One of the most used was that passed by the Orchilla lighthouse, which is located on the Canary Island of El Hierro. Which is why this small island is also known as the island of the Meridian. The French were, who established the zero Meridian at that point in 1634 and that there were also those who most opposed their transfer to Greenwich two and a half centuries later.
Finally, the French also accepted that this peculiar imaginary line went through Greenwich. But when it seemed that the discussion had ended, appeared the GPS.
When developed positioning systems by satellite and, more specifically, the system of Global Positioning (GPS), it was observed that the zero Meridian was not going exactly through where it is assumed that it should pass, but was displaced exactly 102 meters. Is this difference due to what?
Historically, scientists have used two methods for the establishment of coordinates on the Earth's surface: the astronomical and the geodetic survey. Long been known that the small displacement of the Meridian zero can be attributed to the small differences between both methods.
Now, in a recent study, a team of researchers has determined that these differences are due to the peculiar form which has gravity in Greenwich.
The Earth's surface is littered with irregularities in the ground: large mountain ranges, deep ravines or extensive plains. This causes small variations occurring in gravity, at the same time causing variations in measurement of the vertical.
This variation is known as "deviation from the vertical", which is nothing more than a measure of up to what point the direction of local gravity, which in principle should be perpendicular to the Earth's surface has been displaced due to local anomalies, such as nearby mountains.
Traditionally the vertical measurement has been done through a bowl of mercury, so far depended on local conditions. However, GPS system measures the vertical from space, by drawing a straight line passing through the center of the Earth and thus saving the possible effects of gravity.
The authors of the study have been calculated, using gravitational models of high resolution, that in Greenwich. this deviation is the sign and magnitude just explaining the 102 meters of displacement of the zero Meridian.
In addition, to verify that Greenwich apparent length change is indeed an effect located due to the direction of gravity and not a global change in the system of the length of the world, the authors of this study also analyzed the differences in the coordinates of other points, observing the same differences attributable to gravity.
The mission New Horizons, an unmanned space mission of the American Space Agency (NASA) has shown a single image of the misty layers of the atmosphere from the icy mountains of Pluto. On the practical ground we see the usefulness of this technology in the control center to guard coastal Italina.
It's just one of the many responsibilities of the coast guard:
"Keeping illegal fishing under control, search and rescue, as well as the fight against pollution, or piracy and terrorism. All this, without special missions that other police forces can assign us to the national security ", says the Commander of the Department of civilian operations, Antonio Lofu.
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All kinds of incidents are handled from here. A place extremely busy in recent months with the infinite number of calls for help from immigrants who cross the Mediterranean.
"When we have a few days of Sun and the State of the sea is favorable, the outputs soar. In such a way that, sometimes, we have to deal with 25 or 26 alarm situations at the same time", adds Sorop Façe, Deputy Chief of search and rescue coordination.
Every time an alarm sounds, time is crucial. For the rescue of lives, the coast guard needs to know exactly at what point in the open ocean the boat is in question.
Without satellite technology, this whole process of search and rescue would be almost impossible.
"This is the area which manages the operations of rescue of the immigrants. Here, on the map, you can see the side of Sicily and Libya. In general, immigrants use satellite phones to send distress calls as soon as their boats away 10, 20 or 30 miles off the Libyan coast. Using this technology, we can determine the trace of merchant ships and fishing boats in the area. "And from here we decided what unit (boat) is best positioned to provide assistance", explains Syrian Fae.
All types of satellites is involved in one way or another in maritime safety. Some, like the European fleet Sentinel, have radar and optical instruments. Others, such as the Canadian exactEarth, collects the messages of the system of automatic identification of ships.
Carsten Tobehn, of the European Space Agency, explains what happens there above: