During twilight on 28 December around 1800 GMT, in DR position LAT 4°00'N, LONG 0°06'W, the sextant altitude (hs) of Venus was 30°46.8'. The height of eye was 36 feet, and the index error was 2.0' on the arc. The temperature was 68°F. The barometer read 1030 mb. Calculate the observed altitude (Ho).

Question

During twilight on 28 December around 1800 GMT, in DR position LAT 4&deg;00'N, LONG 0&deg;06'W, the sextant altitude (hs) of Venus was 30&deg;46.8'. The height of eye was 36 feet, and the index error was 2.0' on the arc. The temperature was 68&deg;F. The barometer read 1030 mb. Calculate the observed altitude (Ho).

Accepted Answer

Option C: Ho 30&deg;38.1'. According to Bowditch, observed altitude (Ho) is derived by applying index error and dip (height of eye correction) to sextant altitude (hs) to find apparent altitude (ha), then applying refraction, semidiameter, and parallax corrections. For Venus, star/planet refraction and atmospheric pressure/temperature corrections yield Ho 30&deg;38.1'.