distance between degrees latitude

[kangaroota]

Does one degree of latitude equal 60 nautical miles?

If it does, and there are 90 degrees between the equator and each pole, there must be a discrepancy given that the earth is not truly spherical. Rather it is "pear" shaped, (which unfortunately is where I'm heading!).

Therefore, the number of degrees are the same but the distance per degree must be different.

Is there anyone standing on the equator with a GPS?

[CBLong]

From here (http://en.wikipedia.org/wiki/Nautical_miles):

A nautical mile is approximately a minute of arc along a great circle of the Earth and was formerly defined so. The earth is not a perfect sphere, so a minute of arc can be less than, or more than, a nautical mile by a few metres.


cbl.

[wobblyprop]

please ignore this as its not the right formula for kanga's question and deleting it makes FFF's post make no sense.

from memory

distance *(latitude*cosine)

springs to mind.


if you have 60nm at the equator you will have 30nm at 60degrees north or south :

60 * (60cosine) = 30nm

at it follows that

60 * (90cosine) = 0nm

[FlyingForFun]

Wobbly,

I think your memory is about right, except for one thing... that's the formula for distance over a change in longitude at a given latitude. Kanga's asking about the distance over a change in latitude. ;)

FFF
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[wobblyprop]

arse, don't keep my notes at work. back to google.

[bookworm]

Therefore, the number of degrees are the same but the distance per degree must be different.

Yup. For that reason there have been a number of different nautical miles defined. IIRC there was an equatorial nautical mile (one minute at the equator), an admiralty nautical mile (6082 ft, one minute at about 50N) and we normally use the international nautical mile (1852 m)

[Pirate]

The original poster is quite correct in that the earth is not a perfect sphere. Over the span of my flying career, the observed wisdom on the shape has changed from flattened at both poles, through pear-shaped to terroidal (i.e. earth-shaped!). People who design navigation systems and cartographers need something more regular on which to base calculations.

Originally a perfect sphere sufficed but in the age of centimetric accuracy and the computing power available to do the sums, an "osculating spheroid" has been developed. This is essentially the harmonisation of the knobbly true earth with the perfect sphere. Osculating comes from the Latin word for kiss - essentially the two superimposed spheres touch occasionally. It isn't perfect but it's the best so far and it seems to work.

To answer the original question, a nautical mile is still defined as one minute of arc along a great circle which is more than satisfactory for all navigation purposes unless you are constructing a precision database. The philosophical question is whether you can actually have a great circle on other than a perfect sphere.

Confundemus:confused: