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View Full Version : How much is a point?

almeyer
07-23-2007, 09:53 PM
As in, 2 points off the starboard bow?
Thanks,
Al

Hwyl
07-23-2007, 10:09 PM
eleven and a quarter degrees. It's the difference between old compass points like NNE and NE

Nicholas Carey
07-23-2007, 10:11 PM
32 points to the sailor's compass.

360 degrees of arc to a circle.

360 degrees divided into 32 divisions makes it 11 degrees 15 minutes of arc per point.

The points have names: Naming the points, in order is called Boxing The Compass (http://en.wikipedia.org/wiki/Boxing_the_compass). More at http://www.gwpda.org/naval/boxco000.htm#Compass

http://www.gwpda.org/naval/boxc01.gif

http://www.gwpda.org/naval/boxc02.gif

almeyer
07-23-2007, 10:22 PM
Dang, that was quick! I've spent half the night searching the internet. Should have come here first.
Thanks, folks.
Al

Ian McColgin
07-24-2007, 06:56 AM
The break down is to quarter points. The cardinal points are the four, north-east-south-west.

One handy thing about points is that they are like inches on the horizon. If you extent your hand and arrange the index finger such that the second bone is horizontal, that handy human inch will subtend a quarter point. Your fist, about 4", will subtend a point. Try rocking your fist across the horizon and you'll get the eight points from dead ahead to clear abeam.

I never have understood why we got the 360 degree circle.

The other nice thing about points is that it's really easy to do reciprocals or beam bearings in your head, once you can box the compass - which is not, repeat NOT, a variation on the foxtrot invented by that notorious hooker Compass Rose.

JimD
07-24-2007, 07:59 AM
No its not! Its one quarter of 45 degrees.

Bill Perkins
07-24-2007, 08:40 AM
Cardinals (N,S,E,W ) , Intercardinals (NE,SE,SW,NW ) , Combination Points ( NNE, ENE, etc. ). That's as far as I and the weather reporters go with that . I like that chart of the bearings . Logically I guess I should be prepared to describe them within the nearest 2 points as well .

I've read that the 360 deg.circle derives from the Babylonian calender. They reckoned there were 12 , 30 day months in a year ,the entire circular trip taking 360 days .

I've recently read a good book ; Capt Bill Brogdon's "Boat Navigation for the rest of us " (recommended to me here ). Among much else , the captain shares a technique for figuring reciprocals when you're on the spot .If the current course is less than 180 , add 200 and subtract 20 . Steering more than 180, subtract 200 and add 20 .

rbgarr
07-24-2007, 09:43 AM
No its not! Its one quarter of 45 degrees.

You're joking, right? (A quarter of 45 is 11 1/4, i.e., 11 degrees 15 minutes)

George.
07-24-2007, 09:51 AM
A quarter point is also about the limit that a good helmsman can steer in a small boat at sea. A hlaf point is more like it - the compass card itself will rotate trhough more than that if there is a sea running.

Courses given as, say, "281 degrees" are in practice less useful than something like "North by Northeast, a half east."

Thorne
07-24-2007, 10:47 AM
We got the 360 degree circle from the Babylonians, who figured that there were 360 days in the year and 'sub-divided' the horizon into that number.

From Wikipedia -

Ancient calendars

Old Persian inscriptions and tablets indicate that early Iranians used a 360-day calendar based on the Babylonian (http://en.wikipedia.org/wiki/Babylonian) system and modified for their beliefs and named days. Months had two or three divisions depending on the phase of the moon. Twelve months of 30 days were named for festivals or activities of the pastoral (http://en.wikipedia.org/wiki/Pastoral) year. A 13th month was added every six years to keep the calendar synchronized with the seasons.

http://en.wikipedia.org/wiki/Babylonian_numerals

rbgarr
07-24-2007, 10:58 AM
Courses given as, say, "281 degrees" are in practice less useful than something like "North by Northeast, a half east."

LOL. 281 degrees is "shy West by North", but your point is sort of true, if the helmsman knows what the navigator means.

But not always, especially with autopilots and GPS now:

http://i16.tinypic.com/6gyiivb.jpg

bamamick
07-24-2007, 12:46 PM
could tell us!

Well, maybe it's a bad joke in poor taste, but dang it, I needed a joke just now (rough day at work. Very rough).

Mickey Lake

Nicholas Carey
07-24-2007, 03:01 PM
We got the 360 degree circle from the Babylonians, who figured that there were 360 days in the year and 'sub-divided' the horizon into that number.Probably more importantly, the Babylonians liked 12 for one convenient reason: 12 (and its multiples) can divided into whole units by 2, 3, 4 -- that's is, anything that's a multiple of twelve can easily be halved, quartered or divided into thirds. 360 is a multiple of 12.

The divisibility of 12 is why we have 12 inches to the foot.

robtcw
07-24-2007, 05:44 PM
My foot is more than 12 inches.

George.
07-25-2007, 06:56 AM
360 degrees actually came from the sky. The Babilonians reckoned the year at 360 days, and thus divided up the sky into 360 degrees. Each degree was the distance the sun moved in one day against the fixed stars. That way astrologers could figure out where the sun was at any given day.

