Sextants
Question to the collective knowledge of the group. I am planning on taking
celestial navigation and was looking about for a used sextant to be a backup if
both GPS systems die. I've always been interested but never taken the time to
learn. From reading Passagemaker, it seems the morning sightings were enough
for sound bearings during his trip across the pond. Which star/planet did he
use for that sighting and is the Davis 25 a good sextant that should survive
long term storage with little attention?
Thanks as always
Len Cable
http://groups.msn.com/TrawlerMV/mventropy.msnw?Page=1
Entropy, Dutch Steel
A Davis 25 is what I have and it's perfectly good for a backup system of
navigation. One of it's main advantages is light weight. Be careful if you
buy a used one... I purchased one at a boater flea market for $5 and it was
worth every penny. Had a broken part. I purchased my "real" one new. For
long term storage, just make sure you leave the batteries for the
illuminator out.
As far as sights, use whatever works for you. Sun and moon sights are pretty
obvious, and easier to find than stars.
Keith
I am opposed to millionaires, but it would be dangerous to offer me the
position. -Mark Twain
----- Original Message -----
From: LCCableJr@aol.com
Question to the collective knowledge of the group. I am planning on taking
celestial navigation and was looking about for a used sextant to be a
backup if
both GPS systems die. I've always been interested but never taken the time
to
learn. From reading Passagemaker, it seems the morning sightings were
enough
for sound bearings during his trip across the pond. Which star/planet did
he
use for that sighting and is the Davis 25 a good sextant that should
survive
long term storage with little attention?
Len,
Take up celestial for the fun of it, but IMHO unless you practice a lot
and have an up to date almanac it's no good as a "backup". You can skip
all the rest of this email as I don't have much more of value to say
than that.
We finally blew the dust off of and sold our old Astro III sextant after
I had re-taught myself celestial at least four times. We never did use
it in anger (oops, wrong thread).
Not only do you need the sextant, and the current almanac, but don't
forget the sight reduction tables. Now, you could buy one of those
little calculators that does this all for you (check the Celestaire
catalogue or web site), which in the long run is probably better than
buying almanacs. But, now you need spare batteries for backup. Or you
could rely on your confuser...but do you really trust Bill Gates to come
through when the chips (or satellites) are down?
If it is for recreation then a plastic sextant will do just fine,
although I'd spend a few more bucks and get a metal one. You'd look
really salty and people might even think you know what you're doing if
you got a metal sextant, worked for me. You could scrape the black
paint off and folks might even think you got a really expensive one, or
one off a WW-II aircraft carrier. I'm not a big fan of plastic sextants
for serious use, which is why we had our trusty Astro. If you do use a
plastic one for serious work, be sure to check the instrument error
before AND after your sight, as they are known to be a bit more unstable
than even a cheaper metal one.
Bear in mind also, that in skilled hands you can't get much closer than
a few miles from your actual position, and you are never sure. I think
I'd puff my chest out if I got within 40 miles, which is not very
accurate in these days of precise e-navigation. Anyway, when you take
your sights, reduce them, and figure your actual position, announce it
with authority, and add "plus or minus a mile or two"; others will never
know the difference.
Crossing open water or even oceans is easy; there's not much to hit and
no particular need to adhere to a tight rhumb line. Although many do,
we don't let the GPS or the confuser steer the boat and sometimes we
wander all over the place. Before we got all these new fangled gadgets
in our yachts it was no shame to hail a passing ship or another yacht
and find out where you are! Being off course by miles does not bother
me; it's the hard bits that can do the damage.
When we are near shore we like to know exactly where we are. The
sextant won't do you any good there, except for maybe taking some
horizontal angles of charted land features or working out distance off
from known tall objects. Of course, if all you have is vector charts,
half the good stuff on the land is missing, so you can't really use it
without a GPS anyway. Did I mention that we have a bunch of really old
and totally unused charts for sale?
Well, enough, I couldn't sleep and couldn't resist putting my oar into
this one. Enjoy your sextant, but get a third GPS for real back-up. If
the big one comes and Bush switches off the satellites, knowing where we
are will be the least of our worries. Come to think of it I think I'll
buy a new sextant and keep it with my AK-47.
Cheers,
Maurice
-----Original Message-----
From: LCCableJr@aol.com [mailto:LCCableJr@aol.com]
Sent: Saturday, 03 December 2005 02:59
To: trawlers-and-trawlering@lists.samurai.com
Subject: T&T: Sextants
Question to the collective knowledge of the group. I am planning on
taking
celestial navigation and was looking about for a used sextant to be a
backup if
both GPS systems die. I've always been interested but never taken the
time to
learn. From reading Passagemaker, it seems the morning sightings were
enough
for sound bearings during his trip across the pond. Which star/planet
did he
use for that sighting and is the Davis 25 a good sextant that should
survive
long term storage with little attention?
