us and Ptolemy suffered to remain, it
becomes at once evident that the ancient astronomers were not acquainted
with the entire heavens. For there is a large space in the south, left
free from all the old constellations, and no explanation, why it should
have been so left free, is so simple and satisfactory as the obvious
one, that the ancient astronomers did not map out the stars in that
region because they never saw them; those stars never rose above their
horizon.

[Illustration: THE ANCIENT CONSTELLATIONS SOUTH OF THE ECLIPTIC.]

Thus at the present time the heavens for an observer in England are
naturally divided into three parts, as shown in the accompanying
diagram. In the north, round the pole-star are a number of
constellations that never set; they wheel unceasingly around the pole.
On every fine night we can see the Great Bear, the Little Bear, the
Dragon, Cepheus and Cassiopeia. But the stars in the larger portion of
the sky have their risings and settings, and the seasons in which they
are visible or are withdrawn from sight. Thus we see Orion and the
Pleiades and Sirius in the winter, not in the summer, but the Scorpion
and Sagittarius in the summer. Similarly there is a third portion of the
heavens which never comes within our range. We never see the Southern
Cross, and hardly any star in the great constellation of the Ship,
though these are very familiar to New Zealanders.

[Illustration: THE CELESTIAL SPHERE.

Zenith
* *

* * North Pole
*
* Stars /
. * Always
. Visible
. /
. Visible /
. Hemisphere /
. /
. /
. /
. /
(Earth surface) . / (Earth surface)
South --------------------------------------- North
/ . Celestial
/ . / Equator
Invisible .
/ .
/ Hemisphere .
/ .
Stars / .