Solar sailing

Solar sails have been long-discussed as a way to propel spacecraft.  The sails exploit the fact that solar wind exerts pressure on a mirrored surface.   This can be used for propulsion in a way that is akin to sails used by sailboats.

LightSail® is a solar sail project from the Planetary Society that has championed solar sailing for decades. The Society’s LightSail-2 spacecraft, launched 25 June 2019, is the first spacecraft in Earth’s orbit propelled solely by sunlight. On 31 July 2019, the LightSail-2’s orbit was raised, showing that solar sailing is a viable means of propulsion for CubeSats—small, standardised spacecraft that are part of a global effort to lower the cost of space exploration.

 The image above shows LightSail-2's sail after its deployment.

Navigation in Middle Ages and Modern Times

In 1569 Gerardus Mercator devised a new way to project the land and seas on a map that had perpendicular lines of latitude and longitude.  The Mercator maps dominated nautical cartography by the 1800s and are still widely used, as they are easy to unfold and carry aboard.  

 A major advance in navigation was the invention of magnetic compass, as it allowed mariners to know where the magnetic north was. It is not known if the magnetic compass was invented in Europe or migrated from China but Neckham in 1187 mentioned its use by sailors.  With the compass mariners could steer a course and by taking bearings of coastal objects could fix their position on a chart. Knowledge of the vessel’s position and its velocity allowed periodic fixing of its location, a navigation method known as - Dead Reckoning or DR - from deduced reckoning. DR’s simplicity is still practiced today although electronic navigation has supplanted it as the method of choice.

“O God Thy Sea is so Great and my ship is so small”; sailors cried when their ships ventured beyond the coast.  It was easy to wander in the empty ocean and thus unable to return to a safe harbor if they lost sight of the land.  Ancient sailors such as the Minoans and the Polynesians used celestial bodies to assist them in finding their way. Thus the sun, moon and Polaris the North Star became guiding beacons to ancient and modern sailors alike. Thales of Miletus, and Callimachus of Alexandria recorded and taught Ionian sailors how to navigate by the North Star 600 years before the birth of Christ.  Doing the night sift during our journey was not a chore but a joy as there was nothing that gave us more pleasure than looking and admiring the myriad of stars in the night sky one can observe in the total darkness of the open seas or at sites far from cities.  I will expand on this unique experience in a future post.  

At any time in the year at any point on the earth, the sun and stars are found at certain fixed "heights" above the horizon-a distance that can be measured with as simple an instrument as one's fingers, laid horizontally and held at arm's length.  The development of the Astrolabe permitted for exact measurements of the angle a celestial body, such as the sun, above the horizon and thus sailors could determine the latitude their ship was sailing at.  The astrolabe was the precursor to the sextant. 

Although sailors were recording the angle of the sun above the horizon by simple means, looking at the sun could cause blindness.  It was Isaac Newton (1643-1727) who invented the principle of the double reflection, it was John Hardley who produced a working prototype of the Sextant in 1730, for the purpose to determine the angle between a celestial body and the horizon.  The scale of that instrument had the length of one sixth of the circle, i.e. 60 degrees; hence it got its name from the Latin word for one sixth.  With the Sextant navigators could measure the angle of the sun and other celestial bodies above the horizon with precision and determine a line of position.  If the angle above the horizon of two celestial bodies was recorded an exact fix could be obtained. The invention of logarithms by John Napier simplified the math but calculations were still laborious and time consuming especially in a constantly pitching and rolling sailboat.  Thus this wonderful instrument although still used in ocean passages it has been supplanted by the satellite GPS navigation. 

Although the ship’s latitude could be determined early on, longitude had to wait until very accurate clocks called chronometers could be carried aboard ships.  This advance was made possible thanks to John Harrison a 17th century self-taught and determined clockmaker.  Chronometers were recording the time difference between the prime meridian, Greenwich Mean Time, and that of the ship.  As the earth turns 360 degrees in 24 hours or 15 degrees every hour, if a navigator knew the exact time at Greenwich, and one hour had passed when the sun reached the highest point in the sky at noon the ship was 15 degrees west of Greenwich. If the time difference was 2 hours the ship was 30 degrees west and so on.  The availability of chronometers aboard a ship allowed for the exact determination of the ship’s longitude.

Recently the Global Positioning System (GPS) a satellite-based navigation system was made available for civilian use.  The GPS, which was originally intended for the military, is made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense and uses triangulation method to fix the position of a GPS receiver on the map. 

Today even the smallest of sailboats that venture offshore have GPS units aboard and use them to accurately fix their latitude and longitude.  During our journey a hand held GPS unit, depicted on the right, became the most beloved companion as it allowed us to get exact fixes of our position when sailing in the Balearic, Tyrrhenian and Ionic Seas.