NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Amplitudes
From: Jeremy C
Date: 2010 Jun 1, 19:26 EDT
From: Jeremy C
Date: 2010 Jun 1, 19:26 EDT
George brought up an interesting topic in a previous post about measuring
amplitudes. Over the years I have found amplitudes to be far easier to
shoot and reduce by hand than azimuths, but less accurate as well. I have
taken azimuths within an hour or two of a amplitude of the same body and had
different compass errors which should not have existed, and in all cases, I tend
to believe the azimuth over the amplitude.
Amplitudes are shot when the center of the body is on the celestial
horizon. As George notes, this is not the visible horizon, but some
arc-minutes off of the visible horizon depending on the body. (To be fair,
you may shoot the body when the center on the visible horizon, but a correction
needs to be applied which increases as you and the body get further from
the equator.) I was never taught, or have I read, to take into
account any height of eye when taking the measurement (dip correction).
I was taught to observe the Amplitude of the sun when the lower limb was
1/3 of the sun's diameter above the visible horizon. I was also told that
stars and planets are observed when 1 sun's diameter above the visible horizon,
and the moon when the upper limb is on the visible horizon.
I have shot amplitudes of the sun, moon, and two planets. Shooting
the moon on the celestial horizon is quite the challenge, but it does
work. It is the only body that I prefer to shoot on the visible
horizon. Planets are very difficult because they tend to disappear into
the haze of the horizon below about 2 degrees of altitude. I have never
been able to shoot an amplitude of a star because even mighty Sirius disappears
before reaching amplitude height. The sun is the easiest body to observe
but you need to watch out for "amplitude clouds" as we called them. Those
were clouds forming as the earth cooled and obscured the sun from view as
it decended.
For reduction, I have tended to avoid the tables due to the need to
interpolate. While interpolation is not difficult, it is far more
time consuming than a straight calculation. I find it far easier to just
use the formula as even a basic user of any calculator with trig functions can
pull it off with little trouble.
The other major advantage of amplitudes over azimuths is not needing
exact time as the only almanac entry is declination which tends to change slowly
enough to require little time accuracy.
While I still shoot amplitudes at every opportunity, I rely on azimuths as
my preferred method of compass correction at sea.
Jeremy