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----- Original Message -----
From: Bill 
Date:         Wed, 1 Sep 2004 14:51:44 -0500
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject:      Beginner Meridian Passage Question
============================


first things first

>> are there name [...] ?

1? for the line segment from the north horizon to the pole?
Call it lattitude.
zenith to elevated pole = colatitude
zenih to horizon = 90?
zenith - colatitude = latitude.

2? pole to zenith,
Colatitude (90?- latitude) see above

3? zenith to equator
Latitude

4? equator to southern horizon,
Colatitude.

5? Zenith to horizon
90? by definition



6? the unseen semicircle "through the nadir" ?
The "visible" apparent path of the sun is called "semi-diurnal arc",
So you can call the invisible path "invisible semi-diurnal arc".
The whole path is called "diurnal circle".
semi diurnal =>  arc cos P =  tan Lat * tan decl
A star is circumpolar if the colatitude is smaler than the declination.

(90? - Lat) < decl [same name] => circumpolar and visible.
(90? - Lat) < decl [diff name] => circumpolar and never visible.

Second...
>> When looking at times of meridian passage [...]

Upper transit occurs when LHA (AHg) is 0?
lower transit occurs when LHA is 180?

A circumpolar heav. body does have two transits, upper and lower.
You can know your latitude by observing the two transits of an unknown circumpolar star.
(true altitude upper transit + true altitude lower transit)/2 = latitude.

And the misconceptions...

>> -"I am guessing the same is true for the Southern Hemisphere"
Do not speak about southern or northern hemisphere.
The pole in the same hemisphere of the observer zenith is called : elevated pole.

>>"[...] twice in a sidereal or solar day."
mean solar day is 24h.
mean sideral day is 23h 56mn 4 sec.
It is not the same reference point.



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