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Why Was Finding Your Position at Sea Harder East-West Than North-South?

Category: Sea & Fishing

In 1714, the British Parliament offered £20,000 to anyone who could solve one problem for ships. That was an enormous sum at the time. The odd part is that sailors already knew half the answer. Out of sight of land, they could work out how far north or south they were. It was the other direction, east and west, that nobody could pin down[1].

Why was one direction easy and the other so hard? This article follows the problem to the answer that worked, a clock that could keep time on a rolling ship. It ends with a question that sounds simple and is not: who decided where zero degrees of longitude runs?

1. Two Angles Give Every Place on Earth an Address

Think of a grid on graph paper: "three across, five up" picks out one square. The Earth is round, so its grid is made of angles instead of lengths, and two numbers do the job.

Latitude says how far north or south of the equator you are. The equator is 0 degrees and each pole is 90 degrees[2]. One degree of latitude covers about 111 kilometers (about 69 miles) of the Earth's surface[2]. Longitude says how far east or west of a starting line called the prime meridian you are. It runs from 0 degrees up to 180 degrees east and 180 degrees west, and the two meet on the opposite side of the globe[3].

Both are measured in degrees, and the grid is the same everywhere. The two numbers are not equally easy to get, though.

2. Why the Sun and Stars Tell You How Far North You Are

As you travel north or south, the sky changes. The Sun at noon stands higher or lower above the horizon, and so does any given star. Measure that height with the right instrument and you can work out your latitude. This is why latitude could be found at sea long before longitude could[4].

The key is that the sky itself marks the north-south direction. It gives you something to measure against that does not depend on a clock or on anyone's agreement. East and west have no such marker, and that is where the trouble starts.

3. Why Knowing the Time Back Home Reveals How Far East or West You Are

The Earth turns through 360 degrees in 24 hours, which is 15 degrees every hour[3]. Put another way, the Sun's position shifts by one degree of longitude every four minutes[4]. So the difference between two places' local times tells you how far apart they are, east to west.

Here is how a navigator could use it. Watch for the moment the Sun is at its highest: that is local noon. Then look at a clock that still shows the time at the starting port. Say the clock reads 2 p.m. when the Sun peaks. The gap is two hours, and two hours is 30 degrees, so the ship is 30 degrees west of the port[3]. (It is west because the Sun reaches its high point later for people farther west, so the port is already past noon.)

The Sun is easy to watch. The hard part is the clock. It has to keep the home port's time, accurately, through weeks of rolling and pitching, heat and cold and damp. Latitude needed only a measurement. Longitude needed a machine that did not yet exist.

4. How a £20,000 Prize Pushed People to Build a Sea-Going Clock

The 1714 Act of Parliament set rewards by how close a method came. £10,000 was offered for fixing longitude to within one degree (the Act says "sixty geographical miles"), £15,000 for two thirds of that distance, and £20,000 for half of it, which is 30 geographical miles[1].

Turn that into time and you see why it was so demanding. Half a degree of longitude is two minutes of time (four minutes per degree, halved). A clock aiming for the top prize could drift by only about two minutes over an entire ocean crossing, with no chance to reset it.

The clockmaker John Harrison spent decades on this. His fourth sea clock, called H4, sailed to Jamaica in 1761 under the care of his son William. After roughly two months at sea it was only about 5 seconds slow[5][6]. Five seconds of time works out to roughly one nautical mile of longitude near the equator[6]. A second trip to Barbados in 1764 confirmed that H4 had kept time within the strictest limits of the 1714 Act[7].

Harrison's reward did not come easily. The Board of Longitude resisted paying the full prize, asked for more proof, and by some accounts treated the first success as luck[5]. Parliament made a final payment in 1773[5], when Harrison was 80[6]. Even so, the clock had shown that the east-west puzzle could be handled with a machine and a bit of arithmetic.

5. Why Zero Degrees of Longitude Runs Through Greenwich: A Choice, Not a Discovery

There was one more thing to settle. The equator is a natural zero for latitude: it lies halfway between the poles. Longitude has no such natural line. Nothing on the Earth's surface says "begin here," so the dividing line between east and west had to be chosen[8].

In October 1884, delegates from 25 nations met in Washington, D.C., to pick one prime meridian for everybody[9]. On October 13, Greenwich won by 22 votes to 1. San Domingo (today's Dominican Republic) voted no, and France and Brazil held back[10][9].

The main argument was practical. Nearly two-thirds of the world's ships were already using charts based on Greenwich, so choosing it caused the least disruption[8]. A naval delegate from the United States told the conference that more than 70 percent of the world's shipping already used that meridian for navigation[11]. The choice was not unanimous, and it did not change things overnight. France, for instance, kept its own national time until 1911, and used the Greenwich meridian on all its nautical documents from 1914[9].

So the zero line is an agreement, backed by a clock-and-chart habit that had already spread. Everything else on the grid, including the numbers you read off a phone map, hangs from that agreement.

