To understand how one determines the
timescale of a horizontal sundial one starts with
the equatorial sundial. It consists of a disc and
of an axis, which is normal to the plane of the
disc (figure 8). The axis is parallel to Earth’s axis
so that the disc is parallel to the equatorial plane
of the Earth, as well as to the tangential plane at
the poles. It follows from this that the equatorial
sundial is a simplified form of a globe which
simulates Earth’s orientation and has an extended
axis.