Thank you. Clearly this video is intended for beginners...which is what I am. It's a shame all the "experts" have so littlle of a life they feel compelled to comment here.
The title states Mark 25, but a Mark 15 is shown. The mark 15 is darker & distorts in the sun more quickly than the Mark 25. The Half mirror on the 15 is much better for shooting stars, but less convenient/easy for shooting the sun compared to the beam converger on the 25. I see about 7 arc minutes of index error change in about 5 minutes with a mark 15. If I don't check IE before & after every sight, I don't trust the sight. West Marine also carries the Astra (Chinese aluminum) sextant for about $800. It's a more accurate instrument. It is FAR more stable than any of the Davis models. Even the $50 Davis Mark 3 lifeboat sextant is more accurate than the mark 15, but the mark 3 is much more difficult to read accurately, it lacks a micrometer wheel & it's current version has insufficient shades for shooting the sun. These are very serious flaws. With the Astra, you can also purchase a bubble attachment that allows you to shoot a sight when the horizon is not visible. Both half mirror & convergers are available for the Astra. A Mark 15 or 25 is good to learn on & good for cheap emergency back up. If you want to figure a position within a mile or two reliably, get an Astra or better. Also, before you buy one, learn how to do a sight reduction. If the math overwhelms you, then the sextant is not for you. At a minimum, learn to do a Noon sight. That is the easiest & fastest way to get a fix using a sextant.
Jim, this is all very great information. Thank you so much for providing more detail. This video is very old so much of it to your comment is wrong and out of date.
The 25 has a "whole horizon" mirror while the 15 has a "split horizon". The 25 also has an led to illuminate the scale at night time. Curious thing, though, is it seems that the "full horizon" mirror system can make it harder to shoot stars, than the "split horizon" mirror system (the mirror coatings can dim certain wavelengths) yet the 25, which has this system, is the one with the night light on the scale.
that's because the video is MISLABELLED, he's holding a Mark 15 (as he says in the video), his error is at the end when he suggests buying a 'Mark 25' which is the more expensive sextant by Davis Instruments and comes with higher quality, an LED light and full convergence mirror (the 15 has a half mirror). The Mark 25 is a grey plastic that's claimed to be sturdier and costs about $45 more.
@@MrRocque the 25 comes with a more expensive horizon mirror rather than a 15 but that's not always an improvement as it can make star shots more difficult. But it does make a sextant easier fora beginner to use.
Yes you can, with somewhat limited accuracy given the precision of your azimuth. The trouble is that the chances of the GP of the sun being on your chart to plot are very slim (sextant altitude must be 88+ degrees for a plotting sheet). A computerized chart would be able to do this, but it's still only 1 LOP and to get a fix you need at least 2 bodies. The use of the assumed position with AZ and intercept are much easier to plot and with computers, easy to find.
I hope that if and when people get a plastic sextant, they understand the challenges of using it with it's tendency to deform due to thermal changes. (this equals constantly changing index error) Sure it's usable if you take care, but I don't know of any ship that carries one or any pro navigator who recommends them.
i have one question: If i find the sextant altitiude and correction to true altitude and zeith distances. after that , take the bearing of the sun and draw the bearing line on chart , I can find my position ? this method right or false? thank you!
If it is high noon, the sextant reading, corrected for index error, limb error, and declination of the sun will get you pretty darn close on the latitude. Some other corrections can be made to dial it in even better. The exact time of local high noon, will allow you to calculate longitude. Every hour off of GMT (Zulu time) noon will equal 15 degrees of longitude. Other minor corrections can be done here as well. At other times of day, you need to do sight reduction calculations, either using spherical triangle geometry or HO229 tables, after looking up declination & GHA from a nautical almanac for that exact time, down to the second. Every second changes the Ground position by about 1/4 mile in the lower latitudes. The reduction calculations give you azimuth & intercept accurately. If you try to get the azimuth with a compass reading, that is off by a degree or two because you took it on a rolling/pitching boat & the GP of the celestial body is thousands of miles away, then your position error will likely be in the hundreds of miles. Also consider that you need to know true bearings, so if you don't know the exact magnetic variation at your position, that adds error of maybe 10 or 15 degrees in many places. That's a lot.
Thank you. Clearly this video is intended for beginners...which is what I am. It's a shame all the "experts" have so littlle of a life they feel compelled to comment here.
Glad you found it helpful!
