Sextant App

[CyKlop]

Originally Posted by ItDepends

What???? Sextant and clock most certainly gets you lat and long.

While you are correct, it should also be noted that the precision of the clock is quite important. Let's examine this...

Sun goes around the earth every 24 hours, i.e. 360 degrees every 24 hours.
that's 15 degrees per hour.
that's 1 degree every 4 minutes.
that's 60 miles (at the equator) every 4 minutes. So if your clock is off by 4 minutes, you are 60 miles off even if you did everything else perfectly.

Now, obviously, our modern, quartz crystal, or else satellite synchronized watches are far more accurate than that, but if you're relying on an old wind-up watch or clock... However, if you're going to rely on one of those, why not just have a GPS puck that's the size of a stop watch and that gives you exact latitude and longitude?

Mind you, I love celestial navigation, and keep my practice up just because I enjoy it, so this is not a put-down of celestial. It is, however, a (very fast and brief) summary critique of it as a practical navigation in this day and age.

[goboatingnow]

I had an old Casio programmable calculator that I programmed to do the sight reduction maths. Quite nice. It died and I went back to paper. Be easy enough to build a spreadsheet app or something these days. It’s a pity there isn’t a digital sight reduction table set.

I practice my sextant sights when I can. Then compare them to GNSS fixes to see how good my sights are. I typically start with 15 miles and end around 5 miles.

What’s a “wind up clock “ I think my grandfather had one of those. I just use my 25 year old citizen WR200 which gets the German atomic DF777 signal 400 miles out into the Atlantic and similarly the US one the other side.

[PineyWoodsPete]

Originally Posted by StoneCrab

With the director of Earth and Space dead, no offense regarding your friend, it explains a lot of our current problems.

Good story, thanks for sharing

Right, and too many since who have thought they were and acted accordingly...

We old relics are allowed to tell a story or two, so here's how we first met Coleman and Trudy: In '79, on Tangier Is, trying to dock our Endeavour 32 in a four-poster/tiny-dock slip, with wicked crosswind - them next to us enjoying their sundowners in their 26' footer. Louise and me, and our 11 and 14 year old girls - all with a looped line. Couldn't get the upwind posts looped first try, and went downhill from there. Finally secured, Trudy raises her glass: "To the crew of Xanadu! Laurel and Hardy couldn't have been better entertainment!!

After a week of sailing with them, and Hurricane David approaching, they welcomed us to anchor in the little well-protected cove in front of their house on Kent Is. - the children to stay in their home, us to "take care of the boat": Three well-set anchors out, middle of the night, gusting over 60kn, water rising and lifting their mooring, their boat driven ashore, breaking rudder, VHF full of distress calls, shrieking wind and shotgun rain making it untenable out of cabin, and there's not one damned thing you can do. If an anchor breaks loose, which fortunately didn't, forget it. Stay ashore, folks, if you can!

[Macadventure]

I know you asked for an Android App, however crossing the Atlantic in 2018 I used The Celestial plugin for OpenCPN. From memory, my satellite connection was not required to do this.

[JAFO]

Originally Posted by GrowleyMonster

Moon shots as they are available.

Well, at least someone finally (albeit briefly) mentioned the moon.

Need/want to find your longitude via sextant, without a chronometer? The lost art of Observed Lunar Distance sights (as used by Joshua Slocum & Cpt. Cook) has you covered.

https://www.starpath.com/resources2/brunner-lunars.pdf

[Adelie]

Originally Posted by JAFO

Well, at least someone finally (albeit briefly) mentioned the moon.

Need/want to find your longitude via sextant, without a chronometer? The lost art of Observed Lunar Distance sights (as used by Joshua Slocum & Cpt. Cook) has you covered.

https://www.starpath.com/resources2/brunner-lunars.pdf

Let's be clear there are moon shots done like any other celestial body where altitude to the horizon is measured & time from an accurate timepiece recorded and then there are lunars which are a different animal.

Lunars
The traditional lunar combines altitude shots for the moon and various bodies and shots between the moon and those bodies and if I recall correctly you need a timepiece but it does not need to keep accurate time, a stopwatch may do.

The Chichester Lunar involves shooting a body almost due north or south of you to get your latitude and the moon altitude. Duplicate calculations of the moon for before and after the time you think you shot it combined with your known latitude will give you longitude and I believe also re-establish somewhat accurate time (+/-40-80sec).

