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George wrote, inter alia:
>
> 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.

"Definition of Oblique Ascension (O.A.)

As it rises, a star or planet, not on the equator, forms an angle with that
part of the equator which is rising at the same time. This is called its
Ascensional difference. (A.D.) This A.D. added to the R.A. if it have S.
declination, and subtracted therefrom if it have N. declination, gives its
Oblique Ascension. In the Southern hemisphere, reverse; add, if N.;
subtract, if S.

The equator is always at right angles to a line between the North and South
Poles. Any meridian circle can be considered as the horizon of a place on
the equator go degrees distant from that meridian - hence, from that point
such meridian can be called the horizon of the pole.

At either pole a planet on any parallel of declination moves along an arc
parallel to the equator, to the horizon of the pole. It has neither
ascension nor descension, but remains, day and night, above or below the
horizon, according as it is in North or South declination. Viewed from a
place on the equator, a star will by the axial rotation of the Earth, be
carried along an arc parallel to the equator: hence it rises and sets at
right angles to the horizon of that place. All places in latitudes north and
south of the equator, have a prime vertical that cuts the equator at an
angle equal to the latitude of the place; and the horizon cuts the equator
at an angle equal to the complement of the latitude. Stars and planets rise
and set obliquely, since they follow arcs parallel to the equator - to which
the horizon is oblique. The semi-arc of a body on the equator is always go
degrees, or 6 hours; the whole arc is always 180 degrees or 12 hours. On the
equator days and nights are equal, and the semi-arcs of all bodies are
equal; but in latitudes north or south of the equator the arcs above and
below the horizon are unequal, although together these make 180 degrees or
12 hours. The difference between 90 degrees and the diurnal or nocturnal
semi-arc of a body is thus its Ascensional Difference; and its Right
Ascension, plus or minus this Ascensional Difference, is its Oblique
Ascension."

Guess where that little gem comes from? The Astrology Encyclopedia, no less.
http://www.astrologyweekly.com/dictionary/oblique-ascension.php

I should warn George that not all information to be found on the web should
be taken at face value. There's an awful lot of doubtful stuff out there.

   

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