How do I navigate with the stars

Navigating by the stars



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Since I was asked about this team several times, I would like to give a brief overview here. Since the beginning of seafaring, the sailor has orientated himself to the sun and stars, whereby the methods became more and more refined over time. To determine the current position, you need 2 coordinates, geographical longitude and geographical latitude. Although the width has been determined without major problems since the invention of the octant and its further development into the sextant, the determination of the length was not possible for a long time due to the very imprecise time on board the ships. The latitude can be calculated quite easily using the "midday latitude" or at night as long as the Kim is still visible by bearing the stars, whereby in waters in the northern hemisphere the polar star is quite pleasant.

How do I measure with a sextant:

A sextant is nothing more than a very precise angle measuring device, equipped with 2 mirrors, an index mirror as a half mirror or as a transparent mirror and zenith mirror, a small lens telescope, adjusting screw, a set of solar filters to protect the eyes and a scale for reading the measured angle, with With the drum sextants used today and plenty of practice, a very precise reading is possible and an accuracy of the position of approx. 2 nautical miles can be achieved. Note: This accuracy can only be achieved on board larger ships with relatively calm decks. The arrangement of the index and zenith mirror enables the observer to keep both the star and the horizon in view. Now you aim at the desired celestial object, by turning the adjusting screw you pull the lower edge of the star or the sun to the cheek line, because you cannot hold the sextant exactly straight with your hands, we fine-tune it by swinging to the left and right until the lower edge of the star is only at the highest point on the chiming line. Now quickly note the time, read the number of degrees and the measurement is done. Several measurements or determination of the lunar distance are best carried out in pairs, with the navigator taking the measurements and a helper taking down the time and the result. However, the result cannot yet be used for the calculation, since some correction values ​​are still introduced here.

  1. The eye level: The higher the eye of the beholder is from sea zero, the further away the notch is due to the curvature of the earth.
  2. Atmospheric refraction: As the air pressure drops with increasing altitude, the refractive index of the atmosphere changes. The stars are too high, the diameters of the sun and moon vary depending on the time of day.
  3. Star diameter: Since the lower edge is placed on the notch when measuring, the value at the center of the sun and moon must be taken into account.
  4. Horizontal parallax: arises from the distance between the center of the earth and the position of the observer on the earth's surface, but only has an effect on the sun, moon (strong) and planets due to the "short" distance. This correction can be omitted when measuring stars.
  5. Possible index errors of the sextant should no longer be an issue with today's devices and with proper handling and care.

These correction values ​​are taken from a nautical yearbook, for example as published by the Federal Maritime and Hydrographic Agency in Hamburg.

In this yearbook you will also find tables about the 4 planets suitable for navigation, Venus, Mars, Jupiter and Saturn, as well as tables of the 80 common fixed stars that are used for navigation. Now you take the measured celestial angles, leaf through the yearbook, roll over log tables, deal with sine and co-sine and, after a miserable calculation, get your current location. The basics of these calculations fill entire books, but explaining them would probably lead too far here.

As already said above, with today's sextants, thanks to the accuracy of today's clocks and plenty of practice, the current position can be determined at approx. 1-2 nautical miles.

Nowadays with GPS-based course calculator, gyrocompass, navigation computer, etc., the current position can be read at any time with a deviation of a few meters from the screens, which is also necessary because port lay times are becoming more and more expensive and the time window for arrival is becoming ever narrower.

The disadvantage of sky navigation should of course not be overlooked, if the sky is overcast as it is very often the case in the Atlantic or the Great Circle, then in earlier times only "coupling" helped, ie the influence of currents, wind pressure, waves, speed and Including compass misalignment in the location and course calculation in order to arrive at a reasonably accurate cutlery. In the vicinity of the land, the deviation from the destination was then determined by means of landmarks, lighthouses, lightships, and more recently by means of radio beacons.

 

Meldauf & Steppes - Textbook of Navigation

Montenbruck - Basics of the ephemeris calculation