- The diagram is not-to-scale.
The Earth is point compared to the celestial sphere.

The observer on the Earth is a point compared to the quasi-infinite plane defined by the horizon.

- The horizon plane
is just a plane
tangent to
the Earth
at the location of the observer.
It is actually entered on the Earth since, as aforesaid, the Earth is point compared to the celestial sphere.

- The horizon itself is the
circle formed by the
intersection of the
horizon plane and the
celestial sphere.
The horizon is a great circle since it cuts the celestial sphere in half.

- The observer ideally sees everything above the
horizon and nothing below it.
- In the Earth-at-rest perspective,
the celestial sphere
rotates westward
once per sidereal day.
- The sidereal day
(86164.1 s on average) is the rotation period
relative to the inertial frame
of the fixed stars: i.e., it is the
physical rotation period
The sidereal day is about 4 minutes shorter than the solar day.

- The solar day
is defined as solar noon
to solar noon
and is currently about 86400.002 s.
- The civil day
is defined as exactly 86400 s = 24 hours.

- The sidereal day
(86164.1 s on average) is the rotation period
relative to the inertial frame
of the fixed stars: i.e., it is the
physical rotation period
- All natural astronomical objects
are carried by the celestial sphere
as their primary motion.
- The north and
south ends of the
horizon define
circumpolar circles
which are small circles
parallel to the celestial equator.
- The circumpolar circle angular radius
is the angle from the
NCP/SCP
of the
north/south
circumpolar circle.
The circumpolar circle angular radius equals the observer's latitude which can be either north or south latitude. So the circumpolar circles are latitude or observer dependent.

You can see this must be true by just rotating the celestial axis onto the horizon around an axis passing the intersection of horizon and celestial equator.

- Astronomical objects
within the circumpolar circles
never cross the horizon: i.e., they never
rise or set.
These astronomical objects are
circumpolar objects.
Other astronomical objects must
rise and set as they are carried by the rotating
celestial sphere.
Since the circumpolar circles
are latitude dependent so are which
astronomical objects are
circumpolar objects.
- For a Northern Hemisphere observer,
the north
circumpolar objects are always
above the horizon,
and so always in the sky though they are usually
unobservable in the daytime.
The south circumpolar objects are always below the horizon and are

**NEVER**observable.This is why you

**NEVER**see the Southern Cross in Las Vegas, Nevada---it is ∼ 30° from the SCP and the circumpolar circle angular radius for Las Vegas is ∼ 36°.Southern Hemisphere observers have a mirror image situation with the celestial equator defining the mirror.

- At the
North Pole/South Pole,
all astronomical objects are
circumpolar objects.
At the equator, none are.
- Zenith
is the projection of the observer location (the dot on the
Earth on the diagram) from
the Earth center onto the
celestial sphere.
Nadir is the antipodal point on the celestial sphere to zenith.

- The meridian
is a great circle passing through the
north celestial pole (NCP)
south celestial pole (SCP), and
zenith.
- Due north
and due south
are given by the projections of
the celestial axis
onto the horizon plane.
The projections
are in the plane
of the meridian.
- Due east
and
due west
are points
where the horizon intersects
celestial equator.
Due east/due west
is
clockwise/counterclockwise
from due north
looking down on the horizon plane
from the zenith
side of the celestial sphere.

Caption: The celestial sphere and the horizon.

Features:

Image link: Itself.

Local file: local link: celestial_sphere_002_horizon.html.

File: Celestial sphere file: celestial_sphere_002_horizon.html.