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What Is a 'Nautical Mile'? (2011)

science.howstuffworks.com

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Re: What Is a 'Nautical Mile'? (2011)

#11

Post by taejo »

chrisseaton wrote: post #9
codeduck wrote: post #5

> A minute of arc on the planet Earth is 1 nautical mile. Only if that minute of arc lies on a great circle route. As an interesting addendum, when measuring distance made good on a nautical chart, you will use a divider to measure the straight line distance, then lay this onto the latitude scale to convert it to nautical miles. If you were to use the longitude scale, you would be off by approximately the cosine of y…

I can believe that it's true that lines of longitude are not great circles, but I can't picture why in my head. Isn't a great circle formed by extending the shortest distance between two points on the surface of a sphere?

Lines of longitude are great circles. Grandparent must mean latitude: the equator is a line of latitude, and the only one that is a great circle.

Re: What Is a 'Nautical Mile'? (2011)

#12

Post by codeduck »

pmikesell wrote: post #10
codeduck wrote: post #5

> A minute of arc on the planet Earth is 1 nautical mile. Only if that minute of arc lies on a great circle route. As an interesting addendum, when measuring distance made good on a nautical chart, you will use a divider to measure the straight line distance, then lay this onto the latitude scale to convert it to nautical miles. If you were to use the longitude scale, you would be off by approximately the cosine of y…

Also at the equator - the earth is not a perfect sphere.

No, but it's not oblate enough for the difference to cause any noticeable problems in navigation. The difference between an arc length of a minute at the equator vs a minute on the latitude scale is about 0.2% - less than the margin of error when reading a sextant.

Re: What Is a 'Nautical Mile'? (2011)

#13

Post by codeduck »

chrisseaton wrote: post #9
codeduck wrote: post #5

> A minute of arc on the planet Earth is 1 nautical mile. Only if that minute of arc lies on a great circle route. As an interesting addendum, when measuring distance made good on a nautical chart, you will use a divider to measure the straight line distance, then lay this onto the latitude scale to convert it to nautical miles. If you were to use the longitude scale, you would be off by approximately the cosine of y…

I can believe that it's true that lines of longitude are not great circles, but I can't picture why in my head. Isn't a great circle formed by extending the shortest distance between two points on the surface of a sphere?

An arc along a line of longitude is a deviation in the latitude scale. Lines of longitude lie on great circles since they converge on the poles, but when you move a degree of longitude you are moving west or east along a line of latitude, and the only line of latitude which is a great circle is the equator.

The reason you use the latitude scale is that lines of latitude are equidistant between one another - it is the same distance between two lines of latitude no matter your longitude, but it is not equidistant between two lines of longitude at different latitudes. So even if your chart spans several hundreds of miles, the latitude scale remains constant across that distance regardless of the distortions of the projection used to render the globe onto the two dimensional chart surface.

Re: What Is a 'Nautical Mile'? (2011)

#14

Post by codeduck »

taejo wrote: post #11
chrisseaton wrote: post #9

Earlier quoted context omitted.

I can believe that it's true that lines of longitude are not great circles, but I can't picture why in my head. Isn't a great circle formed by extending the shortest distance between two points on the surface of a sphere?

Lines of longitude are great circles. Grandparent must mean latitude: the equator is a line of latitude, and the only one that is a great circle.

Thanks, I've corrected my previous comment.

Re: What Is a 'Nautical Mile'? (2011)

#15

Post by pmikesell »

codeduck wrote: post #12
pmikesell wrote: post #10

Earlier quoted context omitted.

Also at the equator - the earth is not a perfect sphere.

No, but it's not oblate enough for the difference to cause any noticeable problems in navigation. The difference between an arc length of a minute at the equator vs a minute on the latitude scale is about 0.2% - less than the margin of error when reading a sextant.

What I mean is, at least in the US it is defined as an arc minute at the equator, not just any great circle.

Re: What Is a 'Nautical Mile'? (2011)

#16

Post by codeduck »

pmikesell wrote: post #15
codeduck wrote: post #12

Earlier quoted context omitted.

No, but it's not oblate enough for the difference to cause any noticeable problems in navigation. The difference between an arc length of a minute at the equator vs a minute on the latitude scale is about 0.2% - less than the margin of error when reading a sextant.

What I mean is, at least in the US it is defined as an arc minute at the equator, not just any great circle.

True; I think that's the global definition anyway.

Re: What Is a 'Nautical Mile'? (2011)

#17

Post by cmdkeen »

codeduck wrote: post #5

> A minute of arc on the planet Earth is 1 nautical mile. Only if that minute of arc lies on a great circle route. As an interesting addendum, when measuring distance made good on a nautical chart, you will use a divider to measure the straight line distance, then lay this onto the latitude scale to convert it to nautical miles. If you were to use the longitude scale, you would be off by approximately the cosine of y…

It's a really satisfying thing to work with paper charts, rolling rulers and dividers. These days for proper navigational planning it is all ECDIS (or WECDIS for really fun stuff) and electronic.

Re: What Is a 'Nautical Mile'? (2011)

#18

Post by codeduck »

cmdkeen wrote: post #17
codeduck wrote: post #5

> A minute of arc on the planet Earth is 1 nautical mile. Only if that minute of arc lies on a great circle route. As an interesting addendum, when measuring distance made good on a nautical chart, you will use a divider to measure the straight line distance, then lay this onto the latitude scale to convert it to nautical miles. If you were to use the longitude scale, you would be off by approximately the cosine of y…

It's a really satisfying thing to work with paper charts, rolling rulers and dividers. These days for proper navigational planning it is all ECDIS (or WECDIS for really fun stuff) and electronic.

I loved learning navigation. You peel away a layer and a new, more complicated set of skills presents itself.

Re: What Is a 'Nautical Mile'? (2011)

#19

Post by FabHK »

roelschroeven wrote: post #3
leoc wrote: post #2

The metre was originally supposed to be based on a division of the Earth's circumference as well.

Indeed, IIRC the intention was to define the meter so that the distance between the North Pole and the equator at the meridian through Paris would be exactly 10000 km, making the full circumference along that meridian and the one opposite it 40000 km. EDIT: that information is already in the article, making my comment redundant and unnecessary.

Still bears highlighting, I think:

From pole to equator it's 90 (deg) * 60 (minutes/deg) = 5400 nautical miles or 10000 kilometres.

Pretty much the only way I can remember the conversion factor. Well, that's not true, 1.852 also consists of the country code of the USA followed by the country code of Hong Kong, so that's easy.

Re: What Is a 'Nautical Mile'? (2011)

#20

Post by topbanana »

codeduck wrote: post #18
cmdkeen wrote: post #17

Earlier quoted context omitted.

It's a really satisfying thing to work with paper charts, rolling rulers and dividers. These days for proper navigational planning it is all ECDIS (or WECDIS for really fun stuff) and electronic.

I loved learning navigation. You peel away a layer and a new, more complicated set of skills presents itself.

It's good fun. Plotting tidal vectors to calculate course to steer is very satisfying, especially when you end up in roughly the right place!
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