Sunday, September 6, 2020

Steering by Stars and Satellites


My first boat was a 38-foot Californian that had nothing more than a magnetic compass for navigation. If I wanted to go offshore, particularly out of sight of land, navigation was by dead reckoning (deduced reckoning). This kind of navigation uses your speed, course and time from a known beginning point to figure out where you are. There are all kinds of adjustments needed for magnetic variation, your own compass's deviation, currents moving you one way or another, etc. Dead reckoning can be very precise, but was usually not so in my case. My navigation then involved cruising along the coast until I could see the inlet I was heading for. I would, for example, leave the pass in Orange Beach, Alabama and turn to a compass heading from the chart designed to lead me to the entrance to Pensacola Bay. Knowing my speed and holding a course manually, I knew when to begin looking around for the sea buoy to guide me into Pensacola. People sailed all over the world with this system, often carrying a sextant to determine their position from the stars or the sun, and sometimes calling passing ships to ask for a position report.

My second boat had a LORAN receiver to determine my latitude and longitude, a technology introduced during World War II that had been vastly improved with solid-state electronics. LORAN is short for Long Range Navigation and uses powerful low frequency ground transmitters to show your current position based on the time delay between LORAN signals from different locations. The system has been mostly decommissioned, but there are efforts to keep or revive it in some locations. With the LORAN, I could type in the latitude and longitude of where I was headed. Once I entered the waypoint, I would get a digital display showing the heading to follow, whether I was on the centerline of the course or not, when I would arrive, etc. My wife once said she should be jealous of my affair with "Loran Waypoint", the imagined mistress that I spent hours working with on the bridge of our boat. Cruising mostly in the Gulf of Mexico, I gradually programmed in a large number of saved waypoints. So if I was going from Appalachicola to Clearwater, Florida, it was simply a matter of calling up a saved waypoint or route. Checking today, I find that I still have in my Dropbox a spreadsheet of these waypoints, last modified in 1993, and called loran.xls.

Now, with sophisticated GPS chart plotters, everything has become ridiculously simple using a cursor or touch screen to enter a route and telling the autopilot to follow it. In fact, it can all be done using an app on an iPad, although my iPad is not connected to my autopilot. Whatever the technology improvements though, every system still uses latitude and longitude to pinpoint any position on the earth. The first such "geographic coordinate system" was developed in the third century B.C. and our navigation today is the result of improvements made over more than 2,000 years, including the discovery that the earth is not flat.


The first breakthroughs in ocean navigation came with the concept of latitude. Once it was realized that the earth is basically round, one could imagine being inside the center of the earth, where horizontally in every direction was the equator, which is zero degrees latitude. Using the angles from the equator up or down, every position on the planet has a latitude expressed as degrees, minutes, and seconds north or south from the equator. The north pole is 90 degrees north while the south pole is 90 degrees south. Distances were defined using each minute of latitude as one nautical mile. Traveling due north or south for 60 nautical miles you would move one degree of latitude north or south. Traveling at 10 knots means you are moving at 10 nautical miles per hour.

The easiest way to determine your latitude, in the northern hemisphere, is by measuring the angle of Polaris, the North Star, above the horizon. Since the North Star is always directly above the north pole, the angle of the star above the horizon measured with a sextant is always your latitude. Latitudes in the southern hemisphere can be determined using the "Southern Cross" constellation, and angles of the sun and other stars can also be used in either hemisphere. Instruments designed to measure such angles date back to the Roman Empire. Christopher Columbus primarily relied on dead reckoning to determine how far he had traveled, and he basically just went west. But he carried with him an "astrolabe" which he tried to use to figure his latitude. Measuring was difficult on a rolling ship, and he was not skilled, but when Columbus first spotted land in the Bahamas he was actually only 90 nautical miles south of the estimated position shown in his log, an error of 1.5 degrees of latitude. Because nautical distances are defined by latitude, it is easy to see that the distance from the equator to the North Pole is 5,400 nautical miles, or 60 nautical miles per degree times 90 degrees. I live around the 32 degree latitude, just over a third of the way from the equator to the north pole.

Determining longitude took far longer for man to successfully implement, because it required better clocks (marine chronometers) that were not developed until the 18th century. To make exact Greenwich Mean Time (now called Coordinated Universal Time) available for ships, the National Bureau of Standards in the U.S. established powerful short-wave radio transmitters in 1919 that can be heard around the world broadcasting the time. Station WWV (now in Fort Collins, Colorado) is the oldest continuously-operating radio station in the U.S. and perhaps the world. You can hear what it is transmitting and its announcements of each minute at any time on your nearest short-wave radio or by calling (303) 499-7111 where you can listen for about 2 1/2 minutes before it hangs up on you. As it sounds the tone for each minute, your mobile phone clock will magically roll over to the next minute as well.

 Longitude lines are not parallel like latitude lines. Instead, lines of longitude all meet at both the north and the south poles. These lines essentially just divide the earth into the 360-degree circle that it is. Determining longitude is an exercise in measuring time difference from a known reference point. In 1884, the International Meridian Conference was held with representatives from twenty-five nations. Most of them agreed to adopt the longitude of the Royal Observatory in Greenwich, England as the zero-reference line, and it gradually became the standard point of zero degrees longitude. The rotation of the earth causes the sun, from our vantage point, to march across our sky at a rate of 15 degrees of longitude per hour, crossing the entire 360 degrees of longitude every 24 hours.

So to determine longitude, if the sun is at its highest point of the day where I am located (noon my celestial time), and it is exactly 5 pm Greenwich Mean Time, I would know that I am at 75 degrees west longitude since noon where I am is five hours earlier than noon in Greenwich (five hours X 15 degrees = 75 degrees). Time zones, as we use them, cover large areas on either side of the real spot where the local time would be the same as the celestial time. Eastern Standard Time is centered on the 75 degree line that passes just outside of New York City. Where I am located in Georgia is part of the eastern time zone but our longitude is around 81 degrees west.

Given the technology of today, it is quite easy for a modern boater to have no idea what these numbers mean. A fisherman can simply look up or learn and save the location of reefs or sunken ships where fish can be found and "punch in the numbers" to take his boat out to the correct spot. The best local fishermen still closely guard their secret "numbers", but it is easy now for charter customers to simply pull out their phones to see where they are and save the location for future use. For boaters traveling from one place to another, as we do, the newest technologies allow you to simply touch in a single destination point and the entire route will be calculated and laid out in waypoints for you. It is similar to the turn-by-turn instructions that Google Maps or your car navigation system will give you.

So you would think the old-fashioned ways of navigating are no longer necessary, but last winter as I moved our boat through the Jacksonville, Florida area, there was a notice to mariners that military exercises might disrupt the GPS system for a period of several weeks. Sure enough, some boats reported that their position would suddenly jump to somewhere miles away, perhaps on land, and eventually go back to being correct. In fact, jamming or blocking GPS signals is widely used in certain parts of the world for security reasons. I spoke recently to one pilot for Gulfstream who delivered a jet to the middle east where he had to navigate in and out with GPS simply not working. His younger co-pilot had no idea how to do it. 

You would also think that sextants used for navigation would have gone by the wayside, but there are still sailors and other navigators who know and practice the art of celestial navigation, and modern sextants can easily be purchased online. I doubt it is a growing market. There is also a collector's market for some of the beautiful antique brass sextants. I have an older brass one that belonged to my dad. I carry it around on my boat just in case I ever get lost.

2 comments:

JB said...

Great post.

AB said...

Great - sounds like a book in the making !