NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: transit of venus 1769
From: George Huxtable
Date: 2006 May 27, 03:23 -0500
Coralline Algae wrote-
"I am starting to read "the SHIP retracing Captain Cook's Endeavour
Voyage" which I found while browsing the stacks at our local
library.
On page 27 to summarize: by observing from different parts of the
globe and
measuring the time the planet venus took to cross the sun, it would
enable
the measurement of the suns diameter and also the earth's distance
from the
sun, the Astronomical Unit.
The observations of the transit of venus in tahiti were not as precise
as
hoped since the timing of when venus first met the suns disc could not
be
determined with accuracy as a halo due venus atmosphere and an optical
effect called blackspot."
============
response from George-
Although I didn't travel on that voyage, I was involved as an adviser
to the BBC (who organised it) about navigational matters, and also in
correcting errors in the resulting book, "The Ship", that Coralline
is reading.
Determination of the Sun's actual diameter from its angular diameter
was an indirect result of knowing the Earth - Sun distance, now known
as the Astronomical Unit. Measuring that distance, rather than the
Sun's diameter, was the main aim of the international expeditions to
observe the transit of Venus. Not that what Coralline wrote was
actually wrong, but the emphasis should be different.
Coralline describes the reasons for the imprecision of the
measurements as "a halo due to venus atmosphere and an optical effect
called blackspot". Are those, I wonder, Coralline's own words rather
than words found in "The Ship"? What the book does is quote the
observers' description of "an atmosphere or dusky shade around the
body of the planet". Many later commentators have ascribed this, in
part at least, to the atmosphere of Venus, but in fact that does not
contribute to what is seen. The "black drop" effect is entirely
generated here on Earth, due to the muddying of thermal effects in a
heated daytime atmosphere combined with optical defects in the
instruments, seeking the ultimate resolution of which they are
capable, when heated by the Sun. Those, combined with a deficiency
within the human eye known as "irradiation", which affects sextant
observations also, and makes it difficult to define exactly where lies
the exact boundary between a bright area and a dark area, are
sufficient to explain the black drop: not the atmosphere of Venus!
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.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: George Huxtable
Date: 2006 May 27, 03:23 -0500
Coralline Algae wrote-
"I am starting to read "the SHIP retracing Captain Cook's Endeavour
Voyage" which I found while browsing the stacks at our local
library.
On page 27 to summarize: by observing from different parts of the
globe and
measuring the time the planet venus took to cross the sun, it would
enable
the measurement of the suns diameter and also the earth's distance
from the
sun, the Astronomical Unit.
The observations of the transit of venus in tahiti were not as precise
as
hoped since the timing of when venus first met the suns disc could not
be
determined with accuracy as a halo due venus atmosphere and an optical
effect called blackspot."
============
response from George-
Although I didn't travel on that voyage, I was involved as an adviser
to the BBC (who organised it) about navigational matters, and also in
correcting errors in the resulting book, "The Ship", that Coralline
is reading.
Determination of the Sun's actual diameter from its angular diameter
was an indirect result of knowing the Earth - Sun distance, now known
as the Astronomical Unit. Measuring that distance, rather than the
Sun's diameter, was the main aim of the international expeditions to
observe the transit of Venus. Not that what Coralline wrote was
actually wrong, but the emphasis should be different.
Coralline describes the reasons for the imprecision of the
measurements as "a halo due to venus atmosphere and an optical effect
called blackspot". Are those, I wonder, Coralline's own words rather
than words found in "The Ship"? What the book does is quote the
observers' description of "an atmosphere or dusky shade around the
body of the planet". Many later commentators have ascribed this, in
part at least, to the atmosphere of Venus, but in fact that does not
contribute to what is seen. The "black drop" effect is entirely
generated here on Earth, due to the muddying of thermal effects in a
heated daytime atmosphere combined with optical defects in the
instruments, seeking the ultimate resolution of which they are
capable, when heated by the Sun. Those, combined with a deficiency
within the human eye known as "irradiation", which affects sextant
observations also, and makes it difficult to define exactly where lies
the exact boundary between a bright area and a dark area, are
sufficient to explain the black drop: not the atmosphere of Venus!
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.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
