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From: Noyce, Bill (no email)
Date: Wed Sep 01 2004 - 16:27:44 EDT
> 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.
Half right. The objects that behave this way are those whose
declination is
larger than (90d - Lat). You are correct that "Meridian Passage" is
generally
considered to be when the body crosses the half of your meridian that
stretches
from pole to pole and includes your zenith.
> 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?
Not sure this is relevant, but remember that stellar aberration -- the
annual
movement of stars' apparent positions due to the speed of the earth in
orbit
as a fraction of the speed of light -- will appear to affect the SHA of
Polaris
more than other navigational stars, even though its actual position is
not
affected more greatly. This is simply because you divide the position
change
by cos(declination) to get change in SHA. I would guess that
longer-term
changes in SHA for these stars are related much more to the earth's
precession
than to stars' proper motion, though again I'm not sure this is relevant
to
your question.
Looking at a list of star SHA's, it appears Kochab's SHA is pretty close
to
180d away from Polaris's. I suppose the ways to use this info are:
a) when you think Polaris is east or west of the pole, use its altitude
directly as your latitude;
b) when you think Polaris is directly below or above the pole, add or
subtract its difference (which you have memorized as about 45');
c) at other angles, estimate the sine or cosine (depending on how you
name the angle) and multiply by 45'.
> Thanks,
>
> Bill
You're welcome,
-- Bill
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