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 Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.

#21
05-10-2012
 AdamLein Senior Member Join Date: Nov 2007 Location: SF Bay area Posts: 1,887 Thanks: 3 Thanked 1 Time in 1 Post Rep Power: 8
Re: practicing with sextant on land

Whew, just finished reducing my sight of Venus, shot 40 minutes ago :P Actually it's my first sight reduction done on one of my own sights, and my first star sight! I used an artificial horizon but unfortunately there was quite a breeze running... oh well.

Anyway, since you were mentioning sight reduction the other day I thought I'd share the process I used, transcribed from the front and back of an envelope

I'm thinking I will keep a sort of running fix. I'm starting with a DR of 49°N, 123°W, and will update it whenever I can take a new sight, always shooting from my patio.

Index error was 2.6' on the arc.
Hs at 10:20 pm PDT was 25°40.7'
Index correction of -2.6' gives 26°38.1'
Divide by 2 for artificial horizon, gives Ha = 12°49.0'.

(NB: not sure if that's the right order, but I *think* index error "only applies once" in an artificial horizon measurement)

Correction for refraction (not corrected for temp or pressure since it's probably close enough) is -4.2', which gives 12°44.8'.

Parallax correction for HP = 0.41' is +0.4', which gives Ho = 12°45.2'.

Sight was taken at 0520 on May 10 (GMT). 20m is 1/3 of the hour.
At 0500, GHA was 220°47.6' and Dec was 27°41.9'; d=0.1' so no interpolation is necessary for Dec.
v is 1.9', so GHA correction is 1/3 of 15° + 1/3 of 1.9', or 5°0.6'. This gives a GHA of 225°48.2', and LHA = GHA - APLong = 103° gives an AP longitude of 122°48.2' W.

Page 69 of Pub 249 (87 in the PDF) is for Lat 49° Dec (15°-29°) SAME name as latitude. I look in the 27° column for row 103 and find
Hc = 12°11'
d = 46'
Z = 63° (Zn = 360° - Z = 297°)

The d correction of 46' per degree of declination, combined with 42' of declination, makes for a correction to Hc of +35', so Hc = 12°46.0'.

Hc > Ho gives an intercept of 0.8' away from 297°, which is not great considering where my apartment actually is :P Partially it's the wavy surface of the water I used, and partially I'm not just particularly good at this yet, but I also think there are some uncorrected errors in my sextant. The intercept should have been something like 12 or 13 nm toward.

Maybe tomorrow I'll photocopy a chart from the library and start plotting LOPs on it.
__________________
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27
#22
05-10-2012
 AdamLein Senior Member Join Date: Nov 2007 Location: SF Bay area Posts: 1,887 Thanks: 3 Thanked 1 Time in 1 Post Rep Power: 8
Quote:
 Originally Posted by milutin:869591 you do not need chart, you can print ploting sheet and do it.
True, but I don't actually have a printer either. Actually Bowditch has instructions for constructing a plotting sheet for any latitude, and it's a good exercise.
__________________
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27
#23
01-03-2013
 fredf Junior Member Join Date: Jan 2013 Location: eugene or Posts: 1 Thanks: 0 Thanked 0 Times in 0 Posts Rep Power: 0
Re: practicing with sextant on land

This is a rather old thread but I ran into it while looking for ways to do simple sextant practice sights.

Being a novice I want to use my sextant, on land, and get near instant feedback as to my technique. I am not particularly interested, at this point, in generating LOPs or even doing more than 1 sight at a time. My goal is to hone my sextant skills without the associated complicating factors of the computations. I found:

TeaCup Navigator by Roger Farley ( I am new on this site so cannot post a link) is a a free PC based navigational app with a bunch of problem solving sub-apps useful to a sailor. What I am finding extremely helpful is its Sight Planner in which one can view the azimuth and (most useful) the elevation of sun, moon, planets and major stars visible in the local sky. One can then compare the sextant data with the sky object data from TeaCup for that moment. I also measure, with the sextant, an arc distance between 2 sky objects and, again, compare the readings to the data from the Sight Planner. Quick and easy!

