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Re: Star to star angular measurement, beginner
From: Frank Reed CT
Date: 2005 Mar 11, 23:16 EST
From: Frank Reed CT
Date: 2005 Mar 11, 23:16 EST
Bill you wrote:
"My next query, will there be horizontal refraction as well?"
Luckily no. Consider: which way would it go? North, South? By the symmetry
of the atmosphere alone, there shouldn't be any "sideways" refraction, it has to
be all in the vertical direction (except under "weird" atmospheric conditions
which would probably guarantee cloudy weather, too). Refraction lifts all stars.
It compresses the constellations towards the zenith.
And you wrote:
"In Meeus's chapter on refraction he gives an example of the reduction of
the Sun's vertical observed measurement when near the horizon due to
refraction. He then states, "...the horizontal diameter of the solar disk
is very slightly contracted by reason of the refraction. This is due to the
fact that the extremities of this diameter are raised along vertical circles
that meet at the zenith." He goes on to mention Danjon, "...writes the
apparent contraction of horizontal diameter of the Sun is practically
constant and independent of altitude, and that this contraction is
approximately 0".6."
"In Meeus's chapter on refraction he gives an example of the reduction of
the Sun's vertical observed measurement when near the horizon due to
refraction. He then states, "...the horizontal diameter of the solar disk
is very slightly contracted by reason of the refraction. This is due to the
fact that the extremities of this diameter are raised along vertical circles
that meet at the zenith." He goes on to mention Danjon, "...writes the
apparent contraction of horizontal diameter of the Sun is practically
constant and independent of altitude, and that this contraction is
approximately 0".6."
He's talking here about a small effect that arises from ordinary (vertical)
refraction. Imagine modelling the Sun as a diamond of four stars. One at the top
of the Sun, one at the bottom, and one each on the horizontal limbs of the Sun's
disk. In the absence of refraction, this is a perfectly symmetrical diamond --
if the distance from top to bottom is 30 arc minutes, then te distance from side
to side is also 30 arc minutes. OK, now throw in refraction. All four stars are
pushed towards the zenith though by varying amounts depending on their distance
from the horizon. The stars on the sides of the diamond are both raised by the
same amount, but they're being pushed towards the zenith from slightly different
azimuths. The horizontal distance across the diamond is decreased very slightly.
Meeus quotes an amount, but you can confirm it yourself using your star-star
calculation and this "diamond" of stars that I'm describing.
By the way, parallax, for the Moon, Sun etc. is also an entirely vertical
correction so you can go immediately to the case of lunar distance calculations
from you star-star calculations, if you want. There's one small issue: the
Earth's oblateness yields a slightly non-vertical component to parallax but that
can be dealt with separately.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars