NAVIGATING BY THE STARS - Celestial Navigation, Astronavigation

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                                                     Navigating by the Stars






Joe Rao
Skywatching Columnist
SPACE.com
Fri Sep 19, 2008
 
Astronomy is the oldest of the sciences, and quite possibly the oldest use of astronomy is navigating
by the stars. This craft dates from prehistoric times among humans, and is even practiced by certain animals.
 
For example, during the 1960s, a study undertaken by New York's Cornell Lab of Ornithology demonstrated through use of planetarium simulations that the indigo bunting, a brilliantly blue bird of
old fields and roadsides, migrates at night using the stars for guidance. It learns its orientation to the night sky from its experience as a young bird observing the stars.

Some primitive tribes accomplished amazing feats of pathfinding using only the sky as their guide. The Māori came to New Zealand from eastern Polynesia, probably in several waves between the years 1280 to 1300. With no instruments or tables to consult, they very carefully observed the night sky as well local weather patterns and ocean currents. 

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Relying on the stars


In today's modern world, private and commercial aircraft depend on a complex network of radio, satellite, inertial and other navigation systems. But should any or all of these systems fail, the starry sky can serve as the last resort.


As the late Henry Neeley, a popular lecturer at New York's Hayden Planetarium during the 1950s once noted: "The navigational use of the stars will continue to be a valuable asset for many years to come. In spite of all the scientific aids that have been developed to do the navigating by robot science, the ancient stars will still be a 'must' for navigator or pilot." Indeed, celestial navigation is still an important part of a navigator's formal training and while we might immediately think of sailors in this regard, the pilot of an aircraft can also sight on the stars in an emergency (and often with an advantage over sailors, being high above any obscuring clouds). [By day, ancient mariners used sundials to navigate.]


The Nautical Almanac and Air Almanac are special annual publications printed both in the United States and the United Kingdom and describes the positions and movements of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship or aircraft including the sun, moon and planets. 

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57 important stars


In addition, there is a standard roster of 57 stars used by aviators and navigators worldwide and chosen for their ease of identification and wide spacing. A navigator would try to measure the altitude of one of these stars above the horizon during twilight, when both the star and horizon are visible. This yields a "circle of position" on the Earth's globe; the observer must be somewhere on this circle to see the star at a certain altitude at a given time. Other stars yield other circles of position. The point where they all intersect is the observer's location.


In order to be visible against a twilight sky, the majority of the 57 navigation stars are second magnitude or brighter, although a few third magnitude stars were included on the list simply because they occupied regions where none brighter existed (the lower the figure of magnitude, the brighter the star).


Lastly, it was agreed that all these stars should have proper names. But some stars in the far-southern sky lacked such monikers, so after World War II names such as Acrux, Gacrux and Atria appeared on navigators' charts � contractions of the original Greek designations for these stars.


Lastly (and not to sound like Maxwell Smart): Would you believe this official list omits what many might consider to be the most important navigational star in the sky? Yes, it's none other than Polaris, the North Star!


Why? Merely because it lies practically on the celestial pole, so its altitude above the northern horizon alone pretty much indicates the observer's latitude, so there is no need to utilize it for the "circle of position" method described above.







How Vikings Might Have Navigated on Cloudy Days

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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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                                                           Ancient Navigation






by Liesl Clark
NOVA/PBS

How did the first inhabitants of Easter Island arrive? It is the most remote inhabited island on Earth. The coast of Chile lies 2,300 miles to the east, Tahiti 2,500 miles to the northwest, and the nearest island, with a total population of 54 people, is tiny Pitcairn island, 1,400 miles to the west. The answer lies in the deeply-rooted traditions of Polynesian culture.

The people of the Pacific are intimately tied to the ocean. They sailed the sea hundreds of years before Europeans, using voyaging canoes crafted from island materials and stone tools. The Polynesians approached the open ocean with respect; indeed, the ocean was integrated naturally into Polynesian culture, as they came from small islands surrounded by vast ocean expanses. No other culture embraced the open sea so fully.

