The Vikings already knew that the shadow at midday on a ship reveals the latitude of its position. They carved a notch on the rowing bench where the end of the shadow fell from the coaming when the day's shadow was at its shortest. This enabled them to find the latitude of a place such as Shetland, Iceland or Greenland. The shadow is of course not only dependent on latitude, but also on the seasons. It is well known that the shadow is longer in winter than in summer. But if you set off from Norway to Iceland every year in June and sail back in August, you only need two notches for the rowing bench: one for the outward journey and a second for the return journey.
In principle, navigation did not function much differently until well into the 18th century. With increasingly precise instruments (sextant) and an ever more accurate "Nautical Yearbook" for determining the latitude of the sun (declination), it was possible to determine one's own latitude better and better.
But only the latitude! Ships like Columbus', for example, sailed stubbornly south or south-west from the Canary Islands until the desired latitude was found by observing the sun. They then turned to the west and simply waited until land came into view. The navigator was able to use the ship's speed to at least roughly estimate when this would be the case.
Chronometer vs. lunar distance method
John Harrison achieved a quantum leap in astronavigation. In 1736, he developed his first chronometer, which was so accurate that a position could be determined not only by latitude but also by longitude. However, there were also other ways to determine longitude, for example using the lunar distance method, which measured the angular distance between the moon and other celestial bodies.
The theory had already been put forward in 1524, but it was not until many years after Harrison's development of the accurate chronometer, from 1763, that the tables for the lunar method were so well developed that they were used until the middle of the 19th century. The lunar method was actually the more robust one, as there was no need to carry an expensive and sensitive chronometer on board. However, the calculations were very complicated and could not be carried out every day of the month.
Hard to imagine today, but at the end of the 18th century, the price of a Harrison chronometer accounted for a whopping 30 per cent of the total value of a ship! However, it was only a matter of time before the production costs of a chronometer had fallen to such an extent that the lunar method was finally replaced altogether.
The HO249 method
Until the Second World War, sailors had to struggle with cumbersome logarithm tables, which, incidentally, have not been completely eliminated from astronavigation classrooms to this day. Anyone who has completed the SHS knows a thing or two about it. But why make it so complicated when it could be easier?
It was the Americans who in the mid-1930s published "Tables of Computed Altitude and Azimuth 1936-1945" as "H. O. Pub. No. 214". Another milestone in the history of astronavigation. The US long-range bombers needed to determine their position quickly, and so the ingenious HO249 tables "for Air Navigation" were created in just three volumes, which are still used today by sailors who are not interested in logarithm tables. Although there are also the more precise HO224 nautical tables, consisting of six volumes, the less comprehensive tables for aviation are completely sufficient for sailors.
But what is so ingenious about it? Well, instead of having to do the calculations yourself with the help of logarithm tables, the elevation angle to the sun (Hc) and the direction to the sun (azimuth) to be measured there have already been calculated in advance for an almost unimaginably large number of positions. Both values can be easily read from the books. So the calculation is already done for the navigator!
If the sailor now happened to be at one of these calculation locations, he would also have to measure the elevation angle (Hc) printed there on his own sextant. This would of course be a huge coincidence, and so in practice a different angle is observed than the nominal angle of the calculation location. Therefore, the own observation angle at the sextant (Ho) must now be compared with the one from the HO249 table (Hc). The aim is to find the difference between the actual and target values. The difference between Hc and Ho then indicates how far away you are from the calculation location (intercept). In other words, you prove that you are not at the calculation location, but rather calculate the exact deviation from it. The calculation location is first plotted on the nautical chart and then the calculated deviation either towards the sun or away from the sun along the azimuth. The result is then a bearing line.
Electronic astronavigation
Bobby Schenk was one of the first to skilfully use the programmable pocket calculators for astronavigation that had just come onto the market in the 1980s. The advantage: if you can calculate your own Hc and azimuth for any location using a pocket calculator, you no longer need any tables!
Today, there are modern apps that do exactly what Bobby's "Astro Classic" could already do: determine the direction to the sun (Zn, azimuth) and the expected elevation angle to the sun (Hc) at the estimated position (EP, estimated position) at the touch of a button. Some apps are beautifully illustrated and calculate a position directly for two consecutive measurements and the sails in between. This even saves drawing.
The use of Astro programmes is of course practical and convenient. Even if you remain dependent on the power supply for your computer, tablet or smartphone. Most people hardly understand the background to the calculations when using an app anyway. Instead, they are simply delighted with the result. Enter two solar altitudes that are approximately two to three hours apart, together with the exact observation time and the distance sailed in between. The app then spits out the ship's position!
As a result, the personal satisfaction of using a sextant and computer is not very great. If you need a computer programme for astronavigation, you might as well navigate with GPS, Galileo or other satellite systems. However, those who do without electronic aids and find their way to their destination will be much happier with their own knowledge and skills. In the upcoming YACHT webinars, interested sailors will have the opportunity to refresh this knowledge or to acquire it in the first place.