The Letcher Lunar involved shooting a round of stars and the moon using a timepiece of dubious accuracy, then adjusting time for all the shoots until the moon line runs thru the LoPs for all the other bodies. This will improved the accuracy of your timepiece to about 30-60s, which should be good enough for landfall in most places that don't have a lot of offlying rocks and shoals.

For lunars you only need to approximate accurate time and the correct date, although you can usually determine the date from the moons position relative to other stars just by inspection from the Nautical Almanac.

[SeanPatrick]

I see some posts here which seem to be discouraging the use of an app or program to reduce sextant sights and I must say that perplexes me a little. Of course, I am biased. But, to me, using a sight reduction app as opposed to paper tables isn't much different than using a chart plotter as opposed to paper charts. And I see even less difference between using an app versus a handheld calculator - the only real difference being where the formulas are stored.

I'm a celestial navigation nut. I can reduce sights at least a half dozen ways and I can rattle off the formulas for lunars off the top of my head. But The method I use most is an app, computer program or spreadsheet. I mean, why not? It makes the whole process faster and easier. And, as is being discussed over on NavList right now, saving time in the sight reduction process allows one to take more sights and get a more accurate idea of one's position. The discussion over there is centered around the 1938 Fairchild Maxson Line of Position Computer. This device was apparently used by Howard Hughes on his flight around the world. You could think of it as a precursor to the apps we have today.






Using an app to reduce sights still gives one an independent position check. There are even systems in use today which automate the entire process. These "star trackers" have been in use on watercraft, aircraft and spacecraft for many years now. They automatically measure the positions of the stars in the sky and calculate the craft's position and orientation. I recall an article about advances in technology which may put these types of devices within reach of the average consumer in the near future. I thought it was posted on this forum, but I can't seem to find it right now. Point is: who wouldn't want something like that? An automatic, independent position finding device that's as easy to use as GNSS? Sign me up.

I'm all for anything that keeps interest in celestial navigation alive. And if an app gets someone to start using a sextant, I think that's great! Maybe that person will become even more interested in the whole process and will want to learn how to do the reductions the 'old-fashioned' way. But if they are content to keep using a program to do the tedious work, well that's fine, too.

BTW, if anyone is interested in programming a calculator, computer or whatever device to do sight reduction, DR calculation, etc., I can recommend the following resources:

• Astronomical Algorithms by Jean Meeus (My main reference for making my spreadsheet.)
• The Nautical Almanac by the USNO & UKHO (Has sight reduction formulas and more.)
• The IERS website (For information about accurate time.)
• A Manual of Spherical and Practical Astronomy vols. I & II by William Chauvenet (Everything you could ever want to know about astronomical calculations and instruments.)
• The links page of my website has some great resources, too

[Benz]

^^I'm not getting the same denigration-of-apps vibe that you are; but I do see that some value celestial nav as an electronics-free method of navigating, as much as is possible in these days when wind-up chronometers are expensive antiques and can't be easily serviced.
While I see no shame in using spreadsheets or apps to reduce a sight, I find doing the entire process on paper extremely satisfying. And since I usually draw plotting sheets and keep a running fix on those, of which my celestial LOPS are just a part, using an app would only remove a small portion of the pencilwork and break up my rhythm.
You mention a chartplotter, but surely no one is trying to plot celestial LOPs on a plotter? How is that even done? It would take me longer to input a lat/long derived from my plotting sheet into a chartplotter (which I don't have on my boat) than to simply plot it on my paper chart. Do chartplotters have an azimuth/intercept function?

[SeanPatrick]

The app/program I mentioned earlier (Navigational Algorithms) will indeed plot LOPs in OpenCPN with just a couple mouse clicks. It will also plot SAR patterns.

But I wasn't really thinking of using the plotter for celestial. I was just using it as an example vs. using paper charts as an analogy for using a sight reduction app vs. tables.

[double u]

I see 2 reasons to delve into celestial these days:
1: as an emergency tool, if everything else fails (GPS, electricity,...)
2: "for old times sake"

in neither case I see any sense in involving apps, computers or, !!, plotters

[Jim Cate]

^^^^

OK, that's simple: don't use any of them... but don't knock those who do. I reckon that's up to the individual navigator.

