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Old 04-08-2012
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Re: practicing with sextant on land

That was exactly it, the moon wasn't on the meridian. Glad to know I wasn't as far out as I thought. I'm looking forward to more practice this weekend.

I'll have to check a printed copy of the Nautical Almanac to see if the Android App "Celestial" includes all the data. Still lots to learn yet. I've done surveying with compass and chain but that's 2D and things on the ground don't move, whereas celestial navigation is 4D (3D with time) plus the objects don't stay still.

cheers,
-Tony
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Old 04-09-2012
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Re: practicing with sextant on land

Good analogy: one way to think about meridian passages is that the object should "stay still", at least in the sense that it isn't rising or setting for a moment. Near meridian passage, the rate of change of altitude with time is the lowest it will be for the whole time the object is visible. This means that small errors in your Ho should be less important than they would be at other times... on the flip side, it means it's harder to estimate the exact moment of meridian passage (since you do that by finding the time of maximum altitude) which means your longitude errors will be bigger.

I also don't have much experience with this stuff while underway. I know that navigators generally plan ahead what stars they will shoot and where they expect to see them. You do this by figuring out what stars will be visible a bit after sunset or a bit before sunrise. The easiest way is to look at Pub 249 Vol 1, which picks out seven stars for you. All you need to know is your approximate latitude and the GHA of the vernal equinox, listed as "Aries" in the almanac.

Regarding the almanac, I use "The Online Nautical Almanac". Not sure about its accuracy, but then again, I don't really care, and it's been accurate enough for my recreational needs Pub 249, of course, doesn't change from year to year, and the official one can be downloaded for free from Maritime Safety Information

s/v Laelia - 1978 Pearson 365 ketch
Old 04-09-2012
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Re: practicing with sextant on land

Quote:
Originally Posted by milutin View Post
i am using this almanach. i checked its accuracy with another boater HO paper almanac and it is ok, no errors.

Nice! And unlike the link I posted, it has twilight/sunrise/moonrise/etc. tables. The link I posted only has the GHA/Dec tables... but it is nice to be able to just stick in any date and get the tables right away.

s/v Laelia - 1978 Pearson 365 ketch
Old 04-09-2012
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Re: practicing with sextant on land

Hah, looking at some of the other tables available on milutin's link:

"Temperature and Pressure Factors for Refraction (Corrected Refraction = Tabulated Refraction x Factor". I like corrections on the corrections :P The take-home lesson seems to be, never shoot stars near the horizon. Above 25°12', the correction is less than 2 minutes, and above 45°33', the correction is less than 1 minute.

"Moon's Semi-Diameter Corrected for Augmentation", which is indexed by Alt and HP, followed by "Latitude Correction to Moon's HP". Awesome. You use your latitude to correct the tabulated HP, which you use to correct the moon's semi-diameter, which you use to correct your sextant altitude (which you use to calculate latitude!) Take-home lesson: the moon is not worth it.

That said I'm now getting excited to try the Lunar Distance method for calculating time. I guess I'll wait until the next first-quarter, though. One advantage is that it's sextant practice that doesn't require a horizon.

s/v Laelia - 1978 Pearson 365 ketch

Last edited by AdamLein; 04-09-2012 at 01:29 PM.
Old 04-23-2012
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Re: practicing with sextant on land

Thanks AdamLein for the detailed explanation on the intercept method. In the interlude I've been off reading more about celestial navigation and the St. Hilaire's intercept method. I'm slowly wrapping my brain around the methods. To make things easier, I've been reading about coastal navigation and running fixes. Fitting the vector representing the travel distance and bearing between two lines of position, I think, is similar to St Hilaire intercept method with the Azimuth and bearing between the deduced reckoning latitude and the observed latitude. Now that the weather has turned sunny, I've had a chance to work the mechanics of the sextant and have more data to play with. The Celestial app for Android is a bit of a black box and I need to figure out what corrections it's making and not making.

I've tried a couple of artificial horizons, one was a mirror levelled with a digital level and the second was a bucket of water. The bucket was more convenient to move to sunny areas but is rippled by any breeze. The mirror took a few minutes to level each time it was moved but isn't affected by brreeze.

Last edited by tony9; 04-23-2012 at 03:41 AM.
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Old 04-23-2012
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Re: practicing with sextant on land

Quote:
Originally Posted by tony9 View Post
Fitting the vector representing the travel distance and bearing between two lines of position, I think, is similar to St Hilaire intercept method with the Azimuth and bearing between the deduced reckoning latitude and the observed latitude.
I think I see what you mean. In a running fix you move an LOP, keeping it parallel. In the intercept method, you do something similar although you don't draw the first LOP, just the one that results after it is moved to the intercept.

Quote:
I've tried a couple of artificial horizons, one was a mirror levelled with a digital level and the second was a bucket of water. The bucket was more convenient to move to sunny areas but is rippled by any breeze. The mirror took a few minutes to level each time it was moved but isn't affected by brreeze.
Apparently you can get an artificial horizon from Davis that is a liquid tray but also has a wind shield. It's a bit decadent for my tastes.

One thing you might try is heading to English Bay and taking sights from the beach. Sometimes the thing you're shooting will be over the western horizon and then you are set. If you want to shoot something that's over the land, you can use the Short Dip tables; I think it's Table 22 in Bowditch but I'm not sure. I've been meaning to go and do this for a while now, but never seem to get around to it. If you feel like meeting up one evening for some twilight sights, let me know.

s/v Laelia - 1978 Pearson 365 ketch
Old 05-04-2012
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Re: practicing with sextant on land

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Originally Posted by AdamLein View Post
If you feel like meeting up one evening for some twilight sights, let me know.
Just getting back to the forum, sorry for the delay in responding. That would be great to meet up on evening and compare notes. Are you thinking English Bay? Or somewhere inland?

I haven't hauled the sextant out for awhile. I had my bucket of water all set up to take some evening shots of Venus about a week ago then when I went out with the sextant, the clouds had rolled in.

-Tony

Last edited by tony9; 05-04-2012 at 09:28 PM.
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Old 05-04-2012
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Quote:
Originally Posted by tony9:867379
when I went out with the sextant, the clouds had rolled in.
Ah, so it's your fault then :-)

Anyway yes, i was thinking of the bay.

s/v Laelia - 1978 Pearson 365 ketch
Old 05-06-2012
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Re: practicing with sextant on land

It's sunny out today so filled my bucket with water and took some shots before and after apparent noon. My calculated latitude came out within 5 km of actual. Longitude was about 50 km too far east, but I haven't interpolated between the shots to find out the exact time of meridian, I just used a shot that was closest to apparent noon.

Most of the time is waiting for the water in the bucket to settle down. I wonder if covering the bucket would introduce a refraction error of the sun going through the glass and me looking at the sun through the glass. I don't want to use motor oil a that could get messy on cleanup. I tried levelling a mirror but gave up on that, just too finicky.

I took some shots last night of the moon and venus and have some data to play with. Polaris and mars were too faint to make out in the reflection in the bucket of water though I could see them in the sky. I still need to figure out the LOP method.
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Old 05-08-2012
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Re: practicing with sextant on land

Quote:
Originally Posted by tony9 View Post
Polaris and mars were too faint to make out in the reflection in the bucket of water though I could see them in the sky.
I have this problem, too. Venus yes, but nothing else is visible. The ridiculous street lamp across the street from my apartment doesn't help.

s/v Laelia - 1978 Pearson 365 ketch

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