On 12 June your 1845 DR position is LAT 21° 47' N, LONG 46° 52' W when you observe a faint unidentifiable star through a break in the clouds. The star bears 282.5° T at a sextant altitude (hs) of 14° 22.3'. The index error is 0.5' on the arc, and the height of eye is 45 feet. The chronometer reads 09h 43m 27s, and the chronometer error is 1m 46s slow. What star did you observe?
• Altitude corrections for sextant observations: applying index error, dip (height of eye), and main/secondary corrections to get observed altitude (Ho). • Using chronometer time and error to find accurate UTC, then using date and time to select stars that are above the horizon in that part of the sky. • Relating true bearing and altitude from your DR position to the approximate azimuth and altitude of candidate stars from a star identification table or star finder.
• After you correct the sextant altitude to get Ho, is the star relatively low, medium, or high in the sky? How does that help you narrow the choices at that date, time, and latitude? • From your DR position and the star’s bearing 282.5° T, in which quadrant of the sky (NW, W, SW, etc.) are you looking, and which of the four named stars are normally found there in June near that local time? • Once you correct the chronometer reading to get UTC, what approximate Local Hour Angle (LHA) Aries and sidereal hour angle (SHA) / declination (Dec) would you expect for each candidate star at that time, and which one best fits a low western altitude from your latitude?
• Be sure you apply index correction with correct sign ("on the arc" vs "off the arc") and convert your height of eye to dip properly before judging the altitude. • Confirm that the chronometer error (slow/fast) is applied in the right direction when finding UTC so your star data comes from the correct hour. • Check that the chosen star’s declination is consistent with a low altitude sight from latitude 21° N in the western sky around 10h UTC on 12 June.
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