Sextants and celestial navigation

[MalibuC]

I have been attempting to teach myself how to use a sextant. I have read a couple books on the subject (confusing to say the least). I have watched several YouTube videos (more helpful). Purchased a Davis Beam-Convergent Sextant, played with it, read the manual, etc. Searched the night sky, located stars, purchased a nautical almanac, etc., etc. etc. I have taken the sextant to the beach, found a reference star, took my reading, time, index correction, dip correction and started plugging the data into the sight reduction form and stopped when I could not located the refraction correction data in my almanac. Now I am not ready to give up just yet. One issue not yet mentioned is the ability to see the horizon at night...is this only possible on well moon light nights? I could use some pointers here to takeme to the next level. Any advice?

[Dsanduril]

Ahhh, the joys of celestial navigation (only partly facetious, I actually do enjoy it). In general, you don't take sights at night. Thus the importance of knowing the times of various twilights. You have a short period of time when some of the stars and planets are visible and so is the horizon. Very common to take three or so shots (of different bodies) in very quick succession without any calculations, then head below to calculate once the shots are all recorded.

If you want to work without a visible horizon you generally require either an artificial horizon or a bubble sextant.

You don't say what method of reduction you are using, but refraction is usually a separate table in the sight reduction tables rather than in the almanac.

[SkiprJohn]

Aloha Malibu,
Dsanduril has said it well. Using just the almanac you can do Sun sights, meridian passage sights.
You've seem to have started with the most difficult first so take a breather, read the books again and try a few more times.
There are many many books on the subject but my favorite for simplicity is Celestial Navigation by H. O. 249 by John E. Milligan.
Good luck in your successful shots.
kind regards,

[MalibuC]

Originally Posted by Dsanduril

Ahhh, the joys of celestial navigation (only partly facetious, I actually do enjoy it). In general, you don't take sights at night. Thus the importance of knowing the times of various twilights. You have a short period of time when some of the stars and planets are visible and so is the horizon. Very common to take three or so shots (of different bodies) in very quick succession without any calculations, then head below to calculate once the shots are all recorded.

If you want to work without a visible horizon you generally require either an artificial horizon or a bubble sextant.

You don't say what method of reduction you are using, but refraction is usually a separate table in the sight reduction tables rather than in the almanac.

Okay, now starting to make some sense. Take sights at twilight.

Method of sight reduction, not sure. I just downloaded one from the net but there is also one in the Almanac. I have found some neat looking apps for the iPad but I really want to do it the "long handed" method first.

Thanks for responding, it's funny how a little but of info goes so far.

[Dsanduril]

If you check the almanac you'll see listings for civil twilight and nautical twilight (and maybe astronomical twilight). Civil the sun is 6 degrees below the horizon, nautical 12, astronomical 18. You can generally see stars starting about at civil twilight, and can see the horizon until nautical (talking evenings, reverse in mornings). Astronomical is considered "full dark" and astronomers can go to work.

I don't have an almanac here in front of me, can't remember if there is a refraction table in there somewhere.

[PCFithian]

I have just been through this as well, and posted my findings on this forum here: http://www.cruisersforum.com/forums/...ial-98663.html

You may find an issue with the whole horizon (beam converger) mirror as found on the Davis Mark 25 in low light conditions. David Burch discussed this here:
Online Classroom: What style of sextant horizon mirror do you recommend, traditional or full view?

You may also gleen further information from this essay on plastic sextants, also by David Burch:

[goboatingnow]

IN general , the method is ,

Take a vertical sextant angle , by bring the object to the horizon ( hence the need to see the object and the horizon)

AT the same time record the exact time

Correct the Sextant angle for Height of eye ( dip) ,refraction and index error( all in celestial almanac) ( SD and parallax for sun and moon respectively )

This is Ho, the observed altitude

For the intercept method, assume a dead reckoning position

using the time, compute the Declination, GHA and LHA, using the reduction tabels compute calculated altitude and azimuth for the Assumed position

draw the intercept line, This gives you a LOP.

Youll need HO249 for reductions, and a up to date celestial almanac,

theres a good summary here Intercept method - Wikipedia, the free encyclopedia

[muttnik]

Look at the "Altitude Correction Tables" at the beginning of the almanac and the "Additional Corrections Tables" after it which give corrections for different pressures / temperatures.

As stated by others, you need to shoot at twilight. You've only got a limited amount of time before you lose the horizon (assuming you're doing it in the evening, or lose the stars if in the morning) so it's helpful to be aware beforehand which stars are going to be visible first (ie the bright ones, especially to the east), be looking for them and shoot them as soon as you can.

[goboatingnow]

I would suggest that the sun be used first to practice celestial navigation, most people can locate it easily enough..

Dave

[PCFithian]

You may also want to check out this video from Jim Cook:

Calibrating and Using A Sextant - YouTube

[carstenb]

Originally Posted by SkiprJohn

Aloha Malibu,

There are many many books on the subject but my favorite for simplicity is Celestial Navigation by H. O. 249 by John E. Milligan.
Good luck in your successful shots.
kind regards,

SkiprJohn is right. The H.O. 249 book on celestial navigation is very good. Easy to understand and it doesn't make it into some deep dark science.

Buy it it is cheap

And keep at it. Knowing how is great and will also earn yo respect from those that truly can sail.

[AJS]

For an easy to follow tutorial, try Mary Blewitt's book; Celestial Navigation for Yachtsmen. Over the years I have taught celestial navigation to many, most haven't needed much more than Mary's excellent book.

[barnakiel]

Originally Posted by MalibuC

I have been attempting to teach myself how to use a sextant. I have read a couple books on the subject (...) Any advice?

Less reading, more practice.

- start with sun sights,
- shoot stars at dusk/dawn - then you can see the horizon clearly.

Are you a practicing offshore sailor? I have sailed some without ever making a star LOP once. Only ever used sun to get our position.

b.

[Tashtego]

Just a note to say that HO 249 Volume I is an excellent guide for star sights. I, and many others, find figuring out where a particular star is in the sky then finding it with the sextant is not easy! HO 249 vol I gives the altitude and azimuth for the seven best stars for every ten minutes of GMT. Just pre-set the sextant to the altitude in the tables, point the sextant toward the azimuth in the table and there it is. Makes star sights a lot easier especially since you only have a limited amount of twilight both morning and evening. Remember the sun is always up there in the daytime and the moon is visible somewhere useful during the day for 15 to 20 days during the month. There is also the possibility to take a "lunar" measurement to get GMT. The method is very clearly described in "Celestial Navigation in the GPS Age" by John Karl. It is much simpler than it appears (although the computations are long). Karl also describes the other aspects of the art including the use of HO 249 and how to use a scientific calculator (one with sines and cosines) to calculate most everything.

[transformer]

"One issue not yet mentioned is the ability to see the horizon at night...is this only possible on well moon light nights"

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I was already ben warned. to be polite, but i can not resist.

You do not have to be nuclear scientist to figure out that you can not take sights in pitch black when you can not se even your own nose let alone horizon

how many sights you would be able to take waiting for moonlight to "light" your horizon. probably two a month