On 22 July your 1759 ZT DR position is LAT 24° 50.2' S, LONG 05° 16.0' E. You observe an unidentified star bearing 231° T, at an observed altitude (Ho) of 26° 10.0'. The chronometer reads 06h 01m 31s and is 02m 15s fast. What star did you observe?
• Use the chronometer correction (fast/slow) to find the correct GMT of the sight • From date, time, and DR position, determine the Local Hour Angle (LHA) and declination of the body, then compare with star data • Relate the bearing and altitude to which part of the sky (NE/SE/SW/NW) the star must be in, and which stars can actually be seen from 25° S
• First, correct the chronometer time and determine the exact Greenwich date and time of the observation. What is the correct UT/GMT? • From your DR latitude and longitude, what approximate LHA Aries would you expect at that time on 22 July, and what right ascension/declination region does that place over your SW horizon? • Looking at each option, which star(s) are primarily northern vs southern? From 24° S, with a true bearing of 231° and altitude 26°, which stars are realistically visible in that part of the sky?
• Be sure you apply the chronometer error correctly: if it is fast, do you add or subtract? • Confirm whether each candidate star’s declination is north or south, and whether it would be above the horizon at 24° 50.2' S • Check that the star’s azimuth and altitude from tables or calculator are consistent with a bearing of about 231° T and an Ho of 26° 10.0'
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