- The horizontal coordinate system
has its origin is wherever you, the observer, are on the surface of the
Earth.
- The Earth is an infinite
plane from
your perspective that cuts the
celestial sphere
in half
by a great circle
called the horizon.
The plane itself
is the horizon plane.
- The cardinal directions
north (N),
south (S),
east (E),
west (W))
are on the horizon.
- Zenith is the point on the
sky
directly above the observation point.
It's on the vertical from the observer.
- Nadir is the opposite point on the
sky to
the zenith
and you usually can't see since you-know-what is in the way---the
Earth!
- The meridian
is a great circle
that passes through due north,
Zenith,
due south,
and Nadir.
"The meridian"
is how we traditionally refer to what more elaborately can be called the
celestial meridian.
- In the
horizontal coordinate system,
you locate an
astronomical object using
altitude
(the angle from the
horizon along a
great circle through
the zenith)
and azimuth
(an angle along the horizon
measured east
from due north).
- The
horizontal coordinate system
is a local coordinate system
in space
time.
The altitude
and azimuth
for an astronomical object
depend on your latitude
and time of
day.
Nowadays the
computer
(i.e., your phone app) will
the horizontal coordinates
for astronomical objects.
- Specifying
altitude
is pretty common in casual astronomy:
e.g.,
the Moon is at
altitude
60° in that
direction (pointing with your
hand).
- Measurements of the
altitude
of astronomical objects
transiting
are historically very important in precise
astrometry.
This is because those
altitudes
can be easily and accurately converted into
declinations of the
equatorial coordinate system.
- Image 2 Caption: This image also illustrates
horizontal coordinate system
and explicitly shows the
cardinal directions,
north celestial pole (NCP),
south celestial pole (SCP),
and nadir.
The image also shows how the
cardinal directions
(north (N),
south (S),
east (E),
west (W))
are derived from the north celestial pole (NCP).
The NCP
is projected
down on the horizon plane
to give north and
the others follow from
right angle (90°):
i.e, 90°
clockwise
to east,
180°
clockwise
to south,
270
clockwise
to west,
and 360°
clockwise
back to north.