Longitude: Simple Notes
What is it? Longitude is like measuring how far east or west a place is on Earth. Imagine lines drawn on Earth going from the North Pole to the South Pole – longitude tells you which of these lines you're on.
How we measure it: Like latitude, we use degrees (°) to measure longitude. Again, think of angles, but this time measured east or west.
Starting Point: This is important! Unlike the Equator for latitude, there's no natural "middle" line for longitude. People had to choose a starting line.
Prime Meridian: The chosen starting line for longitude is called the Prime Meridian.
Greenwich: In 1884, everyone agreed to make the meridian that passes through the Royal Astronomical Observatory in Greenwich, near London, England as the Prime Meridian. So, Greenwich is at 0° longitude.
Meridians of Longitude: These are the North-South lines of longitude.
They are like half-circles (semi-circles) that go from the North Pole to the South Pole.
They all pass through the Equator and converge (meet) at the North and South Poles.
East and West:
Longitude is measured east and west of the Prime Meridian.
Meridians go eastwards up to 180° East.
Meridians go westwards up to 180° West.
180° East and 180° West are actually the same line on the opposite side of the Earth from the Prime Meridian.
Distance of a Degree of Longitude:
The Earth is round (spherical), about 25,000 miles around (circumference).
There are 360 degrees of longitude in total (all the way around the Earth).
At the Equator, 1 degree of longitude is about 25,000 miles / 360 degrees ≈ 69.1 miles.
Important: Unlike latitude, the distance of a degree of longitude changes as you move towards the poles.
Meridians get closer together as they go to the poles.
So, the distance of 1° of longitude gets shorter as you go away from the Equator towards the poles.
Examples of Length of 1° Longitude:
Equator (0° latitude): 69.172 miles (longest)
25° latitude: 62.73 miles
45° latitude: 49 miles
75° latitude: 18 miles
Poles (90° latitude): 0 miles (meridians meet here)
Using Longitude for Distance?
Because the length of a degree of longitude changes so much (especially away from the tropics), we don't use longitude to easily calculate distances like we do with latitude (using the 69 miles average).
Key Job of Longitude: Time!
Longitude is super important for determining local time.
It helps us understand Greenwich Mean Time (GMT), also called World Time.
As the Earth rotates, different longitudes face the sun at different times, which is why places at different longitudes have different times of day. We'll learn more about this later!
In simple words: Longitude is like measuring how far east or west you are on Earth using lines that go from pole to pole, starting from a line called the Prime Meridian that goes through Greenwich. While it's not great for measuring distances directly like latitude, it's essential for figuring out time around the world!