Earth's Coordinate System

Earth's coordinate system uses latitude and longitude to locate positions on the planet. Latitude is set by the Earth's axis of rotation (or for any rotating sphere).  Each latitude creates a full circle centered about the Poles, which is where the axis of rotation intersects the Earth's surface.  The North Pole is 90ºN, the South Pole is 90ºS, and the Equator, which is halfway between the Poles, is 0º. 

 

The largest latitude circle (greatest radius and perimeter) occurs at the Equator (0º), the smallest at the Poles.  

 

Latitudes may be expressed as positive and negative values. Northern Hemisphere latitudes are positive, and negative latitudes occur in the Southern Hemisphere. 

 

Longitude must be chosen based on political, geographic, economic, aesthetic and/or logical reasons.  For Earth, 0º was selected to go through the British Royal Observatory in Greenwich, England in 1851.  A line of longitude, also called a meridian, creates a half circle starting at the North Pole and ending at the South Pole. All lines of longitude meet at the Poles.


Longitudes may be expressed at positive and negative values.

 

Positive longitudes are in the Eastern Hemisphere, and negative occur in the Western Hemisphere. 


Latitude drawn every 10º. The North Pole is 90ºN, the South Pole 90ºS, and the Equator 0º.

Latitude drawn every 10º. The North Pole is 90ºN, the South Pole 90ºS, and the Equator 0º.

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Notice that the Earth rotates counterclockwise in the Northern Hemisphere but clockwise in the Southern Hemisphere.  To be more consistent, Earth's rotation is described as moving from west to east since this works in both hemispheres.  And it is easy to remember since the east coasts of continents experience sunrise before the west coasts.


Time zones are based on longitude, and each are roughly 15º wide (360º / 24 hours = 15º). There may be changes due to geopolitical boundaries. The time zone that includes the Prime Meridian is extended 7.5º to the east and west, making this special longitude in the center. 


Supporting Articles


Latitude and Longitude,
 

Converting Between Sexagesimal and Decimal Coordinates,

 

and Finding Latitude and Longitude with Stars

Longitude drawn every 20º.  The Prime Meridian is 0º, and the International Dateline is 180º.

Longitude drawn every 20º.  The Prime Meridian is 0º, and the International Dateline is 180º.

Make a Sundial

A Clock, Compass, and Calendar All in One!

Although this may seem like a simple activity, it is so important to give this a try since these are such fundamental concepts and skills to begin to explore.  Very few of us take the time to watch shadows move, let alone mark, measure, and use them to understand our natural world.  Plus, nothing beats a bit of hands-on science!

 Before running the activity, check the weather forecast for cloud cover - clear skies are best to run this activity.
 

Suggestions: If you are running this at school, it works great if there are multiple classes working on it through the day making measurements.  The next class is when you learn to use it as a clock, compass, and calendar.  If a single class, I gave homework to work in teams to make measurements throughout the school day.  
 

If you have a block of time to spend at the sundial, bring additional tools with you.  A globe (or a ball and chalk to mark on the ball), compass, pencil and notebook/drawing pad are good.  Use the globe/ball to help visualize why the Sun appears to be moving as it is for your location - and any location on Earth.  Also, explore how latitude and longitude come into play with direction and timing of the motion of the Sun.  For example, did the Sun rise north of east, east, or south of east that morning?  Why is the Sun south of you at local noon (if you are north of 23.5ºN)?  Where would it be if you were south of 23.5ºS?

Use bright string to show past shadows.  Connect the ends of the shadows with a different color.

Use bright string to show past shadows. Connect the ends of the shadows with a different color.

Challenges

Use a globe or global map to find the latitude and longitude of your hometown, the capital of your state, and the capital of your country.  For each of these points, what is located on the exact opposite side of the globe?  How does latitude and longitude make this easy to calculate?


Based on your longitude, calculate how many hours ahead or behind you are from a clock located on the Prime Meridian.  This is your time zone. Tip: The time zone that includes the Prime Meridian extends 7.5º on either side of this longitude.  Check your answer at https://www.timeanddate.com/time/map/ 


Use this interactive latitude and longitude website to practice locating places on Earth.


Use this interactive latitude website to visualize how latitude lines are drawn on a map.


Use this interactive longitude website to visualize the eastern and western hemispheres.

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Questions

  1.  Why is the International Date Line (180º longitude) the beginning of the world's day and not the prime meridian?  Hint: Think about local noon, which occurs when the Sun is highest in the sky that day, and it occurs as the Sun is crossing that longitude.
  2. Why are there only 180º of latitude yet 360º longitude?
  3. Why is the Western Hemisphere behind the Greenwich Observatory clock, and the Eastern Hemisphere ahead?
  4. What is the longitude of the North Pole?  What time zone is it in?
  5. If you were standing at the South Pole, what direction would be your first step?
  6. If you look in the direction of your latitude circle other than at the Poles, what directions could you be looking?
  7.  If you look in the direction of your meridian (line of longitude), what directions could you be looking?

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