d a kind of
"popular science."

The Equator, as everyone knows, is an imaginary line or circle girdling
the Earth half-way between the North and South poles. If you imagine a
transparent Earth with a light at its very centre, and also imagine the
SHADOW of this equatorial line to be thrown on the vast concave of
the Sky, this shadow would in astronomical parlance coincide with the
Equator of the Sky--forming an imaginary circle half-way between the
North and South celestial poles.

The Equator, then, may be pictured as cutting across the sky either by
day or by night, and always at the same elevation--that is, as seen from
any one place. But the Ecliptic (the other important great circle of the
heavens) can only be thought of as a line traversing the constellations
as they are seen at NIGHT. It is in fact the Sun's path among the fixed
stars. For (really owing to the Earth's motion in its orbit) the Sun
appears to move round the heavens once a year--travelling, always to the
left, from constellation to constellation. The exact path of the sun is
called the Ecliptic; and the band of sky on either side of the Ecliptic
which may be supposed to include the said constellations is called the
Zodiac. How then--it will of course be asked--seeing that the Sun and
the Stars can never be seen together--were the Priests ABLE to map out
the path of the former among the latter? Into that question we need not
go. Sufficient to say that they succeeded; and their success--even with
the very primitive instruments they had--shows that their astronomical
knowledge and acuteness of reasoning were of no mean order.

To return to our Vernal Equinox. Let us suppose that the Equator and
Ecliptic of the sky, at the Spring season, are represented by two lines
Eq. and Ecl. crossing each other at the point P. The Sun, represented
by the small circle, is moving slowly and in its annual course along the
Ecliptic to the left. When it reaches the point P (the dotted circle)
it stands on the Equator of the sky, and then for a day or two, being
neither North nor South, it shines on the two terrestrial hemispheres
alike, and day and night are equal. BEFORE that time, when the sun
is low down in the heavens, night has the advantage, and the days are
short; AFTERWARDS, when the Sun has travelled more to the left, the days
triumph over the nights. It will be seen then that this point P where
the Sun's path crosses the Equator is a very critical point. It i