On 4 July you observe the lower limb of the Sun at a sextant altitude (hs) of 25° 29.8'. The index error is 3.1' off the arc. The height of eye is 48 feet (14.6 meters). What is the observed altitude (Ho)?
• Index correction (IC): how to apply a sextant error that is "off the arc" versus "on the arc" • Dip (height of eye) correction: how height of eye in feet/meters affects the sextant altitude • Main Sun correction from the Nautical Almanac for a lower limb sight at about 25° altitude on 4 July
• First, think about whether an index error that is 3.1' "off the arc" should be added to or subtracted from the sextant altitude to obtain the apparent altitude (Ha). • Next, decide the sign of the dip correction for a 48 ft height of eye—does it make the observed altitude larger or smaller? • Finally, when you look up the combined Sun correction for the LOWER limb at about 25° on 4 July, think about whether the total correction is roughly a small positive number, a small negative number, or close to zero.
• Be clear on the sign of index correction: off-the-arc vs on-the-arc changes whether you add or subtract. • Estimate the magnitude of dip for 48 ft (about 14.6 m) and check that you are using the correct units/table for feet or meters. • Confirm you are using the lower limb Sun correction for the correct altitude band (~25°) and correct date (4 July), and that you apply it with the proper sign.
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