NavList:

   

Reply

Regarding George's comments concerning oblique ascension, I would
preliminarily note its use in astrological calculations, particularly
those dealing with rectification of the horoscope, a subject beyond the
scope of this List - I think. Oblique ascension is not used in "modern
navigation", according to any published definition coming to my
attention, although it's difficult for me, at least, to define what is
meant by modern navigation - at one time navigation and astrological
tables were somewhat interchangeable and the use thereof potentially a
means of converting one to the other, as respects ascensions.

It is otherwise my thinking that right ascension is so termed simply
because of its measurement eastward, or clockwise, from the vernal
equinox, as opposed to the westward measurement of sidereal hour angle. I
cannot, however support this opinion without further research for
references.

Henry

On Fri, 26 Aug 2005 07:37:59 +0100 george huxtable
 writes:
> 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@huxtable.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