What is a sextant and how do you use it?

The sextant features a scale marked in degrees. By adjusting a movable arm, you can measure the angle between a celestial object and the horizon. To take this measurement, you position the arm so that the celestial object is reflected in the mirror and aligns with a stationary section of the graduated scale.

How do you take sights using a Sextant?

“Please note that the information in this video is not considered professional advice or education. It is provided for basic information purposes. Please obtain proper training before attempting to use a sextant.

Video 3 – In this video I’m going to show you how you can use it to measure the altitude of a celestial body. You will need a properly adjusted sextant so if you haven’t watched the previous video about errors and corrections I would recommend watching that first and it’s on the card at the top of the screen.

What we’re goind to do is measure the altitude of this red body – I’ve just coloured it red to make it easier to see the technique we’re going to use. The altitude is just the angular distance between the body and the horizon measured from the observers position. In this case the sextant is the observer. So the first step is to set the index arm and mirometer drop to zero or as close to zero as possible.

If you have a small index error of course, zero on the scale will not quite be zero according to the instrument. This point it doesn’t really matter as you’re going to apply index error at the end. While set at zero the instrument is basically just a telescope – you can look through the sights and see an image.

As the instrument is like a telescope you’ll obviously need protection when looking at bright objects. This mainly applies when you look at the sun but do be careful if you’re looking at anything really big as they can be surprisingly bright. You can use these shades as eye protection. There are a set in front of each mirror named accordingly – so you have the horizon shades and the index shades. You will need to use both for proper protection.

If I aim straight at our body and apply one of the index shades You can see half of the image is suitably dimmed. If we now swap over and apply a horizon shade instead you can now see the other half of the image is dimmed.

Applying both a horizon shade and an index shade gives you full protection. It’s safest to start with all of the shades across and then you gradually remove them until you see a clear image. With experience you will gradually learn the best combinations for different situations. If the body is dim enough you may not need any shades. I’m going to remove them all for this example. So, for the sight, you start liiking straight at the celestial body with shades adjusted as necessary.

With the sun it’s easy but with starts they may take a little bit of finding. You will want the body aligned right in the middle of the image, half of it reflected and half of it true. But, here is one of my little tricks, while you continue setting up the sight, actually get the image in the middle of the horizon mirror instead. It will make it way easier to stay focused especially on a small star or something hard to see. We’ll align it properly when
it comes to the fine adjustment at the end.

Next, you want to get hold of the clamp at the bottom of the index arm. This will allow you to move the index arm freely. Have a little practice wiggle just to see how the image in the horizon mirror actually moves when you’re moving the index arm. Then, you want to start to bring the entire instrument down aiming it towards the horison. At the same time you want to use that clamp to move the index arm and keep the celestial body in the centre of the horizon mirror.

Once you reach the horizon you want that to be in the middle as well. For the final part
you bring the celestial body to the centre of the image. You would normally find there’s a little optical quirk here that means you can see both the horizon and the body if you have it right in the centre.

This doesn’t work in my example as this is all computer generated. Once you’ve got it as close as possible with the clamp,you can then use the micrometer to do the final fine adjustment to to bring the body in line with the horizon. At this point if you use the shades, you might find that you need to remove one of the horizon shades to make the horizon easier to see. Just be certain you are removing the correct shade otherwise a bright body could burn your eyes. Where the star is easy to line up a tiny dot of light but with a large body like the sun or our red sphere here, can be a lot harder. Almanacs usually give and upper limb and a lower limb measurement so you can line up the top and the bottom respectively. In this case, I’ve aligned the lower limb with the horizon.

When you think it’s all lined up you should slowly rock the sextant from side to side keeping it pointed straight ahead. The celestial body will appear to follow a path in the image. All you’re doing is checking the sextant was perfectly upright when you took the site. If it was upright the body will just kiss the horizon at the lowest point on its rocking path, if it wasn’t upright the body would drop slightly below the horizon. If this happens you just need to use the vernier again to make the lowest point on the path just kiss the horizon.

Now you’ve got the site properly set in the sextant and you can just read off the measurement from the Skylark and the vernier. Any index error that needs to be applied to get the correct reading. For example, say you measure a bodies altitude at 13 degree 25.2 minutes and you have an index error of 1.5 minutes off the arc you need to add those 1.5 minutes to your measurement. Your altitude then becomes 12 degrees 26.7 minutes.
Likewise if your error was on the arc instead, you just subtract it from your measured altitude.

And now that brings us to the end of this sextant tutorial series…….”

Contact The Tyneside to purchase your own sextant.