🔍 Key Concepts
• Great circles on the Earth and what makes one "primary"
• How altitude of a celestial body is measured in celestial navigation
• What it means to "reduce" celestial observations to a reference line
💭 Think About
• Look at each statement (A, B, C) and ask: Is this a definition of the equator, or is it describing something related to the equator?
• For choice A, think about: From what reference is altitude usually measured in celestial navigation – from the equator or from the horizon?
• For choice C, ask yourself: When you plot or reduce celestial observations for position fixing, are you reducing them to the equator, or to another reference like a specific latitude or assumed position?
✅ Before You Answer
• For each choice, decide is this strictly true about the equator itself, or is it mixing in other navigation concepts?
• Confirm that you clearly understand the definition of a great circle and how the equator fits that definition.
• Before choosing D (All of the above), make sure every single statement (A, B, and C) is fully correct about the equator, not just partially related.