Longitude

Lines run from the top to the bottom of Earth.

These lines show how far east or west you are. They are called lines of longitude.

These lines help us tell time. The Earth turns every day. This turn changes where the sun looks to be.

If you know the time in two places, you can find your spot. You can use the moon to help. You can also use special clocks.

Long ago, people used the stars to find their way. Now, we use tools from space. These lines help us map our world.

Longitude tells us how far east or west a place is.

Imagine lines running from the top of Earth to the bottom. These lines are called meridians. One special line is the prime meridian. It marks 0 degrees. Most of it passes near Greenwich in London.

Finding your longitude is a way to find your place using time. The Earth rotates every day. This means the sun looks different in different spots. A change of 15 degrees in longitude means a one-hour change in time.

Long ago, people used the moon to find their way. They looked for a lunar eclipse. An eclipse is when one object blocks the light of another. They would compare the time of the eclipse to a known time.

Later, people used special clocks called chronometers.

John Harrison was a maker who built these. They kept very steady time even on a wavy sea. In the 1900s, ships used radio signals to find their way. Now, we use tools from space called GPS. These tools help us find our exact spot on Earth.

Longitude is a way to measure how far east or west a place is on Earth.

You can think of these positions as imaginary lines called meridians. These lines run from the North Pole to the South Pole. One special line is called the prime meridian, which marks 0 degrees. It passes near the Royal Observatory in Greenwich, located in London.

Locations east of this line have positive numbers. Locations to the west have negative numbers. This system helps us map the surface of our planet accurately.

Finding longitude is a way of using time to find a location. Because the Earth rotates, the Sun appears at different spots at different times. A change of 15 degrees in longitude means the local time is one hour different. To find your spot, you must compare your local time to an absolute time. In the past, people used events like a lunar eclipse for this. A lunar eclipse happens when the Moon passes into Earth's shadow. By seeing when the eclipse happened in two different places, people could calculate the distance between them.

Ancient thinkers first studied these ideas. Hipparchus lived in the 2nd century BC. He used a system of 360 degrees and thought the Earth was a sphere. He also knew that comparing eclipse times could reveal longitude. Later, Claudius Ptolemy worked in the 2nd century AD. He created a mapping system using curved lines called parallels. He used a prime meridian that went through the Canary Islands. Many years later, astronomers like al-Battānī used the naked eye to watch eclipses. He could find the difference in longitude between two cities with very little error.

For a long time, finding longitude at sea was a hard job. Ships would toss and turn in big ocean waves.

Simple clocks did not work well on a moving boat. The British government passed the Longitude Act in 1714 to help. They offered big prizes to anyone who could solve the problem. A clockmaker named John Harrison spent over thirty years building special tools. He created five chronometers, which are very steady clocks. These clocks helped sailors know the time even during a stormy voyage.

Technology has changed how we find our way many times. In the 1800s, people used telegraphs to send time signals through wires. Later, wireless radio signals were sent from places like the Eiffel Tower in Paris. These signals helped ships check their clocks while they were traveling. After World War II, ships used radio beacons to find their positions. This worked even when the sky was too cloudy to see the stars. Today, we use a system called GPS to find our exact spot on Earth.

Longitude is a geographic coordinate used to specify an east-west position on Earth or other celestial bodies. It is expressed as an angular measurement, usually in degrees, and is often denoted by the Greek letter lambda (λ).

To visualize this, imagine imaginary semicircular lines called meridians that run from the North Pole to the South Pole. These meridians connect all points sharing the same longitude. The starting point for this measurement is the prime meridian, which is set at 0°. By international convention, the International Reference Meridian passes near the Royal Observatory in Greenwich in London. Locations to the east of this line are assigned positive longitudes, while locations to the west are assigned negative ones.

There is a deep scientific connection between longitude and the measurement of time. This connection exists because the Earth rotates on its axis. As the planet turns, different parts of the surface face the Sun at different times. Because of this rotation, a difference of 15° in longitude corresponds exactly to a one-hour difference in local time. To find a specific longitude, a person must compare their local time to an absolute measure of time. Historically, this absolute time could be found by watching a celestial event, such as a lunar eclipse, that is visible from two different locations. In more modern times, time signals could be received via telegraph or radio.

The study of longitude began with ancient Greek astronomers. In the 2nd century BC, Hipparchus developed a coordinate system based on a spherical Earth. He divided the circle into 360°, a standard still used today. He also suggested that comparing the local time of a lunar eclipse at two different places could reveal longitude. Later, in the 2nd century AD, Claudius Ptolemy created a mapping system using curved parallels to reduce distortion. Ptolemy used a prime meridian through the Canary Islands so that all longitude values remained positive. While his mapping system was sound, his data was often inaccurate. This caused him to overestimate the length of the Mediterranean by about 70%.

After the fall of the Roman Empire, interest in geography slowed in Europe, but Hindu and Muslim astronomers continued the work. They added many new locations and improved upon older data. For example, the astronomer al-Battānī used simultaneous observations of two lunar eclipses to find the longitude difference between Antakya and Raqqa. He achieved an error of less than 1°, which was the best possible using the tools of his time. He used an astrolabe to measure the altitude of a "clock star" to determine local time. By the 12th century, European interest revived as travel increased. Astronomers began using lunar eclipses to find the longitude differences between cities like Toledo, Marseilles, and Hereford.

Finding longitude at sea proved much harder than finding it on land. On land, errors were often less than 1° by the 1720s. However, sailors faced two massive problems: they needed immediate results, and the ocean environment was unstable. In a heavy ocean swell, it is very difficult to make precise observations. Furthermore, the pendulum clocks invented by Christiaan Huygens in 1657 did not work well on moving ships. To solve this, the British Parliament passed the Longitude Act in 1714. This law offered large financial rewards for methods that could determine longitude within 1° or 0.5°.

Two main solutions eventually emerged to meet these challenges. The first involved lunar distances, which used mathematical tables to compare the position of the Moon to stars. The second involved the development of the marine chronometer.

John Harrison, a Yorkshire carpenter and clock-maker, spent over thirty years building five different chronometers. These were highly accurate mechanical clocks that could keep time despite the motion of a ship. Harrison fought for many years to receive the top reward of £20,000, finally receiving an extra payment in 1773. While chronometers were initially very expensive, they became cheaper in the early 19th century and eventually replaced lunar distance methods around 1850.

As technology advanced, new ways to transmit time emerged. In 1839 and 1844, the first telegraphs were established in Britain and the United States. People realized they could use the telegraph to send time signals, which made determining longitude much easier. In the early 20th century, wireless telegraphy, or radio, allowed ships to receive time signals while at sea. For instance, signals were sent from Halifax, Nova Scotia, starting in 1907, and from the Eiffel Tower in Paris in 1910. After World War II, ships used radio navigation systems based on fixed beacons. These systems remained the standard for commercial shipping until they were replaced by GPS in the early 1990s.