Great circle perpendicular to Earth’s axis — naming and notation What do we call the great circle whose plane is perpendicular to Earth’s axis of rotation?

Introduction / Context:
In geodesy and astronomy, the Earth’s axis of rotation and the equator define the primary reference framework. The equator is both a geographical feature on Earth’s surface and a geometric concept on idealized spheres and ellipsoids. Recognizing equivalent terminology avoids confusion across surveying, cartography, and celestial mechanics.

Given Data / Assumptions:

• A “great circle” is the intersection of a sphere with a plane through the sphere’s centre.
• Earth’s axis of rotation is idealized as a fixed line.
• Standard geodetic naming conventions are used.

Concept / Approach:
The plane perpendicular to Earth’s rotation axis and passing through the centre intersects the sphere in a great circle. That great circle is called the equator. On Earth, this is the terrestrial equator. On latitude systems, its angular value is 0° latitude. All three terms refer to the same construct in different vocabularies (geometric, geographic, and cartographic).

Step-by-Step Solution:

Verification / Alternative check:
Globes, maps, and celestial spheres all mark the equator as 0°, confirming the equivalence among the listed terms.

Why Other Options Are Wrong:
Each of (a), (b), and (c) is correct; hence the comprehensive answer is “All of the above.” “None of these” is clearly incorrect.

Common Pitfalls:
Mixing the geographic equator with the magnetic equator (a different concept); confusing geodetic with geocentric latitude (a subtle but separate issue).

Final Answer:
All of the above