The sun, the sea­sons and the nav­ig­a­tion on the sea - Blo­g­post 7 from RV SONNE, March 26, 2018

by Manfred Schlösser

On Board RV SONNE, Chile
March 26, 2018

Blog #7 - The sun, the seasons and the navigation on the sea

Spring and Autumn

While spring starts in the north­ern hemi­sphere, au­tumn be­gins here in the south­ern hemi­sphere. On 20 March was the equi­nox, the day when the sun is at its zenith at the equator at noon. Viewed from the north­ern hemi­sphere, the sun then con­tin­ues to rise north dur­ing the year and the days get longer. This will last un­til the 21st of June, the be­gin­ning of sum­mer. Then the sun stands at noon on the Tropic of Can­cer at 23.4 de­grees north and then moves back to­wards the equator. Around Septem­ber 21, the same game starts again. The sun is in the equator, and it is again equi­nox. The un­der­ly­ing as­tro­nom­ical event is based on the fact that the Earth's axis is tilted by about 23.4 de­grees with re­spect to its or­bit around the sun and thus provides for chan­ging solar ir­ra­di­ation peri­ods, just the sea­sons. 

Compass, sextant and bearing plate

For good seaman­ship, for every seago­ing ship it was a must to have naut­ical charts, a sex­tant for pre­cise meas­ur­ing angles, com­pass, lot, log, bear­ing plate and manu­als with data on tides, cur­rents and wind con­di­tions. Near the coast sail­ors de­term­ined their po­s­i­tions by meas­ur­ing the angle (or bear­ing) of two vis­ible ob­jects which are re­cor­ded on the naut­ical chart. After plot­ting these bear­ings to the naut­ical chart, the own po­s­i­tion could be de­term­ined at the in­ter­sec­tion of the two plot­ting lines. In ad­di­tion, con­clu­sions can be drawn from the meas­ured wa­ter depth in com­par­ison with the depth lines of the naut­ical chart. Al­tern­at­ively, if a light­house was in range, the lis­ted height data from the light­house dir­ect­or­ies were taken. If the height of the beacon is known, it is pos­sible to cal­cu­late the dis­tance to this by de­term­in­ing the angle light­house top, ship and beach. The own po­s­i­tion must be on the arc around the light­house. With a second in­ter­sect­ing line from an­other bear­ing the own po­s­i­tion could be de­term­ined.

Without horizon no astronavigation

With a sex­tant and a pre­cise clock, the height of sun, moon, and stars and as meas­ured by their angles to the ho­ri­zon were used to cal­cu­late plot­ting lines sim­ilar to the method of the light­house el­ev­a­tion angle. The ship´s po­s­i­tion is at the in­ter­sec­tion of two lines cal­cu­lated from the meas­ured angles. A skilled ob­server can achieve an ac­cur­acy of half a naut­ical mile. This method works only, if the ho­ri­zon is vis­ible. So dur­ing the day the sail­ors used the ce­les­tial ob­jects like sun and moon, the stars and plan­ets only in the twi­light phase when the ho­ri­zon is still vis­ible. When the sky is over­cast, the sail­ors had to es­tim­ate their po­s­i­tion from their speed over ground and the com­pass head­ing and an off­set by wind and cur­rent. This method already helped Colum­bus across the At­lantic. It is in­ter­est­ing to note that even today's as­tronav­ig­a­tion is based on the er­ro­neous as­sump­tion that the earth is in the cen­ter and the sun, moon and stars are or­bit­ing around them. Since all move­ments are re­l­at­ive, nev­er­the­less cor­rect res­ults come out. And that is valid for cen­tur­ies.