The title states Mark 25, but a Mark 15 is shown. The mark 15 is darker & distorts in the sun more quickly than the Mark 25. The Half mirror on the 15 is much better for shooting stars, but less convenient/easy for shooting the sun compared to the beam converger on the 25. I see about 7 arc minutes of index error change in about 5 minutes with a mark 15. If I don't check IE before & after every sight, I don't trust the sight. West Marine also carries the Astra (Chinese aluminum) sextant for about $800. It's a more accurate instrument. It is FAR more stable than any of the Davis models. Even the $50 Davis Mark 3 lifeboat sextant is more accurate than the mark 15, but the mark 3 is much more difficult to read accurately, it lacks a micrometer wheel & it's current version has insufficient shades for shooting the sun. These are very serious flaws. With the Astra, you can also purchase a bubble attachment that allows you to shoot a sight when the horizon is not visible. Both half mirror & convergers are available for the Astra. A Mark 15 or 25 is good to learn on & good for cheap emergency back up. If you want to figure a position within a mile or two reliably, get an Astra or better. Also, before you buy one, learn how to do a sight reduction. If the math overwhelms you, then the sextant is not for you. At a minimum, learn to do a Noon sight. That is the easiest & fastest way to get a fix using a sextant.
Jim, this is all very great information. Thank you so much for providing more detail. This video is very old so much of it to your comment is wrong and out of date.
You started talking mostly about the mark 15. But your last words were the mark 25. Could you explain the difference 15/25?
The 25 has a "whole horizon" mirror while the 15 has a "split horizon". The 25 also has an led to illuminate the scale at night time. Curious thing, though, is it seems that the "full horizon" mirror system can make it harder to shoot stars, than the "split horizon" mirror system (the mirror coatings can dim certain wavelengths) yet the 25, which has this system, is the one with the night light on the scale.
You keep saying Mark 15 the video is labeled Mark 25.
that's because the video is MISLABELLED, he's holding a Mark 15 (as he says in the video), his error is at the end when he suggests buying a 'Mark 25' which is the more expensive sextant by Davis Instruments and comes with higher quality, an LED light and full convergence mirror (the 15 has a half mirror). The Mark 25 is a grey plastic that's claimed to be sturdier and costs about $45 more.
@@MrRocque the 25 comes with a more expensive horizon mirror rather than a 15 but that's not always an improvement as it can make star shots more difficult. But it does make a sextant easier fora beginner to use.
Yes you can, with somewhat limited accuracy given the precision of your azimuth. The trouble is that the chances of the GP of the sun being on your chart to plot are very slim (sextant altitude must be 88+ degrees for a plotting sheet). A computerized chart would be able to do this, but it's still only 1 LOP and to get a fix you need at least 2 bodies. The use of the assumed position with AZ and intercept are much easier to plot and with computers, easy to find.
I hope that if and when people get a plastic sextant, they understand the challenges of using it with it's tendency to deform due to thermal changes. (this equals constantly changing index error) Sure it's usable if you take care, but I don't know of any ship that carries one or any pro navigator who recommends them.
Anabasis75
I see them as little more they a "Toy"!
And they cost WAY TOO MUCH!
Give me metal every time!
i have one question:
If i find the sextant altitiude and correction to true altitude and zeith distances.
after that , take the bearing of the sun and draw the bearing line on chart ,
I can find my position ? this method right or false? thank you!
If it is high noon, the sextant reading, corrected for index error, limb error, and declination of the sun will get you pretty darn close on the latitude. Some other corrections can be made to dial it in even better. The exact time of local high noon, will allow you to calculate longitude. Every hour off of GMT (Zulu time) noon will equal 15 degrees of longitude. Other minor corrections can be done here as well. At other times of day, you need to do sight reduction calculations, either using spherical triangle geometry or HO229 tables, after looking up declination & GHA from a nautical almanac for that exact time, down to the second. Every second changes the Ground position by about 1/4 mile in the lower latitudes. The reduction calculations give you azimuth & intercept accurately. If you try to get the azimuth with a compass reading, that is off by a degree or two because you took it on a rolling/pitching boat & the GP of the celestial body is thousands of miles away, then your position error will likely be in the hundreds of miles. Also consider that you need to know true bearings, so if you don't know the exact magnetic variation at your position, that adds error of maybe 10 or 15 degrees in many places. That's a lot.