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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.

   

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