Navigation

When navigating do they use sideral time your standard time to find
longitude.
I`am refering to the sextants.


Thanks Craig

Craig wrote:

When navigating do they use sideral time your standard time to find
longitude.
I`am refering to the sextants.

Thanks Craig

UTC

Greenwich Mean Time (UTC).

Navigators have the Nautical Almanac and Air Almanac to tell them the
altitude of various stars at various times and longitudes.

As I understand it, the usual technique is this:

(1) Estimate your position to about 1 degree, or find the nearest position
listed in the almanac. (If several degrees off, don't worry; you can simply
do the whole procedure twice for greater accuracy.)

(2) Look up the altitude of a star at a particular time at that position.

(3) At that time, measure its actual altitude from *your* position.

(4) If it's off by N arc-seconds, you know you are on a line that runs
perpendicular to the direction of the star and passes N nautical miles from
the originally estimated position. (If the star is too high, the line runs
between the est. posn. and the star; if too low, it's on the opposite side
of the est. posn., away from the star.)

(5) Do the same for another star in a different direction.

Where the lines cross is your position. If it was a long way away from the
estimate, do the whole procedure again starting from a better estimate.

"Michael A. Covington" wrote
in message ...

(4) If it's off by N arc-seconds, you know you are on a line that runs
perpendicular to the direction of the star and passes N nautical miles
from

Sorry, I mean arc-minutes. Sextants aren't *that* accurate!

A nautical mile is an arc-minute on the globe of the earth.

Michael A. Covington wrote:
A nautical mile is an arc-minute
on the globe of the earth.

However, the earth is not a true sphere. It is flattened like a
spinning top and has bumps and hollows like a potato. Thus there is no
constant relationship between angle at the core and distance at the
surface. A minute of arc at one point can be tens of metres different
to a minute of arc at another.

This uncertainty became unacceptable decades ago as navigation became
more precise. The superceded definition remains a convenience, but the
current definition no longer refers to angle. It is a fixed distance.

1 nautical mile = 1852 metres precisely
http://www.bipm.org/en/si/si_brochur...r4/table8.html

PN
From: (Pat Norton)
PN
Subject: Navigation
PN
Date: 9 Nov 2003 06:36:41 -0800
PN
Organization: http://groups.google.com
PN
PN Michael A. Covington wrote:
PN A nautical mile is an arc-minute
PN on the globe of the earth.
PN
PN However, the earth is not a true sphere. It is flattened like a
PN spinning top and has bumps and hollows like a potato. Thus there is no
PN constant relationship between angle at the core and distance at the
PN surface. A minute of arc at one point can be tens of metres different
PN to a minute of arc at another.
PN
PN This uncertainty became unacceptable decades ago as navigation became
PN more precise. The superceded definition remains a convenience, but the
PN current definition no longer refers to angle. It is a fixed distance.
PN
PN 1 nautical mile = 1852 metres precisely
PN http://www.bipm.org/en/si/si_brochur...r4/table8.html

All true enough. Yet, for sea navigation we let the Earth be a
sphere and all the geometry is neat and clean. Note that more or less,
the length of the nautical mile (one arc minute) makes one arcsecond
equal to quite 30m. Many of us here have properties several arcseconds
in size.

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