NavList:

From: Gary LaPook <glapook@pacbell.net>
Subject: Shooting Polaris with a bubble sextant
To: "Navlist" <Navlist@navlist.net>
Date: Friday, May 27, 2011, 12:26 AM

And for everybody with a bubble sextant, this is a good time of year to test the accuracy of your sextant. When Polaris is crossing your meridian its altitude doesn't change by even 0.1' for a half hour because it is traveling horizontally as it crosses your meridian. You can take many sights without having to worry about recording the exact time or working out the Hc for each shot. The altitude doesn't vary more that half a minute of arc for a period 68 minutes and doesn't vary more than1.0 minutes for 96 minutes. Right now (May 26th)  it is crossing your meridian at about 11:30 pm and gets earlier by four minutes every day. To check the accuracy, calculate what you should measure with the sextant from your known location. To do this simply start with your latitude and subtract the polar distance of Polaris (which is 41.3' since its declination now is 89° 18.7' north) to compute Hc at your location as it crosses your meridian. Then to make this directly comparable to your sextant altitude add the refraction correction to the Hc to determine what flight navigators call Hp (precomputed) and this should be the altitude measured by your bubble sextant. By applying the refraction to the Hc with the sign reversed you make it directly comparable to the Hs. If you look at the "Q" correction table for Polaris found in the Air Almanac you will see that the correction is -41' while the LHA of Aries in in the range of 31° 49' to 51° 18' and + 41' while LHA Aries is 211° 42' to 231° 25'. Simply reverse the signs from the Q table and apply to your latitude to compute Hc. Calculate when the LHA of Aries is within these ranges and go out and shoot Polaris. gl