At the equator, a difference of one degree in longitude corresponds approximately to how many kilometres on the Earth surface?

Introduction / Context:

This question evaluates understanding of the geographic grid system, particularly how angular measurements in degrees of longitude convert to linear distances on the Earth surface. Knowledge of the approximate distance represented by one degree of longitude at the equator is a basic geographic fact useful in map reading, navigation and interpreting coordinates.

Given Data / Assumptions:

• The Earth is approximated as a sphere with an average equatorial circumference of about 40 000 km.
• The full circle around the Earth is 360 degrees of longitude.
• The question asks specifically about one degree of longitude at the equator, where lines of latitude and longitude intersect at right angles.
• Small variations due to the Earth being an oblate spheroid are ignored for this approximate calculation.

Concept / Approach:

To find the linear distance represented by one degree of longitude at the equator, we divide the total length of the equatorial circumference by 360 degrees. The formula used is simple: distance per degree = total circumference / 360. Using an approximate value of 40 000 km for the Earth circumference leads to a convenient figure near 111 km per degree. This relationship is standard in geography and assists in estimating distances from coordinate differences.

Step-by-Step Solution:

Verification / Alternative check:

More precise measurements of the Earth equatorial circumference give a value around 40 075 km. Dividing 40 075 by 360 gives approximately 111.3 km per degree of longitude at the equator. This refined calculation still rounds to 111 km in most exam settings. Textbooks and atlases often directly state this approximate value. Therefore, selecting 111 km is verified as correct.

Why Other Options Are Wrong:

The value 101 km is too small compared with the calculated result and would imply an unrealistically small Earth circumference. The option 121 km is too large and would correspond to an unreasonably large Earth. The value 91 km also underestimates the distance and again implies a much smaller circumference. The option 150 km is clearly far above the calculated value and is not consistent with known Earth dimensions. Thus only 111 km matches accepted geographic data.

Common Pitfalls:

Students may confuse distances per degree of longitude with those per degree of latitude, or may try to memorise numbers without understanding the underlying division. Some also mistakenly think that one degree always equals the same distance everywhere, forgetting that longitude spacing decreases toward the poles. Remembering the simple calculation using the approximate circumference makes it easier to deduce the correct value under exam pressure.

Final Answer:

At the equator, a difference of one degree in longitude is approximately 111 km.