- The celestial equator
and
the horizon are both
great circles on the
celestial sphere
(the tilted sphere in the image)
and their intersection points actually define
due east
and due west.
To understand this, note that on the horizon plane (i.e., on the ground), the due east-due west line is perpendicular to the due north-due south line (i.e., the projection onto the horizon plane of the celestial axis). Thus the due east-due west line is perpendicular to the celestial axis and thus intersects the celestial equator.

The image, in fact, illustrates how the cardinal directions NSEW are actually defined.

- On an equinox day,
the Sun is on the
celestial equator,
and so
rises due east
and sets due west.
- The Sun is carried around
by the daily rotation of the
celestial sphere.
- Diagrammatically, it is clear that
equinox days
have exactly 12
hours of
daytime
and 12
hours of
nighttime for any location on
Earth.
- When the Sun is above the
celestial equator
in the Northern Hemisphere,
it is carried about on a small circle
that is still perpendicular to the
celestial axis
(the NCP axis),
but higher up.
It is above the horizon longer than
12
hours.
- When the Sun is below the
celestial equator
Northern Hemisphere,
it is carried about on a small circle
that is still perpendicular to the
celestial axis
(the NCP axis),
but is lower down.
It is above the horizon less than
12
hours.

Caption: The Sun's path on the sky on a day of an equinox for a location/observer in the Northern Hemisphere. The setup is basically that for illustrating the horizontal coordinate system (see Celestial sphere file: horizontal_coordinates.html).

The celestial sphere **ITSELF** is the
tilted sphere in the image.

Features:

Similar File: Celestial sphere file: ncp_altitude_circumpolar.html which has a different pedagogical use.

Image link: Itself.

Local file: local link: sunpath_equinox.html.

File: Celestial sphere file: sunpath_equinox.html.