# The Mystery of Determining Longitude

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Have you ever been to Greenwich? If you haven’t, make sure it is on your bucket list. Not only is it a delightful place with a spectacular view of London and the Thames River, but it is the real birth place of “Longitude” and home to some of the most amazing master mathematicians of the burgeoning modern times of the 17 and 18oo’s.

The zero degree line of longitude was selected and placed in Greenwich in 1751 and then later in 1851 it was moved a few feet to align with a new telescope placed at the now zero deg.  zero minute, zero second longitude position. This designated line placed on earth is a humankind determined and positioned line. It stretches from the true north pole to the true south pole.

The zero mark could have been placed anywhere on earth. It was placed in Greenwich because of the incredible work being done by the Royal Observatory mathematicians and astronomers. They needed to set a zero point and what better than exactly underneath their telescope  The axis of spin of the earth also passes through the north and south poles. This is unlike the zero degree latitude line of the equator. This line is a universally given line and can not by randomly placed by humankind.

In the 1700’s in bars and cafe’s all over the topic of the times was that determining Longitude on the ocean was akin to perpetual motion – it was seemingly impossible. Yet the challenge was there and £20,000 had been put up as cash incentives to inventors to help solve the issue. £10,000 of which was ordered by the King as prize money to the person who could solve it. Longitude was that illusive!

If you understand that in those days because of ships not knowing exactly where they were, there were many shipwrecks costing lives but also huge amounts of money. Thus global positioning was imperative. Latitude had been pretty easily solved much earlier. If you do a noon shot of the sun to determine its angle above the horizon then compare this with tables of the suns angle on a specific day of the year you get your Latitude. But longitude had no such luxury of determination.

I had the luxury of recently visiting Greenwich and was delighted to see and learn about one of my favorite topics. Longitude. Here I am with one foot in the eastern hemisphere and one in the western hemisphere.

The zero degree Meridian Line in Greenwich

What was particularly awesome about my trip was that the Maritime Museum in Greenwich was displaying a whole history tour of “Longitude” and I was actually able to see the time pieces that John Harrison invented to allow ships to keep time at sea and thus solve the Longitude problem and collect the Kings prize.

I highly recommend this short read of the book Longitude.

Variation aka Declination

We discuss Variation/Declination heavily in our NauticEd Coastal Navigation Course. This is the difference between the charted true north and what will read on your compass due to the earths magnetic poles not aligning with the axis poles. Interesting enough, one means used to determining rough longitude int he old days was to take a measurement of the variation. True North can easily be measured by the north star in the Northern hemisphere and magnetic north by a compass. This difference combined with a variation map of the world allowed approximate longitude determination. The method was rudimentary at best and provided no accuracy.

On that same topic and quite interesting also is that a very accurate global map of variation exists today and is programmed into every magnetic field chip used to determine compass directions. Your smart phone can tell you magnetic North and this is measured by the electronics measuring the earth’s magnetic field. To go from magnetic north to true north, the phone needs to know where on the planet it is. Once this is known (usually via GPS measurement) it applies the known table of variations and can then show also true north.

Here is also a great website resource to see your position on earth but also find your variation.

http://www.ngdc.noaa.gov/geomag-web/?id=ushistoricFormId#declination

It’s astounding to know that these guys back in the 1700’s knew all this and were the discoverers of this knowledge. Another part of this whole story is bought to life by Cook’s first voyage in 1769 to Tahiti. He was sent there by the Royal Astronomical society to measure the times that Venus was to transit the sun. This was predicted by Haley more than 40 years earlier. By comparing the measurement of times of the transit from many different locations on the planet, the mathematicians of that time were able to calculate the distance to the sun. AND the nailed it within hundredths of a percent.

Here is the picture I took of Cook’s Monument in Tahiti at the place where he measured the transit of Venus across the sun in 1769.

Cook himself used John Harrisons Chronometer (Watch) on his second voyage around the world. Cook’s logbook notes about 6 months into the trip, said that he believed more and more that Harrisons time piece was to be the way of the future.

I really hoped you enjoyed this little trip through the discovery of knowledge that we all use today virtually in our everyday lives. Whether we are aware of it or not, Latitude and Longitude is touching us manytimes every day in our every way.