Of course, after I acquire a bit more dexterity, I shall use TeaCup Navigator to crunch the data to derive a position.
#24
01-04-2013
 Flybyknight Senior Member Join Date: Nov 2005 Location: Milton, DE Posts: 592 Thanks: 2 Thanked 7 Times in 7 Posts Rep Power: 10
Re: practicing with sextant on land

Quote:
 Originally Posted by RandyBC So I've been having a great time learning celestial navigation with my new Astra IIIb. Since I live far inland and at an elevation of 3500ft I've been practicing my sun sights with dish of water and oil. I believe that there is no dip correction when you practice this way but what effect if any will the altitude have on the final result??

None
#25
01-05-2013
 AdamLein Senior Member Join Date: Nov 2007 Location: SF Bay area Posts: 1,887 Thanks: 3 Thanked 1 Time in 1 Post Rep Power: 8
Re: practicing with sextant on land

Quote:
 Originally Posted by Flybyknight [/B] None
Actually, refraction correction is dependent on air temperature and pressure, both of which are dependent on elevation.
__________________
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27
#26
01-07-2013
 chamonix Senior Member Join Date: Jun 2010 Location: Newmarket Ontario Posts: 254 Thanks: 4 Thanked 2 Times in 2 Posts Rep Power: 5
Re: practicing with sextant on land

Got a sextant for Christmas. So have just learned Celestial navigation. Used three different books to learn. Best book I found was a book called Simple Celestial, by a CDR Chris Kreitlein (ret). Not a lot of theory (but enough), just logical , step by step instructions on sight reduction, for all sights. The reason I'm bringing it up is that it is not a widely distributed book. You have to order it from the guys website ( it's about \$12), yet it is the book that made everything clear to this not so smart guy. I have no connection to the guy, I discovered him on youtube while trying to figure it all out. Just thought I'd mention it, as I found it to be an invaluable resource.
To be honest I haven't taken a sight yet, it's to bloody cold to mess around outside in Ontario. I'll have to wait for spring.
by the way the book had the best sight forms I came across. You can download them free from his website.
#27
01-07-2013
 AdamLein Senior Member Join Date: Nov 2007 Location: SF Bay area Posts: 1,887 Thanks: 3 Thanked 1 Time in 1 Post Rep Power: 8
Re: practicing with sextant on land

Quote:
 Originally Posted by chamonix To be honest I haven't taken a sight yet, it's to bloody cold to mess around outside in Ontario. I'll have to wait for spring.
Can you at least see the sky? I'm starting to forget what stars look like.

Thanks for the tips.
__________________
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27
#28
01-15-2013
 Flybyknight Senior Member Join Date: Nov 2005 Location: Milton, DE Posts: 592 Thanks: 2 Thanked 7 Times in 7 Posts Rep Power: 10
Re: practicing with sextant on land

Quote:
 Originally Posted by AdamLein Actually, refraction correction is dependent on air temperature and pressure, both of which are dependent on elevation.
What are you busting my balls for?
That was not the intent of the guy's question.
You always take into consideration temp & refraction whether on a boat or on top of a mountain.
Bug off.
Dick
#29
01-17-2013
 castorp delivery & race Join Date: Sep 2011 Location: Newport RI, USA Posts: 23 Thanks: 0 Thanked 0 Times in 0 Posts Rep Power: 0
Re: practicing with sextant on land

Teacupnav is a neat app!
thanks for sharing!!
has anyone tried building Farley's octant?
#30
02-28-2013
 PaulnNathalie Junior Member Join Date: Mar 2012 Posts: 5 Thanks: 0 Thanked 0 Times in 0 Posts Rep Power: 0
Re: practicing with sextant on land

Celestaire makes a practice sight for the 3b. I think they're about 50 bucks or so. I got one so I could teach my sig other. It takes a fair bit of practice with them to actually start getting fairly accurate, but like the saying goes: your first 1000 shots will never be as accurate as your second 1000 shots.
The sight has a leveling bubble inside with a coresponding horizontal hairling that acts as your horizon.
One bonus I found after using this practice sight for awhile: I's amazing how steady and accurate your sights are with a scope on after using this practice sight.
I do find that during times of twilight it can be difficult to locate stars of lesser magnitude because of the lack of light that enters into the sight, but overall for the price I think it a worthwhile purchase.
3500 ft is a fair elevation, but remember that all of those atmosphereic corrections within the nautical almanac are average constants anyway.
How do you like the 3b anyway? I think it a fantistic sextant for the price. I've used many different types while in the Canadian Navy, and I think it rivals anything that they've got. How do you think the full two-way mirror compares to the half mirror?

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