Welcome to the NavList Message Boards.


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

Compose Your Message

Add Images & Files
    Double Altitude - Sensitivity to Time Interval
    From: David C
    Date: 2016 Dec 4, 23:44 -0800

    Whle mowing the lawns this afternoon I noticed that the sun was visible more often than it was hidden by cloud. I checked a phone app and at my location the sun would be on the prime vertical at 1716 NZDST (-13). I looked at my watch and it was 1645. Clearly a double altitude sight was in order.

    I took three pairs of sights. Unfortunately about 10 min before the sun was due west the sun disappeared behind a cloud. The cloud cover was such that I decided to put my sextant away. Then I noticed a blunder in the first pair of sights so I was left with two pairs of sights.

    When I analysed the sights I discovered that the approximate formula cos lat = a/15/t is so sensitive to errors in time as to make it  useless. It is possible that I have made an arithmetical error but I found that a four second error in three minutes gives an change in lat of nearly 1 °. It is many many years since I studied differential calculus so I doubt I could work out d lat/d t for the above formula.

    Here is what I did:

    I observed the sun in an AH. I noted the times when the bottom and top limbs touched and then when the top and bottom limbs touched. That is a change of two diameters but I am using an AH so the actual change is one diameter or 32.4'.

    To check the accuracy of the sights I worked the first sight of each pair as an intercept and then as a time sight (with gps lat). I then used the formula cos lat = a/15/t to determine the lat by double altitudes. The intercept was worked with the cos formula, a calculator and the ABC tables. The time sight was worked by calculator.

    Base data

    GPS  position  S41° 06.5'  E175° 05.2'

    Zone -13  (NZDST)

    Zone date 2016/12/5  (what is preferred time format for this forum?)

    Sight 1

    Ho  39°  39.4'

    Time 1  165504

    Time 2 165754

    Intercept  0.2' A

    Az  N 85.7 W

    Long by Chron 175 5.0'


    Sight 2

    Ho  38°  35.9'

    Time 1  170037

    Time 2  170331

    Intercept  0.7' A

    Azimuth  N 86.5 W

    Long by Chron  175° 6.1'

    Double Altitudes

    The change in altitude is 32.4 * 60 arc seconds

    The time interval is 170 secs for the first pair and 174 sec for the second pair.

    These numbers give 40° 19' and 41° 51' for the lat. What I find interesting is not the error in lat but just how sensitive lat is to the time interval.

    Have I made a blunder?

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    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 Settings

    Posting Code:

    Custom Index

    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