library book sale I think (maybe it was on a bookstore clearance table though…I forget) This will explain how it all works, and will have approximate ephemeral data you can use to take actual readings. (exact ephemeral data is typically only published a couple of years out. Generating it was the job of a naval observatory)
You might want to double check your work.
Geeze. Thinking and typing must not be in the cards today. The arm moves 60 degrees, and carries one mirror which doubles the optical angle. The second mirror is fixed, so does not double the angle. So you still get a 120 degree optical measuring range.