For the continental Europeans, on the other hand, the ocean was looked upon as a menacing world that only the bravest explorers ventured upon for long periods of time. And even these explorers felt at odds with the ocean upon which they traveled. One of Magellan's chroniclers described "a sea so vast the human mind can scarcely grasp it." To a Polynesian islander, the world is primarily aquatic, since the Pacific ocean covers more area than land in this region. The Pacific, in fact, covers one third of the Earth's surface.

In island culture, the double canoe and its navigator were integral to the survival of the people. As an island became overpopulated, navigators were sent out to sail uncharted seas to find undiscovered islands. For weeks, they would live aboard boats made from wood and lashings of braided fiber. Thousands of miles were traversed, without the aid of sextants or compasses. The ancient Polynesians navigated their canoes by the stars and other signs that came from the ocean and sky. Navigation was a precise science, a learned art that was passed on verbally from one navigator to another for countless generations.

In 1768, as he sailed from Tahiti, Captain Cook had an additional passenger on board his ship, a Tahitian navigator named Tupaia. Tupaia guided Cook 300 miles south to Rurutu, a small Polynesian island, proving he could navigate from his homeland to a distant island. Cook was amazed to find that Tupaia could always point in the exact direction in which Tahiti lay, without the use of the ship's charts. Sadly, Cook was never able to learn and document Tupaia's navigational techniques, for Tupaia, and many of Cook's crew, died of malaria in the Dutch East Indies. Unlike later visitors to the South Pacific, Cook understood that Polynesian navigators could guide canoes across the Pacific over great distances.

But these navigational skills, along with the double canoe, disappeared with the emergence of Western technology, which mariners the world over came to rely on. In 1976, the Hokule'a', a replica Polynesian double canoe made by a team of Hawaiian canoeists, voyaged from Hawaii to Tahiti using the ancient navigational techniques of their ancestors. Ben Finney, a member of the team, explains their mission: "Since by the 1960s Polynesian voyaging canoes had disappeared and ways of navigating without instruments had largely been forgotten, those of us who objected to Heyerdahl's ...negative characterizations of Polynesian voyaging technology and skills ...concluded that we would have to reconstruct the canoes and ways of navigating, and then test them at sea, in order to get at the truth."

Using no instruments, the canoe team navigated as their ancestors did, by the stars. They had no maps, no sextants, no compasses, and navigated by observing the ocean and sky, reading the stars and swells. The paths of stars and rhythms of the ocean guided them by night and the color of sky and the sun, the shapes of clouds, and the direction from which the swells were coming, guided them by day. Several days away from an island, they were able to determine the exact day of land fall. Swells would tell them that there was land ahead, and the surest telltale sign would be the presence of birds making flights out to sea seeking food. By sailing from Hawaii to Tahiti, Hokule'a's team was able to prove that it was possible for Polynesian peoples to migrate over thousands of miles from island to island.

With the success of this voyage came renewed interest in old world navigation. More double canoes were built, and now several teams are attempting to be the first to reach Easter Island, using ancient navigational techniques. No one has navigated a raft or voyaging canoe from Polynesia to Easter Island since the early settlers arrived here in AD 400.

For the ancient Polynesians, finding Easter Island, a small 64-square-mile speck in this vast ocean, must have been like finding a needle in a haystack; but the Polynesian community today is convinced their navigators intuitively discovered and settled this island. "At the backbone of the maritime tradition lies the outrigger canoe," explains Jo Anne Van Tilburg "the quintessential symbol of Polynesian mastery of the sea. The outrigger canoe is today part of every Polynesian island child's upbringing, except on Easter Island. There, the outrigger canoe was lost sometime in the mid-1800s." Van Tilburg has been instrumental in reintroducing three outrigger canoes to the island. The islanders' loss of their sea-faring past, according to Van Tilburg, "took away the traditional link people had with the sea."

For Van Tilburg, the Polynesian canoe is a metaphor in her theories of how the Easter Islanders transported and erected their 15-ton moai. "It's not much different from erecting a mast on a very large canoe. It's a transfer of technology from one industry to another. The people who built these structures were both sailors and farmers, and they used their sea-faring technology to help them in moving and erecting their moai....Erecting a mast on a ship or a statue on a platform requires similar abilities, skills and tools."



http://www.pbs.org/wgbh/nova/easter/civilization/navigation.html