Index Error Sextant

[Refueler]

For anyone interested .... using the sun for error determination is more accurate than using horizon.

Horizon used for quick reference check.

Sun ? Alamanac gives you SD for each period ... this gives you opportunity to test upper and lower SD to arrive at error.

I've stayed quiet .... reading the posts ... and I haven't seen anyone mention that ALL 3 errors interact with each other ... as an old Sextant User as Deck Officer ... correcting a Sextant was basically remove half of each error in turn ... repeat till error is acceptable.

Just a comment .... 20' of arc is a VERY VERY BIG amount ...... thats 1/3rd of a Degree !! In terms of a sight ... 1' = 1nm approx.

Is there some loose terminology / units getting into this ??

 

[AntarcticPilot]

For anyone interested .... using the sun for error determination is more accurate than using horizon.

Horizon used for quick reference check.

Sun ? Alamanac gives you SD for each period ... this gives you opportunity to test upper and lower SD to arrive at error.

I've stayed quiet .... reading the posts ... and I haven't seen anyone mention that ALL 3 errors interact with each other ... as an old Sextant User as Deck Officer ... correcting a Sextant was basically remove half of each error in turn ... repeat till error is acceptable.

Just a comment .... 20' of arc is a VERY VERY BIG amount ...... thats 1/3rd of a Degree !! In terms of a sight ... 1' = 1nm approx.

Is there some loose terminology / units getting into this ??

Just for reference, it's two-thirds of the width of the Moon. As @Refueler says, it's a LOT.

 

[Sailwahine]

Sea horizon is the norm, as that will be your reference when observing.

Am a bit puzzled about using the sun for IE, as I have not come across any references to that technique. I have used a bright star though, which is probably a bit kinder on the eyes should you inadvertently look over the telescope.

Many years ago I did try correcting one of the early plastic sextants for someone. Mirror screws were turned with a hex key. I brought the index mirror into line, relaxed the pressure on the key and it returned to the original position. Mirror was not moving but the frame was bending!

Using the sun for index error involves using the upper limb and then the lower limb and then averaging the two, I believe since it is farther away than the horizon it is more accurate. I read about this is Mike Scanlan's book Celestial Navigation. It looks a bit more involved so I'll try that if I have to.

 

[srm]

Using the sun for index error involves using the upper limb and then the lower limb and then averaging the two, I believe since it is farther away than the horizon it is more accurate. I read about this is Mike Scanlan's book Celestial Navigation. It looks a bit more involved so I'll try that if I have to.

Thanks for that, I looked it up afterwards. Apparently its a method used by early explorers on land who had to use an artificial horizon for observations.

Sun or star are more accurate for checking the sextant as they are effectively at infinity so there is no parallax between the two mirrors. Observing from the deck or cockpit of a small boat the sea horizon will only be around three or four miles away depending on height of eye, but at that range parallax should be almost negligible.

However, your 20' error is instrumental rather than being due to a close horizon. As mentioned by others above I also came across a reference to plastic sextants having variable IE depending on the direction of rotation of the micrometer drum, so it is probably a common problem due to play between the drive teeth.

Rather than using the sun for correcting side and index errors I would suggest that you try using a bright star as a point reference (not a planet as that will have a SD). I found this method quite straight forward, provided that you have just enough light to read the sextant scale.

 

[Sailwahine]

Thanks for that, I looked it up afterwards. Apparently its a method used by early explorers on land who had to use an artificial horizon for observations.

Sun or star are more accurate for checking the sextant as they are effectively at infinity so there is no parallax between the two mirrors. Observing from the deck or cockpit of a small boat the sea horizon will only be around three or four miles away depending on height of eye, but at that range parallax should be almost negligible.

However, your 20' error is instrumental rather than being due to a close horizon. As mentioned by others above I also came across a reference to plastic sextants having variable IE depending on the direction of rotation of the micrometer drum, so it is probably a common problem due to play between the drive teeth.

Rather than using the sun for correcting side and index errors I would suggest that you try using a bright star as a point reference (not a planet as that will have a SD). I found this method quite straight forward, provided that you have just enough light to read the sextant scale.

Thanks so much for this, I did just check and noticed there is definitely a difference depending on which way I move the micrometer! Will check index error with a bright star as suggested.