On 18 November your 1750 zone time DR position is LONG 110°16.0' W. At that time you observe Polaris with a sextant altitude (hs) of 21°29.8'. The chronometer time of the sight is 00h 52m 43s, and the chronometer error is 02m 18s fast. The index error is 3.2' on the arc, and the height of eye is 49.5 feet. What is your latitude by Polaris?
• Polaris latitude method – how observed altitude of Polaris relates to latitude • Chronometer correction to UTC – using chronometer error and zone description to get the correct time for the sight • Altitude corrections – index correction, dip (height of eye), and main altitude corrections to get Ho from hs
• First, think through how to convert the sextant altitude (hs) to a true observed altitude (Ho). What corrections must you apply, and with what signs? • Once you have Ho, recall how Polaris is used: what is the basic relationship between Ho of Polaris and your latitude, and what small corrections depend on LHA Aries or date? • How do you get the local hour angle (LHA) needed for the Polaris correction from the given chronometer time, chronometer error, and DR longitude?
• Be sure you apply index correction with the correct sign for an error "on the arc". • Use the correct dip correction for a 49.5‑foot height of eye and remember it is always subtractive from hs. • Confirm you are using the date (18 November) and the correct Greenwich hour angle (GHA) Aries / LHA Polaris step from the Polaris tables (Bowditch or equivalent) before deciding which latitude choice matches.
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