The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Guns, Politics, Gunsmithing & Reloading Walter's Own General Discussions Astrolabe accuracy - sea AND land?Go | New | Find | Notify | Tools | Reply | |
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I recently read an article about Columbus' voyage of discovery that included a bunch of information with which I am unfamiliar and ignorant. Apparently Columbus located his ship by using an Astrolabe in conjunction with the sun. But apparently an Astrolable's accuracy/precision is insufficient on its own. A Portuguese astronomer-mathematician, Rabbi Abraham Zacuto, created a "nautical almanac" whose tables either corrected [some sort of] errors or imprecision inherent in Astrolabes. I am marginally familiar with sextants and theodolites as devices that work in concert with chronometers to accurately locate the user. What the heck is an Astrolabe? How does it work, especially in the absence of precise identification of the time? What is an Astrolabe's standard of accuracy? What does a nautical almanac do? Does one nautical almanac [tables] function correctly worldwide, or are distinct almanac-tables necessary for different locations, or longitudes, or latitudes? Would the nautical almanac Columbus relied upon be functional today? Columbus used an Astrolabe on the ocean. Since it is keyed to the sun, is it functional on land? It's so simple to be wise. Just think of something stupid to say and then don't say it. Sam Levinson | ||
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An astrolabe (Greek: ἀστρολάβον astrolabon, "star-taker")[1] is an elaborate inclinometer, historically used by astronomers, navigators, and astrologers. Its many uses include locating and predicting the positions of the Sun, Moon, planets, and stars, determining local time given local latitude and vice-versa, surveying, triangulation, and to cast horoscopes. It was used in classical antiquity, through the Islamic Golden Age, the European Middle Ages and Renaissance for all these purposes. In the Islamic world, it was also used to calculate the Qibla and to find the times for Salah, prayers. There is often confusion between the astrolabe and the mariner's astrolabe. While the astrolabe could be useful for determining latitude on land, it was an awkward instrument for use on the heaving deck of a ship or in wind. The mariner's astrolabe was developed to address these issues. http://en.wikipedia.org/wiki/Astrolabe   The almanac, or ephemeris, shows the functions of time, celestial position, and latitude. I believe you need two out of three to compute the needed data. A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea. The Almanac specifies for each whole hour of the year the position on the Earth's surface (in declination and Greenwich hour angle) at which the sun, moon, planets and first point of Aries is directly overhead. The positions of 57 selected stars are specified relative to the first point of Aries. In Great Britain, The Nautical Almanac has been published annually by HM Nautical Almanac Office, ever since the first edition was published in 1767. [1] [2] In the United States of America, a nautical almanac has been published annually by the US Naval Observatory since 1852.[2] Since 1958, the USNO and HMNAO have jointly published a unified nautical almanac, for use by the navies of both countries.[2] Almanac data is now available online from the US Naval Observatory.[3 http://en.wikipedia.org/wiki/Nautical_almanac Sailing along a given latitude to make a specific landfall is still used today. | |||
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I know little about an astrolabe and its functions, however there are a couple of related books, interesting and closely related to navigation, Columbus and this query. Might I suggest perusing 1421 and 1434 by Gavin Menzies. Fantastic reading which I could not put down. One of the books (can not remember which) spurred a memory of a stopping and idling in an antique store on the Oregon coast years ago. While in that store I found a brick of bees wax, well worn and weighing several pounds, with time-blurred Oriental characters engraved into the wax. I knew not that which I then held in my hand and, much to my dismay now, wish I had purchased same. In one of Menzie's books are references to this bees wax cargo lost on the Oregon coast centuries ago. Also, there are rocords of found relics and Chinese ships much inland on the Oregon coast. Both books are chock-full of navigational details in specific. Geez, I got side-tracked. Any way, good reading, Stephen | |||
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I don't know about the mariners astrolabe, but the common astrolabe was a device that showed the motions of the planets and the earth. The issue of correctly locating one's position is exactly that of knowing the longitude. This equates to knowing two things: the exact time at your present location, and the exact time at GMT. this translates to having two very accurate clocks. Not an easy task on board a ship! In 1714 the English Parliament offered 20,000 pounds ($12 million in todays currency) to anyone who could solve this problem. You might want to read "Longitude" by Dana Sobel. It is a good read. Should be able to find it in the local library. Peter. PS Galileo attempted to use the planets of Jupiter to solve the longitude problem. Be without fear in the face of your enemies. Be brave and upright, that God may love thee. Speak the truth always, even if it leads to your death. Safeguard the helpless and do no wrong; | |||
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As Peter mentions, the book "Longitude" is a really good read for anyone even remotely interested in navigation. It does a superb job of not only explaining technicalities but is also a darn good story in its own right. Also, as Tin Can mentions, "sailing by latitude" is still common, though slightly less so due to GPS (but keep in mind there are still thousands of working boats that don't have GPS). Basically you sail North or South until you are at the same latitude as your destination, then sail East or West until you get there. It is easy to find your latitude by using the sun at local noon, and at night in the Northern Hemisphere the latitude is equal to the angle formed between the North Star and the horizon. (Off on a tangent, there are 57 primary navigational stars and IIRC 50 of the 57 have Arabic names) Lastly, I have a book somewhere called "Latitude Hooks and Azimuth Rings" which showed you how to make early navigational instruments, but I can't remember what it said about astrolabes but you may want to see if your local library can locate a copy for you to check out. for every hour in front of the computer you should have 3 hours outside | |||
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The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Guns, Politics, Gunsmithing & Reloading Walter's Own General Discussions Astrolabe accuracy - sea AND land?
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