Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Fw: Shooting Polaris with a bubble sextant
    From: Gary LaPook
    Date: 2011 May 27, 14:44 -0700
    I sent this before at 12:26 this morning but it never showed up on the website so I am sending it again.

    gl

    --- On Fri, 5/27/11, Gary LaPook <glapook@pacbell.net> wrote:

    From: Gary LaPook <glapook@pacbell.net>
    Subject: Shooting Polaris with a bubble sextant
    To: "Navlist" <Navlist@fer3.com>
    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

    File:

      
       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site