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).
• Sextant altitude corrections: index error, dip, refraction, semi-diameter, and parallax • How to apply an index error that is "on the arc" versus "off the arc" • Which altitude corrections apply to a bright planet like Venus and which are only for the Sun or Moon
• First, think about how to convert sextant altitude (hs) to apparent altitude (Ha). What two corrections must you apply using the index error and height of eye? • Next, consider what standard altitude correction table you would use for a planet, and which parts of the full sun/moon correction DO NOT apply to Venus. • After working through each correction step-by-step (with correct signs), compare your final Ho with the choices and see which is closest.
• Be sure you apply index error with the correct sign for "2.0' on the arc" (does it get added or subtracted from hs?). • Confirm you are using the planet correction, not the sun or moon tables—Venus is treated as a planet. • Check that your final Ho is lower than hs by a reasonable amount, mainly due to dip and refraction; if it ends up higher, re-check your signs.
No comments yet
Be the first to share your thoughts!