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
    Re: Jack Aubrey's fixing of longitude
    From: Frank Reed
    Date: 2011 Jun 19, 21:04 -0700

    Hi Geoffrey,

    Yeah, that seems very odd. Clearly they're describing land-based instruments (set up on a "carefully leveled patch of sand") so it's not a sextant observation. That alone implies it's not a standard lunar distance observation. In addition, assuming the story is set in the first decade of the 19th century (which is what I understand is implied for that specific novel), lunar distances for the planets were not available. They were tabulated in the standard Nautical Almanac starting only in 1834.

    What else could we do with Venus to get a longitude, if not a proper "lunar"? A carefully timed lunar occultation of Venus can yield an excellent longitude. That would be a rare enough event that the commander of a vessel with an astronomically-sophisticated navigator might try to find a good land location to set up a telescope for the observation. The calculations to reduce it would be long and difficult and so the idea of two calculators both ending with "274" might make some sense. Or, maybe closer to the description, they might have pre-calculated the local time of the immersion or emmersion of Venus and they were reporting the number of elapsed seconds after some chosen start time --maybe the number of seconds after an observation for local time had been taken. The description also makes it plain that this was a daylight observation since you have to know "where to look" to spot Venus. The description of Venus "clear above" the Moon could make sense if it's a crescent moon with Venus emerging from behind the dark limb. But there's just not enough detail in the description to be sure. I googled the phrase "Venus clear above here" and found some previews of the book online. There's not much more in the complete version except a comment that this was done on an island in the Indian Ocean. Anyone care to check: was there an occultation of Venus visible in daylight in the Indian Ocean in the first decade of the 19th century? It wouldn't be a "surprise". :)

    The line about the chronometers strikes me as a little unhistorical. The chronometers (plural) would not be out by some single number unless they're referring to the average longitude implied by two or more chronometers. Rather, one chronometer might be out by 20 miles West, another out by 32 East, for example.


    NavList message boards and member settings: www.fer3.com/NavList
    Members may optionally receive posts by email.
    To cancel email delivery, send a message to NoMail[at]fer3.com

    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