Nautical Notes & Quotes
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Celestial navigation is referred to as astronavigation. It represents the practice of position fixing. Thus, enabling the navigator to transition through space, without the need to consider estimated calculations to assess his/her position.
To that end, one way of defining celestial navigation would be by indicating that it is the use of angular measurements, or sights. These angular measurements are established between different celestial bodies – namely the sun, the moon, the planets, or a star, and the visible horizon. The sun used to be the most frequently utilized. However, navigators would also depend on the moon, different planets, Polaris or one of the 57 individual navigational stars. The coordinates of the stars were incorporated in nautical and air almanacs.
Celestial navigation is an intuitive means of navigation. It existed before the invention of north, south, right and left and is based on a quintessential rule…. having a fixed, known reference point.
The Pole Star was seen as what we today call North. Even nowadays, the North – by extension, the North Star, represents the singular universal point of reference on Earth. The same could be said about the South Star – namely Sigma Octantis – which is characteristic of the Southern Hemisphere. That being said, ancient celestial navigation didn’t provide information regarding your exact position. As a matter of fact, it had little to do with your position vector. Before the invention of watches and calendars, the moon, the sun and the stars were the only timekeepers you could depend on. So, you had to fix two reference points on the sky – such as the moon and a known star. After doing that, you assessed the angular measurement between the two celestial bodies. It is fascinating to see that the angle between the moon and Regulus, for instance, would be the same in the Indian Ocean as it would be in any other location on earth. In addition to that, the angle between the moon and Regulus is unique for that specific time in space, being the same angle, regardless of the place from which you observe it in the world.
At a specific time, each individual celestial body has a geographic position (GP) that is assessed according to its latitude and altitude. In other words, the angle between the visible horizon and the celestial body is linked with the distance between the observer’s location and the geographic position of the celestial body.
It goes without saying that the accuracy of angular measurements evolved throughout the centuries. There is actually a simple method that implies holding the hand above the horizon, as one arm is stretched out. You can use the width of the pinkie finger in order to assess the elevation of the sun from the horizon plane, in this way, anticipating the time until sunset. That’s because the width of the pinkie finger has an angle of over 1.5 degrees elevation, by considering the length of the extended arm. Obviously, there was a significant need for the development of more precise methods of measurement; just to name a few: astrolabe, sextant, and octant.
When it comes to the degree of accuracy, the sextant and the octant are regarded as the most accurate.
That’s because they calculate the angles from the horizon. This eliminates the likelihood of errors potentially triggered by the placement of an instrument’s pointers. At the same time, they both come with a dual mirror system that excludes relative motions of the instrument, presenting a steady overview of the horizon.
In regards to celestial navigation terms, it’s worth pointing out that navigators calculate the distance on the globe by using the following measurements: degrees, arcminutes, and arcseconds. Meanwhile, a nautical mile measures 1852 meters. Concurrently, it is measured as being one minute of angle along a meridian.
Navigation by stars entails one essential thing: knowing how to find the North Star. Of course, the reason why this is so important is that it indicates the north. The North Star – namely Polaris – has a given position; to that end, it has always been situated within one degree of the celestial North Pole. Many people consider Polaris to be the brightest star in the sky. However, this is a common misconception. But what is true, is that Polaris is the brightest star in the Ursa Minor constellation. Hence, one way of pointing out the North Star is by locating the Ursa Major in the Big Dipper. Next, you should focus on pointing out the star located at the very bottom of the dipper spoon, while not touching the handle. As soon as you discover that specific star, redirect your sight at an angle to discover the Little Dipper – also referred to as Ursa Minor. The star located at the top of the Little Dipper is what you’re looking for – namely the North Star.
Once you determine the position of the North Star, assessing your latitude is easier. In fact, the easiest way in which you can achieve that is by using a sextant or a quadrant.
But how did sailors navigate by the stars before the development of such handy tools? What you have to do is extend your fist towards the horizon. Afterward, what you must do is place your fists hand-over-hand until reaching the North Star. Each fist equals roughly ten degrees. Since Polaris is always situated within 1 degree of the celestial North Pole, if the measured angle to Polaris is 10 degrees from the horizon, this means one thing: the navigator is situated at approximately 10 degrees north the equator. Navigators would usually increase the accuracy of this calculation by resorting to almanac corrections or simple tables.
Most people refer to the North Pole as simply the North. Nevertheless, it is widely unknown that there are actually two north poles – namely the geographic North Pole and the magnetic North Pole. When utilizing a compass, the needle will point towards the magnetic North Pole. The reason why there are two North Poles is that the Earth is surrounded by a magnetic field. This magnetic field has a north and a south pole – which is precisely the end of the magnetic field. Since the axis of the Earth doesn’t coincide correctly with the magnetic field, there are two North Poles and two South Poles.
Moving on to longitude, it can be assessed in a similar way as the latitude, with some differences, of course. If one can measure the exact angle to Polaris, then engaging a similar type of measurement to a star of the western or eastern horizons will help you find the longitude. Nonetheless, the thing is that every hour, the Earth turns 15 degrees. This is why longitudinal measurements rely on time. A measure of a few minutes only can lead to significant navigation errors. Before the development of reliable chronometers, the navigator would calculate longitude measurements depending on the transit of the moon, or the location of the moons of Jupiter. This is what made the invention of the chronometer in 1761 by John Harrison a noteworthy discovery in the field of celestial navigation. You can aim at calculating the longitude by pointing out the precise local time when the sun is at its peak point in the sky. In order to calculate noon, for instance, you would need to resort to a small, vertical rod driven into the ground. You should do the reading when the shadow points north – in the northern hemisphere. Afterward, the local reading should be subtracted from GMT (Greenwich Mean Time) or the time in England, London. In other words, a noon reading (1200 hours) in the proximity of central Canada would be at roughly 1800 (6 pm) hours in London.
In regards to the six-hour difference between the two, it constitutes a quarter of a day, or, simply put, 90 degrees of a 360-degree circle (the Earth).