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
    Right Ascension.
    From: George Huxtable
    Date: 2005 Aug 26, 07:37 +0100

    For some time, I've wondered why Right Ascension is called Right Ascension.
    I've asked, but nobody has offered to enlighten me.
    
    Now I have a bit more information.
    
    Recently, I picked up a modern paperback reprint (Conway, 2005) of "The
    Sailor's Word Book", of Admiral W H Smyth, 1867. This, cheap from Amazon at
    only ?7, is exceptional value for a work of  744 pages! A mine of
    fascinating stuff about words used at sea, many being very obscure in terms
    of modern usage. Quite fascinating to any seeker after useless knowledge, a
    category in which I often find myself.
    
    Smyth defines Right Ascension as  "An arc of the equator between the first
    point of Aries, and the hour circle which passes through any planet or
    star; or that point of the equinoctial, which comes to the meridian with
    any heavenly object, and is therefore similar to terrestrial longitude".
    
    So far, so good. I wasn't looking for the definition, though, but for the
    reason it was called "Right Ascension".. And there's a clue in Smyth,
    because he describes another sort of Ascension, "Oblique Ascension". This
    is- "An arc between the first point of Aries and that point of the equator
    which comes to the horizon with a star, or other heavenly body, reckoned
    according to the order of signs."
    
    He follows that with "It is the sum or difference of the right ascenscion
    and ascensional difference", which isn't very useful, because "Ascensional
    Difference" is defined as "The equinoctial arc intercepted between the
    right and oblique ascensions (which see)", which has brought us round in a
    circle. However, it implies to me that the two arcs, right ascension and
    oblique ascension are both arcs measured around the same plane, that of the
    equator and therefore of the celestial equator.
    
    This led me to Bowditch, which in Vol 2 of the 1981 edition defines Right
    Ascension in similar terms to Smyth, and Oblique Ascension as- "The arc of
    the celestial equator, or the angle at the celestial pole, between the hour
    circle of the vernal equinox and the hour circle through the intersection
    between the celestial equator and the Eastern horizon at the instant a
    point on the oblique sphere rises, measured eastward through the hour
    circle of the vernal equinox through 24 hours". He concludes, helpfully-
    "This expression is not used in modern navigation".
    
    One wonders what Oblique Ascension was actually used for. It seems to me
    that all stars that rise or set at the same moment when seen from a
    particular latitude will have the same Oblique Ascension, and that, as the
    latitude changes, the oblique ascensions of different stars will change,
    differently. Indeed, as the observer's latitude changes, some stars must
    lose their oblique ascension altogether, once they become circumpolar, so
    never rise or set at all (if I've understood it right).
    
    So, as often happens, it has raised more questions, without answering the
    original one of why Right Ascension was so called.
    
    I have found no mention of Oblique Ascension in any historical navigation
    texts I've read. If any reader can suggest what it was used for, I would be
    interested.
    
    George.
    ===============================================================
    Contact George at george---.u-net.com ,or by phone +44 1865 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.
    
    
    

       
    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