Thanks as always
Len Cable
http://groups.msn.com/TrawlerMV/mventropy.msnw?Page=1
Entropy, Dutch Steel
http://lists.samurai.com/mailman/listinfo/trawlers-and-trawlering
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-----Original Message-----
From: Maurice on AKAMA
Take up celestial for the fun of it, but IMHO unless you practice a lot
and have an up to date almanac it's no good as a "backup".
REPLY
Although Maurices's post was half in jest, the sextant, plastic or
otherwise is indeed a good backup.
Bowditch has tables for distance off using a vertical sextant angle.
These tables also take into account the curvature of the Earth and
therefore how much of a distant mountain is below the visible horizon.
As Maurice said, the danger is not in the open ocean but closer to
shore.
Assuming total electronic failure and a flaky magnetic compass, the
sextant can still provide you with noon time elevation of the sun for a
latitude sight.
Remember the Viking, Arabs and early European navigators all relied on
taking a vertical angle of the sun at noon and then sailing down their
latitude to reach distant destination.
Similarly sighting on Polaris gives you a pretty close approximation for
true north to check your compass. At least that way you will know how
much error it has.
Closer to shore the horizontal angle between two or three known objects
is a very precise method of determining your position.
Conversely if you have an accurate compass and one vertical angle you do
have an approximate fix (range and bearing)
The few tables you need from Bowditch can be laminated for preservation
even if the boat gets rolled and everything inside gets soaked or of you
are in an open life boat.
The navy used to teach these techniques under the heading of "lifeboat
Navigation" don't know if they still do. But such techniques still have
a place on recreational vessels.
Arild
From reading Passagemaker, it seems the morning sightings were enough
for sound bearings during his trip across the pond. Which star/
planet did he
use for that sighting and is the Davis 25 a good sextant that
should survive
long term storage with little attention?
The tricky part about using a sextant to "back up" your GPS is that
achieving a good result with astro requires regular practice. If you
are going to rely on astro for serious use, it might be a good idea
to take sights and work up your position every few days (daily?) in
parallel with your GPS. Don't get discouraged when you come up miles
out either, just keep practising. So your sextant won't require "long
term storage" will it? :)
The Davis 25 is a perfectly satisfactory instrument. Metal sextants
are nice, and make you feel like Captain Cook, but any difference in
accuracy will almost certainly be less than your "personal error" in
taking and working the sight.
Sun sights are are very traditional, especially the noon altitude,
but star sights at dawn and evening are perfectly possible from a
yacht and have the advantage that it's relatively easy to get
multiple position lines in a short time if you're well organised. The
particular stars you'll use will depend on location, time of year and
other circumstances. You can look up and record the altitude and
azimuth of the stars you want to use in advance, which makes it
easier to find them. In many ways getting organised to record your
sights, especially working solo, is the bit that takes the most
practice.
Don't forget that you'll need sight reduction tables, maybe a star-
finder, and a current Nautical Almanac that you'll need to replace
annually. Oh, and an accurate timepiece too.
Also don't forget to brush up on your traditional coastal position
fixing and navigation if you're assuming that your GPS will be
unavailable, and keep suitable charts that show traditional coastal
landmarks. Ocean navigation is fairly relaxed; It's that nasty hard
stuff round the edges of the sea that'll sink you!
I love astro, but I'd probably go with multiple redundant GPS units
as a backup...
Best regards, Robert Bryett
Sydney, Australia.
rbryett@mail.com
Call me a teck-weenie, but I have 2 Garmin GPS units, 2 Deluo GPS units, 1
Magellan GPS unit and a Micrologic LORAN C aboard. I gave my sextant away a
few years ago and concluded the best backup for a GPS is another GPS! YMMV!
Regards....
Phil Rosch
Old Harbor Consulting
M/V "Curmudgeon" MT44 TC
Currently lying Bond Creek, NC
Friday, December 2, 2005, 5:59:26 AM, LCCableJr wrote:
Lac> Question to the collective knowledge of the group. I am planning on taking
Lac> celestial navigation and was looking about for a used sextant to be a backup if
Lac> both GPS systems die. I've always been interested but never taken the time to
Lac> learn. From reading Passagemaker, it seems the morning sightings were enough
Lac> for sound bearings during his trip across the pond. Which star/planet did he
Lac> use for that sighting
Haven't read the article you refer to, but you can take mid-morning, noon and afternoon sights on the sun, and advance the earlier lines of position to the time of the last sight to get a running fix.
A single sight only gives you a line of position - you need two lines of position crossing to get a fix.
A (partial) exception is a noon sight - the maximum altitude of the sun can be used to calculate the latitude, and the time at which that altitude was reached will give the longitude. However, to do this, you need to take a series of sights around local apparent noon to determine this information.
--
Peter Bennett, VE7CEI Vancouver, B.C., Canada
Lien Hwa 28 (AKA Polaris 30) "Sea Spray"
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca