During evening twilight on 28 December a sextant altitude (hs) of the planet Venus was 29° 43.2'. The height of eye was 40 feet, and the index error was 2.0' on the arc. What was the observed altitude (Ho)?
• Sextant altitude corrections: sequence from hs → ha → Ho • Meaning and sign of index error ‘on the arc’ vs ‘off the arc’ • How dip (height of eye) and the planet main correction affect the final altitude
• First, decide whether an index error noted as 2.0' on the arc should be added to or subtracted from hs. What is the rule? • Next, estimate the dip for a 40‑foot height of eye. Should dip be added or subtracted from the sextant altitude, and is its magnitude closer to 2', 4', 6', or 10'? • After applying index error and dip to get the apparent altitude (Ha), think about the combined correction for a planet from the Nautical Almanac tables. For an altitude near 30°, is that correction a small addition or subtraction, and roughly how big (a few tenths of a minute, about 1', or several minutes)?
• Be sure you apply index error with the correct sign: ‘on the arc’ vs ‘off the arc’ is critical. • Confirm that dip is always negative (it reduces the altitude) when using a natural horizon. • Check that your total correction from hs to Ho is reasonable: for an altitude around 30° with 40 ft height of eye, the net change should be several minutes of arc, not just a few tenths.
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