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A Guide To Navigating the Stars

How is astronomy used for navigation?

The use of the stars for navigation – by sea and by land – has been documented since the time of the Mesopotamian empires, but it is likely that it has been a common practice since the Neolithic, if not earlier.
Often treated as a complicated subject and only for people who have studied the constellations and the celestial vault for a long time, in its most basic form, the use of the stars for navigation is very simple.

When you navigate “on sight”, you look for a unique and easily recognizable point of reference and use it to orient yourself.
The North Star represents this unique landmark – and coincides with the geographic north of the Earth. Unlike all the other stars in the sky, it appears motionless – all the other stars seem to revolve around it.

Finding the North Star is not overly difficult – on a clear night and with minimal interference from artificial lights, it is sufficient to locate the Big Dipper (which Anglo-Saxons call “Big Dipper”) and extend the line defined by the two stars aligned at the bottom. ‘end. The Big Dipper moves across the celestial vault but is always in the same relative position with respect to the North Star, the fixed point around which it rotates.

On the opposite side of the Great Bear with respect to the Polar Star is the constellation of Cassiopeia, which has a zig-zag pattern, and can help, as a reference, the Polar Star.

The elevation (or distance) between the North Star and the horizon in the Northern Hemisphere is exactly equal to the latitude.
If the North Star is 45 ° above the horizon, our latitude is 45 ° north.

There are many different methods for measuring a star’s elevation above the horizon – and the sextant has been the instrument used for this type of measurement, in one form or another, since time immemorial.
In a very empirical way, and in a very hasty way, it is however possible to measure the elevation in degrees using a fist.
A hand clenched into a fist, and held with the arm outstretched in front of us, is equivalent to about 10 degrees of elevation.
We can therefore determine our latitude more or less precisely by extending an arm in front of us, with the clenched fist in such a position that it rests on the horizon, and then verify how many “fists” separate the Polar Star from the horizon.

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