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Guy Schwarz quoted Tom Cunliffe as saying-

"In chapter 11 of Tom Cunliffe's Celestial Navigation book, he talks
about
Amplitude tables. He says " Every proprietary nautical almanac should
have a page devoted to tables know as amplitude tables.
Checking my 2007 and 2006 Nautical Almanac - Commercial edition, I did
not see anything about amplitude tables. Am I missing it
or is it not there?"

I don't have Cunliffe's book, but he is referring to a different sort
of Almanac that has now disappeared, unfortunately.

The official, UK and US, Nautical Almanac concerns itself only with
things that vary from day to day, and apply only to the year in
question. An exception is the refraction table; but in general, things
that stay constant are omitted, to be found elsewhere. An amplitude
table comes into that category.

In the past, proprietary almanacs such as the late-lamented Reed's of
old, before it was subsumed into Macmillan's, would publish a full
astronomical ephemeris for the year, together with lots of such
subsidiary, "constant", navigator's tables, including amplitudes. In
1993, the constant information was split off into a separate volume,
Reed's Companion. Amplitudes are given in my copy of Norie's tables
(1963), and in the tables which were part of Norie's Navigation, (mine
date from 1914); also in Raper's "Practice of navigation", of 1864,
and probably earlier.

=======================

The amplitude is the azimuthal angle, measured either way (clockwise
or anticlockwise); either from due East, at which a body rises, or
from due West ,at which it sets. The only body that is considered, in
practice, is the Sun, so declinations are given from 0, at 1-degree
intervals, up to 23 1/2 degrees, seldom more. Latitudes are given in
degrees up to the limit that the author considers no sensible mariner
would exceed (usually 66 deg). The resulting amplitude is usually
given to the nearest 0.1 deg, which is plenty good enough for its
usual purpose, checking the compass error. Interpolation is possible,
but seldom worthwhile.

You have to be aware of the special meaning given to "Sunrise" and
"Sunset" in these tables. It refers to the moment at which the true
Sun, if there was no refraction, is bisected by the horizon. So,
because of that refraction, the navigator takes his compass-bearing on
the Sun at the moment when its whole disc floats well clear of the
Sun, with a gap between the lower-limb and the horizon, equal to a
semidiameter. That's easy to estimate by eye, quite well enough for
the job. A subsidiary table is often offered, to adjust the result if
you use the apparent Sun, split by the horizon, but it's hardly worth
the bother of ensuring that you apply it the right way round.

Why bother with tables for amplitude, though, if you have a pocket
calculator?

The simple expession to use is-

sin amp = sin dec / cos lat, but remember again, that applies at the
moment when the lower limb is a semidiameter above the horizon.

===================

Guy continued-

"On to Variation. How was it discovered? My guess is that navigators
would take a bearing on the setting/rising sun and their almanacs
had the amplitude tables and determined something was off? Or am I off
base

Portuguese navigators, setting off deep into the Atlantic around the
time of Columbus, did their navigation from Polaris, and thought that
the compass needle was magically drawn toward Polaris. So when it
started to diverge as they went West, it was a source of great worry
and confusion.

George

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.



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