Lines of longitude on a globe or map are used to measure what?

Introduction / Context:
This question is about basic concepts of latitude and longitude, which form the coordinate system used to locate places on the Earth. Lines of longitude, also called meridians, run from the North Pole to the South Pole. They are used along with lines of latitude to specify positions. The question asks you to recall what exactly these lines measure in relation to the Prime Meridian.

Given Data / Assumptions:

• The focus is on lines of longitude, not latitude.
• We assume the Prime Meridian at zero degrees longitude is passed through Greenwich in London.
• Options include angular distance east or west, angular distance north or south, height, and depth.
• Longitude is expressed in degrees, minutes, and seconds.

Concept / Approach:
Longitude is defined as the angular distance of a place east or west of the Prime Meridian, measured along the equator. Values range from zero degrees at Greenwich up to 180 degrees east and 180 degrees west. Latitude, on the other hand, measures angular distance north or south of the Equator. Height and depth relate to elevation and bathymetry and are not measured by longitude lines at all.

Step-by-Step Solution:
Step 1: Recall definitions. Longitude is related to east west position, while latitude is related to north south position. Step 2: Recognise that the reference line for longitude is the Prime Meridian at zero degrees. Step 3: Understand that longitude is measured as an angle from this meridian, along the equator, to the meridian passing through the point of interest. Step 4: Compare the options. Only option A describes angular distance east or west of the Prime Meridian. Step 5: Confirm that height and depth are measured in units such as metres, not in degrees, so options C and D are clearly incorrect.

Verification / Alternative check:
Think of a global coordinate example such as 77 degrees east, 28 degrees north. The value 77 degrees east is the longitude, measured east of Greenwich. The value 28 degrees north is the latitude, measured north of the Equator. This illustrates that longitude is about east west angular distance, not north south. Any map or GPS display that shows coordinates follows this same convention.

Why Other Options Are Wrong:
Option B describes latitude, not longitude, since latitude is the angular distance north or south of the Equator. Option C, height above sea level, relates to vertical elevation and is measured in metres or feet, independent of longitude. Option D, depth of oceans, refers to bathymetry and is again measured in metres or similar units, not in degrees of longitude. These options do not match the definition of longitude.

Common Pitfalls:
A common mistake is to reverse the meanings of latitude and longitude or to think of both simply as grid lines without remembering what they measure. Another error is to assume that any coordinate system must measure height or depth, which in reality are separate from horizontal location. To avoid confusion, remember the simple rule: latitude equals ladder and measures north south, longitude equals long lines to the poles and measures east west angular distance from the Prime Meridian.

Final Answer:
Lines of longitude measure the angular distance east or west of the Prime Meridian.