🔍 Key Concepts

• Index error (off the arc vs. on the arc) and how it changes hs to get ha • Dip (height of eye correction) from the height of eye in feet or meters • Main altitude corrections for Sun’s lower limb (refraction + semi-diameter, and additional corrections from the Nautical Almanac)

💭 Think About

• First, decide whether an index error that is off the arc is added to or subtracted from hs to get the apparent altitude (ha). What is the sign of this correction? • Next, determine the dip correction for a height of eye of 56 ft (or 17.1 m). Is that correction added or subtracted? About how many minutes of arc do you expect? • Finally, think about the Sun’s lower limb correction for an altitude around 54°. Should this net correction (refraction + semi-diameter) be positive or negative, and is it larger or smaller than 15'? Combine all your signed corrections and compare with the choices.

✅ Before You Answer

• Be sure you treat index error “off the arc” with the correct sign (don’t mix it up with “on the arc”). • Confirm that dip is always a negative correction (it reduces the apparent altitude because your eye is above sea level). • When you look up the Sun’s altitude correction for lower limb, use the correct column for the observed altitude range (around 54°) and apply it with the correct sign.