Tuesday, April 3, 2018

Lines of Longitude explained, with maps

Photo: Thesevenseas via Wikimedia, CC 3.0

From ScienceTrends by Daniel Nelson

You’ve probably heard of latitude and longitude before.
They’re the lines that divide the globe up into different regions, and points on the earth are specified by where the two types of lines intersect.
Without the longitude system, we wouldn’t be able to do many important things, like orient ourselves on the globe or calculate time zones.
A close examination of longitude which shows why this form of measurement is so critical to our lives and society.

Before delving into why longitude is so important, let’s be sure we have our definitions straight.
You may have confused longitude with latitude or vice-versa, so it’s important to know which lines on the globe are latitude and which are longitude.
Latitude is the system of measurement that runs east to west across the globe, diving the Earth into north and south.
It divides the earth into two hemispheres, with 90 degrees of latitude in the northern hemisphere and 90 degrees of latitude in the southern hemisphere.
The Equator, middle of the globe, is at 0 degrees latitude.
Longitude divides the globe into east and west halves, centered on a line called the Prime Meridian, or 0 degrees longitude.
Every other line that runs north to south across the globe is known as a meridian, and it measures one degree of the entire Earth’s circumference.
There are 360 degrees of longitude total, with 180 being west of the Prime Meridian and 180 being east of the Prime Meridian.

courtesy of NASA

Why do we need the Longitude system?

The longitude system is necessary because we have to have a standardized way of tracking the passage of time across the globe.
It would be troublesome if people in one part of the world had no method of determining what time it was in another part of the world.
A longitude system is also important for ocean navigation, as being able to track the passage of time across the various time zones is necessary for orientation.
Scientists can use the system to help them calculate trajectories, monitor weather data, and engineer self-driving vehicles.

Map of Africa with lines of latitude and longitude.
Image: (Eric Gaba – Wikimedia Commons user: Sting) CC 3.0

These problems were understood by various explorers, traders, and mariners throughout the centuries.
Fortunately, tracking the movement of the sun provides a reliable way of measuring time.
The British Government passed the Longitude Act in the early 1700’s, an act which promised a substantial amount of money to the person who could design a way to track longitude at sea.
This problem was eventually solved by John Harrison, who invented a device called the marine chronometer which allowed sailors to determine their longitude position while at sea.
Even after chronometers were proven reliable, many cities and small towns still continued to set their clocks based upon sunset and sunrise.
The problem with this is the fact that variables like altitude impact sunrise and sunset, leading to a situation where cities located on roughly the same lines of latitude had different times.
This problem was compounded by the proliferation of railroads during the industrial revolution.
It was hard to coordinate the schedule of trains because each city would have its own time.
To solve this problem, nations began standardizing time zones based, more or less, upon lines of longitude.

Maritime Time Zones with the GeoGarage platform

One place on the globe had to be chosen as the Prime Meridian, and it was eventually decided that the city of Greenwich would be used as the location for the Prime Meridian.
This is why the Prime Meridian is often called the Greenwich Meridian and the world’s standard time is Greenwich Mean Time (GMT).
The now international 24 hour system developed out of this initial system, and now all time zones are based on the Prime Meridian and lines of longitude.

 A map of the world but it’s just the time zones.

In terms of calculating time with longitude, time zones shift (more or less) every 15 degrees.
Dividing the 360 degrees of longitude by 15 should result in 24 perfect time zones, though in reality time zone borders often follow political or geographical boundaries.
Some time zones even have offsets of only half an hour or 45 minutes.
This means there’s actually quite a few more time zones than twenty-four.
For every line of longitude that the sun passes, approximately four minutes pass.
Another notable line running north to south across the globe is the International Date Line.
The Date Line is about halfway across the Pacific Ocean, in between North America and Asia.
The Date Line isn’t straight, it curves around to avoid cutting across countries and certain political borders.
You need to add a day to your calendar if you cross the Date Line while going east to west, and you need to subtract a day if you cross it going west to east.

Image: Jailbird via Wikimedia Commons, CC 3.0

The degrees of longitude that make up the time zones are set approximately 60 nautical miles, 69 regular miles, or 11 kilometers apart at the equator.
This distance varies and shrinks as the lines of longitude move closer to the poles.
This happens because the Earth is widest at the Equator and becomes more narrow towards the poles, the meridians converge on one another at the North and South poles.
When it comes to calculating one’s global position, each degree of longitude can be divided into 60 minutes, and these minutes divided into sixty seconds.

The longitude of Seattle, Washington, for example, is:
122.3321° W. (122 Degrees, 33 Minutes, 21 Seconds West).
It’s this extremely precise system, when combined with the latitude system of measurement, allows people to figure out their exact place on the globe.
Without these measurements, Global Positioning Systems wouldn’t work.

This video demonstrates how to use observations of the sun combined with local and Greenwich time to determine longitude.
An interesting project for anyone interested in navigation and the geometry of our relationship with the sun.

Reading GPS Coordinates

To read GPS coordinates, know that the latitude coordinates will be presented first in the coordinates.
The lines of latitude run 90 degrees north and south, so check the N or S after the coordinates to see which hemisphere it is in.
The longitude coordinates are given after the latitude coordinates, with “W” representing points west of the Prime Meridian leading up to 180 degrees and “E” representing points east of the Prime Meridian, also leading up to 180 degrees.
Let’s look at the full GPS coordinates of a point in Seattle this time:
47.6062° N, 122.3321° W.
That’s 47 degrees North, 122 degrees West, 33 minutes, 21 seconds.
Now you can see that lines of longitude and latitude are important for both scientific research and our daily lives.

Links :

No comments:

Post a Comment