How can you measure latitude and longitude

Measuring the earth


And yet she is moving ...

Is the earth flat - or is it round? Do the planets rotate around the sun - or do the stars rotate around the earth? Questions that seem obsolete to us in the 21st century. And yet in earlier times people fought bitterly about it until the knowledge of mathematicians and astronomers could no longer be pushed aside.

When Yuri Gagarin became the first cosmonaut to circumnavigate the earth in a space capsule on April 12, 1961, an old human dream came true. Gagarin saw the blue planet with his own eyes from space. He was so impressed by it that he sent the following message to humanity: "I saw how wonderful our planet is, let us preserve this beauty, increase it and not destroy it." The 27-year-old cosmonaut was also the first to see the globe-like shape of the earth for himself.

Astronomers and mathematicians already recognized the spherical shape of the earth in ancient times. Pythagoras (6th century BC) not only took this view, he also advocated spherical celestial bodies.

Central are the observations of the Greek philosopher Aristotle (384 to 322 BC). He closely observed lunar eclipses and noticed that the earth cast a circular shadow on its satellite, the moon. A shadow curved in this way, Aristotle concluded, could only come from a sphere.

He also observed that as ships moved away, the hull was already out of sight before the sails. Finally, Eratosthenes succeeded in measuring the exact circumference of the earth for the first time in the 3rd century BC.

Since the first century AD, the spherical shape of the earth was generally accepted among European scholars. The common people knew about it too. But with the beginning of the Middle Ages, the proponents of the actually outdated disk theory found more popularity again.

The Latin rhetoric teacher Lactantius (245 to 325 AD) described the ball theory as nonsensical, since people would have to fall down on the underside of such a ball. Many medieval contemporaries saw the idea of ​​a spherical earth as a strong contradiction to the Bible. There the creation story outlines the image of a flat earth.

The graticule of the earth

But if the earth was a sphere, how could one find one's way around it and determine one's own position more precisely? Here, too, ancient scientists had the brilliant idea. It was again Eratosthenes who designed a map of the then known world and divided the surface into quadrants, creating a first grid in this way.

The Greek astronomer and mathematician Hipparch (around 190 to 120 BC) first divided the earth into 360 degrees in an east-west direction - the longitude was invented. The cartographer and astronomer Ptolemy (100 to 175 AD) transferred this graticule into his atlas. The creation of the first globe and the determination of position by means of geographical longitude and latitude go back to him.

The graticule of the earth, which can be seen on every map today, is a human invention. An artificial network with imaginary lines, designed to make it easier to determine your position. This network is divided into longitudes and latitudes.

The lines of longitude lead in elliptical orbits from top to bottom, they meet at the north and south poles of the earth, where they condense into a single point. The latitudes, on the other hand, are concentric orbits that never touch. They divide the earth into slices from top to bottom.

The graticule is made up of numbered lines that cross each other. The resulting intersections work like a coordinate system with an X-axis and a Y-axis. The positioning method is based on the fact that each intersection point unmistakably only exists once. Every place in the world can therefore be identified and named with exactly one number, one coordinate, consisting of longitude and latitude.

Determination of latitude and longitude

The age of great circumnavigations began with Christopher Columbus (1451 to 1506). After America, other hitherto unknown countries and continents were discovered. However, the mapping of the coastlines was still very imprecise in the pioneering days. There was a lack of reliable navigation and surveying methods.

In the middle of the 18th century, the sextant was developed, with the help of which the geographical latitude could be determined on the high seas by measuring the height of the sun above sea level.

On the other hand, it was much more difficult to calculate the longitude. Basically, all you need to know about longitude is knowledge of time zones and an accurate clock. Because the longitude is calculated by measuring time. If you want to determine the geographical longitude on land or in the open sea, you need to know both the exact local time at sea and the reference time on land, the so-called Greenwich Mean Time.

At that time the English established the convention to locate zero longitude near Greenwich in London. Since England was the most powerful seafaring nation on earth at the time, Greenwich Mean Time has prevailed on navigation maps and has remained the prime meridian around the globe to this day.

Once the navigating seaman has determined the time difference, he can translate it into the geographical distance. The earth needs exactly 360 degrees for one complete revolution, so the time difference of one hour corresponds to 15 degrees of longitude exceeded.

As early as 1530, a Flemish astronomer suggested using mechanical clocks to calculate longitude from the time difference between the home port and the place at sea. A brilliant idea, but the pendulum clocks available at the time were not up to the rough seas. The consequences were fatal. Ships and cargoes were lost. People drowned because the ships could not determine their exact position and ran aground.

The longitude problem

The number of accidents alarmed the British Parliament. In 1714 it advertised with the so-called "Longitude Act" a price of 20,000 pounds for those who could present a practicable solution for determining the degree of longitude.

It was not until 1736 that the Scottish watchmaker John Harrison succeeded in developing a clock that ran so precisely that the time could be read reliably even in adverse climatic conditions and violent ship movements at sea. This enabled seafarers to determine the time difference between their home port and their place at sea at the highest point of the sun.

Harrison devoted his entire life to developing this marine chronometer, which he continued to improve in the following years. In 1759 Harrison presented his fourth model to the amazed public: the H4. With a diameter of 13 centimeters and a weight of 1.45 kilograms, the chronometer had shrunk to the size of a pocket watch. The watch did what it promised. In 1773 Harrison was awarded the premium.

The mathematician Tobias Mayer from Marbach am Neckar was also involved in calculating the longitude. Mayer worked with the so-called moon distance method. The moon was the pointer of an imaginary clock, the dial the starry sky.

A certain distance between the moon and another celestial body corresponded to a very specific point in time. So the sailor had to measure the angular distance between the moon and certain stars. The time at sea determined in this way could then be compared on lunar tables with the Greenwich Mean Time specified there.

The degree of longitude determined by the position of the moon was so precise that the geographical longitude at sea could be determined with an accuracy of 0.5 degrees. In 1755 Mayer submitted his moon tables to the English government. But the mathematician died prematurely. It was only his widow who successfully fought £ 3,000 from the British reward for Mayer's calculations. A German was also involved in the solution to determine the longitude.

Satellite-based positioning

To this day, sextants and chronometers are sufficient to determine latitude and longitude on the high seas. But now an electronic, computer-aided navigation system the size of a cigarette packet is sufficient to be able to determine your own position with pinpoint accuracy with the help of satellites.

The so-called Global Positioning System (GPS) consists of a network of 32 satellites that orbit the earth at an altitude of 20,200 kilometers. The satellites send radio signals from which a GPS receiver calculates the respective location. Measuring the earth and determining your own position are now child's play thanks to the use of high technology.