Jim

[goboatingnow]

Originally Posted by JAFO

Well, at least someone finally (albeit briefly) mentioned the moon.

Need/want to find your longitude via sextant, without a chronometer? The lost art of Observed Lunar Distance sights (as used by Joshua Slocum & Cpt. Cook) has you covered.

https://www.starpath.com/resources2/brunner-lunars.pdf

I’ve used this method which hasn’t been taught formally since about 1920 ( that’s when it disappeared from Bowditch ) the table of corrections are difficult to find these days.

It’s an inaccurate method but interesting

[goboatingnow]

Originally Posted by double u

I see 2 reasons to delve into celestial these days:
1: as an emergency tool, if everything else fails (GPS, electricity,...)
2: "for old times sake"

in neither case I see any sense in involving apps, computers or, !!, plotters

Why. Electricity doesn’t arbitrarily fail , there are 4 GNSS systems up there

You use and learn celestial these days cause it’s a hobby. Nothing else.

[GrowleyMonster]

Originally Posted by JAFO

Well, at least someone finally (albeit briefly) mentioned the moon.

Need/want to find your longitude via sextant, without a chronometer? The lost art of Observed Lunar Distance sights (as used by Joshua Slocum & Cpt. Cook) has you covered.

https://www.starpath.com/resources2/brunner-lunars.pdf

Well, I didn't mean Lunars, just observations of the moon and reduction to an LOP, but yeah, being able to work Lunars could be darn handy in the rare event of a lightning strike frying everything on the boat with a transistor in it. Good call, and I have seen that file before. Good explanation.

In the pre-internet age I read about Lunars and set about trying to find a 1914 or earlier edition of Bowditch so I could use the tables. I never found one at a price I didn't mind paying. I did early on realize that there is one unique time when a particular difference in LHA or GHA between the moon and another body could occur, and by assuming different shot times I could calculate the hour angles for the assumed shot times and in a couple of iterations find the time that it had to be for the observations to have been made. Similarly, I knew I could calculate the time when the observed difference in height of the moon and another body would have been possible. The time when the height of one body would depend on the unknown Longitude, but the time of a given difference in height between the moon and another body would be a unique time and not be dependant on Longitude, and so with a time found, Longitude would be found.

I did have a scientific calculator, one that even had a button for converting to and from decimal degrees or hours, so the math wasn't excruciatingly tedious once I had my keystroke routine nailed down. This method proved usable, but errors of course are greater than the single direct Lunar Distance observation method. I started to work out how to calculate the Lunar Distance from a body so I could do the same thing, assume observation times and see what time would give the Lunar Distance actually observed, and finally decided my high school dropout brain had its fill of spherical trig, and it would be unlikely that I would ever not have a usable time standard available with which to get a proper celestial fix. At that time of course Celestial was still in common use outside of LORAN coverage areas. The ship's sextants got a lot of use. At that time I mostly sailed as a watchstanding AB and usually had the 4 to 8 watch, perfect for working on my Celestial skills, and usually the Mate would tell me go ahead and plot my LOPs on the chart. From childhood I knew how to take LAN but it wasn't until I started going to sea on ships that I really sank my teeth into reducing ex meridian sights to LOPs. The ship's chronometers and hack watch were usually used but I used my unrated but still accurate digital watch, and of course we could always pick up the time ticks on HF at any time of day, or the top of the hour BBC time signal. Lunars by whatever method just didn't seem worth continued pursuit when I had other interests. I couldn't envision not being able to know the time well enough to navigate.

BUT, I just now downloaded the 1914 edition of Bowditch from Internet Archive and I might dip my hand into time and Longitude by Lunars again. Honestly I have never talked to anyone who actually practiced Clearing the Lunar Distance. Well, come to think of it, I don't know any of my yachty friends to be celestial practitioners at all. But yeah, for the true nautical prepper, or a total Luddite, the ability to do lunars would give the desired redundancy and independance from all things electric or electronic. Time checks via HF radio would not be in keeping with either system. Nor computers or calculators. If you really want to be a true hard core Navigator, I guess you really do need to be able to work Lunars with the tables. My head is already hurting, thinking about it.

[SeanPatrick]

If you need any help, let me know!