GPS, Sextants and Astrolabes

Global Positioning Systems (GPS), used by most navigators today, began replacing the sextant on June 26, 1993. On that date, the U.S. Air Force launched the 24th Navstar satellite into orbit, completing the network of 24 satellites known as the Global Positioning System, or GPS. With a GPS receiver costing less than a hundred dollars today, you can pinpoint your latitude, longitude, and altitude to within 30 feet. And before the sextant, until the mid 17th Century, mariners used an astrolabe to measure the angle of the sun. This link to a site with more than you ever wanted to know about Astrolabes.

Josh is concerned about finding his way without a GPS, because he will have to:
1) Measure the sun’s altitude above the horizon with a sextant, beginning a few minutes before his onboard clock shows noon. He records the angle to within seconds of arc, measuring the sun every 30 seconds until it passes noon and begins to decline. His readings will show the sun rising higher and higher as it approaches noon, and then starting to decline as it passes noon. He takes the highest angle as being noon, sets the ship’s local clock to noon and notes the time on the ship’s clock which is set to Fiji time.
2) Using tables for this hemisphere, for this approximate location, for this time and date, Josh will convert the sun’s angle into a line of latitude.
3) He’ll calculate the difference between the time local noon occurred, from the time in Fiji. The number of minutes is the number of degrees of longitude that he has traveled since leaving The Gate.
While this is a laborious process, subject to many possible errors beginning with the user’s familiarity with a sextant, it pales in comparison to how many years it took to understand how to navigate a ship like this.

For those who want some detail on how a GPS works: The US Department of Defense maintains 24 operational satellites in geo-synchronous orbits (4 clusters of 6 orbital planes). The geo-synchronous nature of their orbits maintains each satellite above precisely the same spot on the earth’s surface at all times. Any spot on earth has at least 6 and sometimes as many as 12 satellites visible in a direct line of sight. It is thus a simple matter of trigonometry to calculate the position of any location on earth, relative to the satellites that can be ‘seen’ from it. The position of a given location is expressed as a latitude and longitude, according to the Ptolemaic grid he devised in the 2nd Century A.D. The system is available to non-military users and is accurate to within a range between 30 and 100 feet. And seen is a relative term, as the satellites maintain their positions approximately 23,000 miles above sea level.

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