Prime Meridian vs. Worldwide Reference Meridian
Not like the equator (which is midway between the Earth’s north and south poles), the prime meridian is an arbitrary line. In 1884, representatives on the Worldwide Meridian Convention in Washington, D.C., met to outline the meridian that may symbolize 0 levels longitude. For its location, the convention selected a line that ran by way of the telescope on the Royal Observatory in Greenwich, England. On the time, many nautical charts and time zones already used Greenwich as the place to begin, so conserving this location made sense. However, in the event you go to Greenwich together with your GPS receiver, you’ll must stroll 102 meters (334 toes) east of the prime meridian markers earlier than your GPS reveals 0 levels longitude. Within the nineteenth century, scientists didn’t take into consideration native variations in gravity or the marginally squished form of the Earth once they decided the placement of the prime meridian. Satellite tv for pc expertise, nonetheless, permits scientists to extra exactly plot meridians in order that they’re straight traces working north and south, unaffected by native gravity adjustments. Within the Nineteen Eighties, the Worldwide Reference Meridian (IRM) was established because the exact location of 0 levels longitude. Not like the prime meridian, the IRM just isn’t a set location, however will proceed to maneuver because the Earth’s floor shifts.
Traces of longitude, additionally known as meridians, are imaginary traces that divide the Earth. They run north to south from pole to pole, however they measure the gap east or west.
The prime meridian, which runs by way of Greenwich, England, has a longitude of 0 levels. It divides the Earth into the jap and western hemispheres. The antimeridian is on the alternative facet of the Earth, at 180 levels longitude. Although the antimeridian is the premise for the worldwide date line, precise date and time zone boundaries are depending on native legal guidelines. The worldwide date line zigzags round borders close to the antimeridian.
Like latitude, longitude is measured in levels, minutes, and seconds. Though latitude traces are at all times equally spaced, longitude traces are furthest from one another on the equator and meet on the poles. On the equator, longitude traces are the identical distance aside as latitude traces — one diploma covers about 111 kilometers (69 miles). However, by 60 levels north or south, that distance is all the way down to 56 kilometers (35 miles). By 90 levels north or south (on the poles), it reaches zero.
Navigators and mariners have been in a position to measure latitude with primary instruments for 1000’s of years. Longitude, nonetheless, required extra superior instruments and calculations. Beginning within the sixteenth century, European governments started providing large rewards if anybody might clear up “the longitude downside.” A number of strategies have been tried, however the very best and easiest option to measure longitude from a ship was with an correct clock.
A navigator would examine the time at native midday (when the solar is at its highest level within the sky) to an onboard clock that was set to Greenwich Imply Time (the time on the prime meridian). Every hour of distinction between native midday and the time in Greenwich equals 15 levels of longitude. Why? As a result of the Earth rotates 360 levels in 24 hours, or 15 levels per hour. If the solar’s place tells the navigator it’s native midday, and the clock says again in Greenwich, England, it’s 2 p.m., the two-hour distinction means the ship’s longitude is 30 levels west.
However aboard a swaying ship in various temperatures and salty air, even essentially the most correct clocks of the age did a poor job of conserving time. It wasn’t till marine chronometers
have been invented within the 18th century that longitude could possibly be precisely measured at sea.
Correct clocks are nonetheless crucial to figuring out longitude, however now they’re present in GPS satellites and stations. Every GPS satellite tv for pc is provided with a number of atomic clocks that present extremely exact time measurements, correct to inside 40 nanoseconds (or 40 billionths of a second). The satellites broadcast radio alerts with exact timestamps. The radio alerts journey at a continuing pace (the pace of sunshine), so we will simply calculate the gap between a satellite tv for pc and GPS receiver if we all know exactly how lengthy it took for the sign to journey between them.
On the bottom, NOAA’s Nationwide Geodetic Survey manages the Constantly Working Reference Stations Community, which contains 1,800 stationary, completely working GPS stations. These CORS constantly obtain GPS radio alerts and incorporate that information into the Nationwide Spatial Reference System. The GPS place on a smartphone is correct to inside about 5 meters (16 toes), however processed CORS information can present longitude correct to inside a number of centimeters, together with latitude and top positions.
Infographic Transcript: Longitude
- The left picture reveals the Earth tilted on its axis. The equator and the Greenwich meridian line (prime meridian) are labeled. A bit has been reduce away to indicate longitude traces (or meridians) that run vertically across the Earth. These begin on the Greenwich meridian line and are labeled from 0 to 90 levels east. Further traces are drawn from the middle of the Earth to every labeled meridian. These illustrate that the longitude measurement for every level equals the angle made between a line from it to the middle of the earth and one drawn by way of the airplane of the prime meridian. Close to the South Pole, an arrow signifies the route of the Earth’s rotation on its axis.
- The suitable picture reveals the Earth with the North Pole on the heart of the picture. At this angle, the Greenwich meridian line runs straight up and down, with 0 levels longitude labeled on the backside of the picture. The antimeridian (180 levels longitude) is discovered straight on the prime of the picture. The left half of the picture is labeled Western Hemisphere and the best half labeled Jap Hemisphere. Meridians are labeled across the Earth, from 0 to 180 levels on the west facet and from 180 again to 0 on the east.