What is the primary cause of index error of...

🔍 Key Concepts

• Index error on a sextant is the error present when the sextant reads something other than 0° with the index arm set to 0 and the horizon viewed normally • The relationship between the horizon glass (or clear glass) and the index mirror is critical for proper alignment • Understand the difference between instrument error and human error in celestial observations

💭 Think About

• When you set the sextant to 0° and look at a distant horizon, what physical misalignment inside the instrument would make the direct and reflected images not line up? • Which pair of surfaces must be exactly parallel so that a 0° reading truly represents no angle between the two lines of sight? • Among the options, which describes a built-in alignment issue of the instrument itself, rather than a user mistake or a secondary effect?

✅ Before You Answer

• Verify which component is attached to the index arm and moves with it versus which is fixed to the frame • Check which choice describes a condition you can detect and correct when you "check the sextant for index error" before taking sights • Eliminate any options that are primarily about observer technique rather than internal optical alignment of the sextant

What is the primary cause of index error of a sextant?

The horizon glass not being parallel to the horizon mirror

The horizon glass not being parallel to the index mirror

Improperly correcting the other errors in a sextant

Human error in taking a celestial observation

🔍 Key Concepts

💭 Think About

✅ Before You Answer