How to Calculate Degrees of Longitude

Longitude is a geographic coordinate that specifies the east-west position of a point on the Earth's surface. Measured in degrees, minutes, and seconds, longitude helps determine time zones, navigation routes, and the exact location of places. This guide explains how to calculate degrees of longitude using a simple formula and provides an interactive calculator for quick results.

What is Longitude?

Longitude is one of the two coordinates in the geographic coordinate system, the other being latitude. While latitude measures how far north or south a point is from the Equator, longitude measures how far east or west a point is from the Prime Meridian.

The Prime Meridian runs through Greenwich, London, and is designated as 0° longitude. Lines of longitude are measured eastward and westward from this reference line, with values ranging from 0° to ±180°. The 180° meridian is directly opposite the Prime Meridian.

Longitude plays a crucial role in navigation, timekeeping, and cartography. It helps determine time zones, as Earth rotates 15° per hour (360° in 24 hours). For example, moving 15° eastward means gaining one hour, while moving 15° westward means losing one hour.

How to Calculate Degrees of Longitude

Calculating degrees of longitude involves determining the angular distance between two points along a line of latitude. Here's a step-by-step method:

Identify the coordinates of the two points you want to measure between.
Ensure both points have the same latitude (or are on the same line of latitude).
Subtract the longitude of the western point from the longitude of the eastern point.
The result is the degrees of longitude between the two points.

For example, if Point A is at 30°E and Point B is at 60°E, the longitude between them is 60° - 30° = 30°.

If the points cross the Prime Meridian or the 180° meridian, you may need to adjust the calculation to account for the Earth's spherical shape.

The Formula

The basic formula for calculating degrees of longitude between two points is:

Longitude = Longitude₂ - Longitude₁

Where:

Longitude₁ is the longitude of the western point
Longitude₂ is the longitude of the eastern point

For points that cross the Prime Meridian or the 180° meridian, you may need to use the following adjusted formula:

Longitude = (Longitude₂ - Longitude₁ + 360) mod 360

This formula ensures the result is always between 0° and 180°.

Worked Example

Let's calculate the degrees of longitude between two cities:

City	Latitude	Longitude
New York	40.7128° N	74.0060° W
London	51.5074° N	0.1278° W

Since both cities are on different lines of latitude, we'll calculate the longitude difference directly:

Longitude = 0.1278° W - (-74.0060° W) = 74.1338°

The degrees of longitude between New York and London is approximately 74.1338°.

Note: This calculation assumes both points are on the same line of latitude. For more precise measurements, especially over long distances, spherical geometry calculations may be needed.

FAQ

What is the difference between longitude and latitude?: Latitude measures north-south position from the Equator, while longitude measures east-west position from the Prime Meridian. Together they define any point on Earth's surface.
How do I convert degrees of longitude to miles?: You can estimate the distance by multiplying the degrees of longitude by the cosine of the latitude and then multiplying by the Earth's circumference at the Equator (about 24,901 miles).
Why is the Prime Meridian at 0° longitude?: The Prime Meridian was established in 1884 at the Royal Observatory in Greenwich, London, as the official reference line for longitude measurements.
Can longitude be negative?: Yes, longitudes west of the Prime Meridian are negative, while longitudes east of the Prime Meridian are positive.
How does longitude affect time zones?: Each 15° of longitude corresponds to one hour difference in time zones, as Earth rotates 15° per hour. This creates 24 time zones around the world.