How to Check the Sun's Clock Yourself

  • Find your local noon with a stick. On a sunny day, push a straight stick upright into level ground, or stand a pole on a flat surface. Mark the end of its shadow every ten minutes around midday. The shortest shadow marks local noon, when the Sun is at its highest. Never look straight at the Sun.
  • Compare with the clock. Write down the clock time of your shortest shadow. Is it close to 12:00? Where you live, the clock follows a time zone that covers a wide band of longitudes, and summer time (daylight saving time) can add an hour, so a gap is normal. A gap is not a mistake. It is the same difference between "Sun time" and "clock time" that the navigators were using.
  • Try a friend's town. If you can, ask a friend or relative living far to the east or west to do the same, or read the "solar noon" time for both places in an online sunrise table. Remember the rule: four minutes of time for each degree of longitude.
  • Read the original. The Royal Museums Greenwich "What is longitude?" page and the Linda Hall Library's short biography of John Harrison are good next steps.

Sources

This article is a personal summary based on the public sources listed below.

  1. Royal Museums Greenwich and Cambridge University Library, Board of Longitude teaching sheet quoting "An Act for Providing a Publick Reward for such Person or Persons as shall Discover the Longitude at Sea" (1714), object ref. RGO14/1. https://www.rmg.co.uk/sites/default/files/import/media/pdf/Longitude_Act_3.pdf (The PDF was read in full. It gives the Act's own wording for the three rewards: £10,000 for one degree or sixty geographical miles, £15,000 for two thirds of that, and £20,000 for the half. The line that sailors could already find latitude but not longitude is the sheet's own summary.)
  2. NOAA National Ocean Service, "What is latitude?" https://oceanservice.noaa.gov/facts/latitude.html (Read through a page summary tool, not word for word. It supplies the equator at 0 degrees, the poles at 90 degrees, and about 111 kilometers per degree. The mile figure in the text is a conversion.)
  3. NOAA National Ocean Service, "What is longitude?" https://oceanservice.noaa.gov/facts/longitude.html (Read through a page summary tool, not word for word. It supplies 0 to 180 degrees east and west, the 360 degrees in 24 hours (15 degrees per hour) reasoning, and the noon-versus-clock method with the 2 p.m. / 30 degrees west example, which this article repeats.)
  4. Royal Museums Greenwich, "What is longitude?" https://rmg.co.uk/explore/what-longitude (Read through a page summary tool. It supplies latitude from observations of celestial bodies, longitude as a time difference, and one degree every four minutes. The page did not give instrument details, so none are described here.)
  5. Linda Hall Library, "John Harrison" (Scientist of the Day). https://www.lindahall.org/about/news/scientist-of-the-day/john-harrison (Read through a page summary tool. It supplies the Jamaica voyage, "only 5 seconds slow" after about two months, the Board's resistance, and the 1773 payment. Its stated total of £22,065 does not match the sum of its own three amounts, so this article gives no total.)
  6. Wikipedia, "John Harrison." https://en.wikipedia.org/wiki/John_Harrison (A secondary source, read through a page summary tool. It gives about one nautical mile of error for the 5 seconds and the 1773 Parliament payment. Its referenced sources were not read here.)
  7. Royal Museums Greenwich, "Who was John Harrison?" https://www.rmg.co.uk/discover/explore/longitude-found-john-harrison (Read through a page summary tool. It supplies the 1764 Barbados trial and the statement that H4 "had kept time within the most stringent limits of the 1714 Act." It gave no seconds figures, so the numbers come from the sources above.)
  8. Royal Museums Greenwich, "What is the Prime Meridian, and why is it in Greenwich?" https://www.rmg.co.uk/stories/time/what-prime-meridian-why-it-greenwich (Read through a page summary tool. It supplies "no natural line" for east-west, the choice of Greenwich for least disruption, and nearly two-thirds of the world's ships using Greenwich charts.)
  9. The Greenwich Meridian, "The adoption of a Prime Meridian and the International Meridian Conference of 1884." https://www.thegreenwichmeridian.org/tgm/articles.php?article=10 (Read through a page summary tool. It supplies the 25 nations, the 1911 and 1914 French dates, and an 1879 count of ships. The summary did not show the vote count, which comes from the next source.)
  10. Linda Hall Library, "Lewis Rutherfurd and Cleveland Abbe" (Scientist of the Day). https://www.lindahall.org/about/news/scientist-of-the-day/lewis-rutherfurd-and-cleveland-abbe (Read through a page summary tool. It gives the 22-1 result with San Domingo against. It mentions France as abstaining; the Brazil abstention and the October 13 date come from search results that list the conference vote, and the conference's own record of that day was not read. Treat those details as secondary.)
  11. Protocols of the Proceedings of the International Conference Held at Washington for the Purpose of Fixing a Prime Meridian and a Universal Day (1884), Project Gutenberg edition. https://www.gutenberg.org/ebooks/17759 (Only the part up to the October 6 session was available to the reading tool, so the final vote record was not checked here. The "more than 70 per cent" remark is from a US Navy delegate on October 6, as the summary reported it.)

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