Fortunately for future trigonometry students, when later astronomers figured out that the sun really took 365 1/4 days to go around the ecliptic, they left the number of degrees in a circle alone. :D

Steve Lansdowne
07-25-2007, 09:53 PM
Gee, Al, see what you started!

rbgarr
07-25-2007, 10:08 PM
This always happens, doesn't it!? (At least he got his answer early on.)

ssor
07-25-2007, 10:12 PM
The french figured 400 degrees and a circumference of earth at 40,000 kilometers at the equator.

almeyer
07-26-2007, 10:42 PM
Gee, Al, see what you started!

Rgbarr's right, I got the answer I was looking for, and an education as well. Until it was mentioned about 360 degrees between divided by the 32 compass points, I would have thought that 11 1/4 degrees per point was very odd. Now it makes sense.

If a helmsman can steer within a quarter point, he's a much better sailor than I am (which isn't saying much, considering my still-greenhorn status). I'm doing good if my very-inexpensive kayaker's compass and my also-very-inexpensive GPS are within 5 degrees, and they are often 10 degrees apart. I realize that one is telling me which direction the boat is pointed, and the other is telling me the track I've already made, but surely I'm not making that much leeway?

Al

Bill Perkins
07-26-2007, 10:52 PM
Al ;you may want to go to the setup page of your GPS and make sure it's set to read in magnetic rather than true degrees ,if you want it to match the compass ,which is convenient .If you hold the GPS too close to the compass it may also be throwing it off . Bill

Wild Wassa
07-28-2007, 02:31 PM
"Courses given as, say, "281 degrees" are in practice less useful than something like ..."

... something like this?

When I'm the navigator during a race and we cant see the marks because of the swell or the distance, I'll tell the helm and the Skipper exactly how many degrees we are above and below the mark. Often I'll say that we are 1 degree or 2 degrees below the mark and "gain the height now before it bites us." Paying attention to even one degree matters heaps, ... and "how much is 1 point?" Well, 1 point is like being from 'here to Christmas'.

Warren.

JimD
07-28-2007, 02:34 PM
You're joking, right? (A quarter of 45 is 11 1/4, i.e., 11 degrees 15 minutes)

I'm either joking or I'm very stupid.

Bruce Hooke
07-28-2007, 03:00 PM
"Courses given as, say, "281 degrees" are in practice less useful than something like ..."

... something like this?

When I'm the navigator during a race and we cant see the marks because of the swell or the distance, I'll tell the helm and the Skipper exactly how many degrees we are above and below the mark. Often I'll say that we are 1 degree or 2 degrees below the mark and "gain the height now before it bites us." Paying attention to even one degree matters heaps, ... and "how much is 1 point?" Well, 1 point is like being from 'here to Christmas'.

Warren.

Which also illustrates the difference between racing and cruising. Yes, at times a degree or two matters in cruising, but 9 times out of 10 it does not.

almeyer
07-28-2007, 07:42 PM
I'll agree that racing and cruising are different games. And a degree or two can make quite a difference over a long distance, but fortunately I'm sailing in such small waters that if I'm a little off, it won't hurt me too bad.
Bill, I checked my GPS and it's set to mag north.
Al

Ian McColgin
07-28-2007, 10:14 PM
Points are wonderfully descriptive and have their conveniences but navigation is done in degrees.

A compass marked down to quarter points often has degrees around the rim, so you have both anyway. But who can even easily see what they are steering to with all that clutter? Only compasses with a verge ring and central lines that can be made parallel to the fleur de lies are then easily steered to.

If not that, the modern yacht compass that White developed - marked to five degrees - is more precisely and, by interpolation with the advantage of better precision, more accurately steered to, any course, not just those ending in zero or five. Even there, it's well to have a verge ring or dumb compass around the compass to show what your aiming at just as a memonic.

Especially if you keep your skills up so's you can still navigate if DoD takes down all the satilites or your electronics are fried in a lightening strike or just generally turn to green ginge, an uncluttered compass is the route to precise accurace, not some rose with too much clutter or some electro marvel with spurious precision of uncertain accuracy.

Analog forever.

almeyer
07-29-2007, 10:19 PM
Some good comments on this thread. I can see the use of both points and navigating in degrees. The point system would be convenient when noting potential obstacles with respect to the boat. For instance, "There's a crab trap float at two points off the port bow." But for navigating, especially for destinations out of easy eyesight, degrees would seem to be much more practical.

Ian's comment about analog systems also rings true. As an engineer, I use computers every day to perform some fairly serious number-crunching. Easy and convenient, but I have a distrust of a lot of the programs, so I'll spot-check some numbers with a hand-held calculator. If I get the same answer with both the program and the hand-held, I'm fairly confident. I would suspect the same holds true for navigating, although my experience to date is somewhat limited. GPS is a neat toy, but it would be just my luck that the batteries crap out, or some other glitch happens, when I need it most. It's good to have a backup that you can rely on.
Al

John R - Kitenui
07-30-2007, 06:57 AM
Al - Tide set, more than leeway can cause amazing differences between 'course over ground' and 'boat heading'

Nicholas Carey
07-30-2007, 01:23 PM
Al - Tide set, more than leeway can cause amazing differences between 'course over ground' and 'boat heading'Once upon a time, I was coming under power around Point Wilson into Port Townsend (WA) from the Straight of Juan de Fuca on a flood tide with a strong tidal current running.

The compass heading and the true track over the ground displayed by the GPS had a slight disagreement :D of about 45 degrees or so. To hold the actual course we wanted, the boat had to be all cattywumpous WRT to our track over the bottom.