On 16 January, you take a sight of a star....

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Question 1 of 2066636ef128f7522a1c522da3

On 16 January, you take a sight of a star. The sextant altitude (hs) is 4°33.0'. The temperature is - 10°C, and the barometer reads 992 millibars. The height of eye is 42 feet. The index error is 1.9' off the arc. What is the observed altitude (Ho)?

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Question 1 of 20

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🔍 Key Concepts

• Index correction (IC): how to apply "off the arc" vs "on the arc" to hs • Dip (height of eye correction): why it is always subtractive from hs for a sea horizon • Refraction and temperature/pressure corrections for a star sight from the Nautical Almanac tables

💭 Think About

• Start from hs and apply each correction step‑by‑step in the correct order. Decide first whether the index error makes the sextant read too high or too low. • Convert the 42‑foot height of eye into dip using sight reduction tables or the dip formula, then think about whether this makes the apparent altitude bigger or smaller. • Use the approximate star refraction correction for a small altitude and then adjust it slightly for the very cold temperature and low pressure. Ask yourself: does colder, lower‑pressure air increase or decrease the refraction correction magnitude?

✅ Before You Answer

• Be very clear on the sign of the index correction for 1.9' "off the arc" before doing anything else. • Confirm that dip is always subtracted from the sextant altitude when using a sea horizon. • After applying all corrections (IC, dip, refraction, and small temperature/pressure adjustment), check that your final Ho is reasonably close to hs (only a few minutes different, not tens of minutes).

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