On 15 February at 0610 ZT, in DR position LAT 56°53.0'N, LONG 157°02.9'E, you observe Polaris at a sextant altitude (hs) of 56°10.4'. The index error is 2.5' on the arc, and the height of eye is 18 meters. What is the latitude?
• Polaris (North Star) altitude gives an approximate value of your latitude in the Northern Hemisphere • How to correct sextant altitude: apply index error, then dip (height of eye), then other small corrections to get observed altitude (Ho) • On 15 February, the Polaris correction from the nautical tables is small but not zero—this adjusts Ho to get an accurate latitude
• First, carefully compute the apparent altitude by correcting hs for index error and dip. Is the corrected altitude higher or lower than hs? • Compare your final Ho of Polaris to the DR latitude. Should the latitude be slightly north or south of DR based on whether Polaris is higher or lower than expected? • Use the appropriate Polaris table/correction (for date and longitude/Local Hour Angle of Aries) and see how that adjusts your first estimate of latitude
• Be sure you use the correct sign for index error: "on the arc" vs "off the arc" • Verify the dip correction for 18 meters is taken from the correct column and applied with the right sign (always subtract dip from hs) • After all corrections, check whether your final latitude is reasonable compared with DR latitude 56°53.0'N—it should not be many degrees away
No comments yet
Be the first to share your thoughts!