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Global Director of Education

P.S – If you are interested in Navigation, take our Coastal Navigation Course

# Simplistic Explanation of Latitude and Longitude Determination

The posting here is not a course in celestial navigation by any means. However it’s meant to simplify a few principles for you so that you’ll at least have some sort of celestial orientation. And… perhaps it’ll inspire you to learn the aging art.

This was written by Grant Headifen, Educational Director of NauticEd. NauticEd provides online sailing courses and Sailing Certifications accepted by charter companies worldwide.

Latitude: In the northern hemisphere, finding latitude is simple using one of the greatest gifts to human kind – The North Star. What ever angle the northern star is at from the horizon, that’s your latitude.

Imagine you’re an ant sitting on the top of an apple looking at a spot directly above you on the ceiling then the spot is 90 degrees from the surface you’re standing on. If you’re standing half way around the apple then you’d barely see the spot but it would be horizontal to the surface you’re standing on and so the spot would be at zero degrees. And if you were ¼ of the way down the apple then the spot would be at 45 degrees etc. ie the northern star is the spot on the ceiling to us.

You can also find latitude using other celestial sightings but they involve table lookups and are slightly more complicated. Not meant for this post and also note that there are a few more complicated variables not taken into account during this simplistic explanation like the height of your eyeballs above the earths surface etc etc. But at least you’ve now got the principle.

Longitude: Now this is a fun one and in an incredibly easy principle. But years ago (early 1700’s) while the principle was easy then the execution was difficult. Read on to see why.

The earth rotates through 360 degrees in 24 hours. That’s 15 degrees per hour. By convention, when the sun is at it’s highest point in Greenwich, it is noon in Greenwich. That means that at a place that is 15 degrees to the West of Greenwich the sun will be at it’s highest point one hour later. Six hours after Greenwich the sun will be at it’s highest point somewhere in over the USA and 12 hours later the sun will be at it’s highest point in New Zealand.

Animation of time zones

So if we know the time in Greenwich and sun just reached its highest point where we are then we can calculate our longitude.
Lets do a few examples. If it is 6 pm in Greenwich and the sun just peaked overhead here, then I am 6 x15 degrees to the west of Greenwich which is 90 degrees West which is right near St Louis Mo.

If the sun peaked overhead in Los Angeles what time would it be in London.?Well LA is 118.15 degrees West (from Google earth). Divide that by 15 degrees per hour and we get 7 hrs 53 minutes. Now since the times zones are created in bands this would round up to 8 hours. Thus it would be 8pm in London.

You’re sailing in the Greek islands in the Mediterranean and a little bird just told you your latitude is 34 deg 54 minutes north but failed to tell you the longitude. Fortunately you have your handy sextant and just as you take a shot, the sun just reached its apex overhead. You look at your watch and the local time is 12:10:48 pm. Where are you?

Since you’re in time zone B you are 2 hours ahead of Greenwich. Thus the time in Greenwich is 10:10:48 am. And since the sun peaked just now (=noon) then you are 12:00:00 minus 10:10:48 = 1 hour 49 minutes and 12 seconds from Greenwich. Putting this into decimal time this is 1.82 hours. Multiply this by 15 degrees per hour and we have 27.3 degrees East or 27 degrees, 18 minutes East.

You’re in the harbor north of the town of Kos on the Island of Kos.

That was incredibly easy, so why all the hoopla back in the 1700’s? The King of England even offered up a ₤10,000  reward to anyone who could solve the issue of Longitude. The above math was well known but the issue was telling the time. No one could accurately keep time at sea. After 27 years of work on the project, John Harrison, finally invented the Chronometer more commonly known as the watch. The watch was not susceptible to the sudden crashes of waves at sea and thus kept proper time.

James Cook on his second trip around the world in 1772 sailing on Rendezvous, took Harrison’s watch with initially much skepticism. Stating that he’d give it a try. After six months at sea, Cook stated that the Chronometer would almost certainly become the way of the future for Navigators. Cook then went on to reposition many of the Islands in the Pacific including Tahiti, his favorite island. His map of New Zealand astounds people even today with its accuracy.

Again there were a few simplistic assumptions taken in that explanation. But now, at least you understand the principle of longitude determination from a noon shot of the sun. You can also determine your latitude from a noon shot of the sun as well using tables and a bit of math. Again beyond this posting.

If you’d like to delve deeper into these topics, NauticEd provides online sailing lessons and an Introductory Celestial Navigation Sailing Course, or maybe you’re just happy with your handy boring ol GPS.