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From: Bill (no email)
Date: Wed Sep 01 2004 - 15:51:44 EDT
I am a bit confused on meridian passages.
So that we using the same frame of reference, assume a position of 40N 90W.
As I understand it the observer's meridian in the Northern Hemisphere runs
from the north horizon, through the celestial pole, through the observer's
zenith, through the celestial equator, continuing directly south to the
horizon. An unseen portion of this (great?) circle continues on through the
nadir and back to the northern horizon.
First, are there terms for the line segments from the north horizon to the
pole, pole to zenith, zenith to equator, equator to southern horizon, and
the unseen semicircle though the nadir? I suspect much of what follows is
matter of semantics/terminology, hence the above queries.
Second, the concept of meridian passage for objects with a declination
greater than the observer's latitude causes me some confusion as they never
dip below the horizon, so under proper viewing conditions Polaris, Dubhe et
al could be observed crossing the meridian twice in a sidereal or solar day.
(They do this in fact, whether I can see it or not. While not observable
from my position, the Sun would cross the unseen part of my meridian 12
hours after my meridian passage.)
When looking at times of meridian passage for various bodies with navigation
software only one time of meridian passage is listed, even for bodies with
declinations greater than my latitude that would cross over my observable
meridian twice. A spot check for meridian passage of bodies that don't dip
below the horizon seems to indicate that meridian passage is when the Hs/Ho
is greatest. Put another way, when it intersects the meridian to the south
of the north celestial pole. Yet another look, when its LHA is 0. I am
guessing the same is true for Hs/Ho for the Southern Hemisphere, but
direction is opposite.
Would anyone be so kind as the slightly shrink my large sphere of ignorance?
As a side bar, Susan Howell/Practical Celestial Navigation's chapter on
meridian sights states that in a lifeboat situation the position of Polaris
relative to the north celestial pole can be estimated by the position of
Ruchbah, as it is on the same side of the PN and directly in line with
Polaris and the PN. While published in 1976, it has been revised since.
Inspection of the current almanac star pages would indicate the SHA of
Polaris is 320+, while Ruchbah is 338+. Inspecting the rate of change in
one year, it appears that Ruchbah's SHA is changing faster than Polaris's,
so the relationship may be quite different in 2004 than it was in 1976. Am
I missing something? If not, any current rules of thumb (other than finding
a visible star with a like or 180 opposite SHA) for estimating the angle of
Polaris to the North Pole?
Thanks,
Bill
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