The following picture shows the Celestial view.
The red line is the path of the Sun.
The blue line is the horizon. The
black line is the celestial equator.
The line in light blue color is the boundary
of the sky.
Let the position O be the observer standing in the plane of the horizon.
So back in the Mercator View, the observer
was standing on the celestial equator, and hence that is the reason why
the path of the sun and the horizon are in sinusoidal shapes. This
Celestial View is what we normally see. Half of the sky of the earth
is below the horizon, from the observer's standpoint. Also, the letters
SP and NP refer to South Polar and North Polar. Hence, imagine a
straight line connecting from SP to NP. The light blue line rotates
around the line SP and NP. In other words, SP and NP marks the ends
of the axis of this view.