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On Sun, 21 Oct 2007, Dan Allen wrote:

Since the invention of double reflecting instrument
(in the mid XVIII century), almost all instruments
made were double reflecting.

> So a quadrant is 90 degrees;

Used before the mid XVIII century.
In fact, the most common instrument at that time
was the cross-stuff. (Not reflecting at all).

The real (non-reflecting) quadrants were used
in Columbus time, mostly from shore and with very little
success.
They measured 90 degrees.

> An octant is only 45 degrees;
> these could only measure 45 degrees,
> but did not some octants
> use the double reflecting principle to
> measure 90 degrees?

A typical octant (XVIII-XIX centuries) was just
a cheaper version of a sextant. It was
double reflecting. It had approx 45 degrees
arc and measured 90 degrees.
(Nowadays wooden quadrants of XIX century are
more expensive than sextants:-)

They were used mostly for altitudes (you don't need
more than 90 degrees for that), while the first sextants
were designed for the Lunars. Later it was found that
sextants are also convenient for other tasks (horizontal
angles), and in XX century mostly sextants were produced.

Other instruments capable of taking larger angles were
apparently proved non-practical.
I can confirm this with my own expericnce of measuring
angles of about 130-140 (my sextant has 140d theoretical
capability).

Alex.


> Dan
>
>
> >


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