Image 1 Caption: A diagram
giving---for the cognoscenti---a
proof by inspection
for the
Northern Hemisphere
that the altitude
above due north of
the north celestial pole (NCP)
AN_NCP equals the
observer's latitude
on Earth L: i.e.,
    AN_NCP = L .
Note that a right angle
can be rotated into a right angle.
Also note that B = L by the
converse of Euclid's parallel postulate.
-
A Bit More Detail on the Formula for AN_NCP:
- Image 1 is a cross-sectional
view of the Earth.
- The Earth's axis
is extended to the north celestial pole (NCP)
and the Earth's equator
is projected outward to the
celestial equator from
the Earth's center.
- The NCP
and celestial equator
are on the
celestial sphere
which is infinitely remote from the
Earth which is like
a point
relative to the
celestial sphere.
- All lines
starting from the Earth
toward the NCP
or celestial equator
are effectively parallel---precisely
because
the Earth is like
a point
relative to the
celestial sphere.
- L is value of
the latitude of a general observer
on the Earth's surface
in the Northern Hemisphere.
It's NOT 45° or any specific angle---it
just looks that way.
- The observer is tiny, and so the
Earth's surface
to them is the
infinite
horizon plane which
cuts the celestial sphere in
half.
The cut line is a
great circle
and is, in fact, the horizon itself.
- Image 1 clearly shows for a general
point on the
Earth's in
the Northern Hemisphere that
we obtain the above formula AN_NCP = L.
- Actually, the same formula holds for the
Southern Hemisphere
if you count south
latitudes as
negative.
We prove this statement in
section
The General Formulae for Declination-Altitude Conversions on the Meridian
which appears below or in the extended version of this figure
(i.e.,
Celestial sphere file:
declination_altitude.html),
where the general formulae
for
declination-altitude
are given and derived
and where the circumpolar sky
as a function
of latitude
is explicated in section
The Circumpolar Sky as A Function of Latitude.
- Note in the Southern Hemisphere
the NCP
is below the horizon,
and so its
altitude is
negative.