st as are the Equator and the
two Tropics, going round the earth, and begins at 66 deg. 32' north latitude
and is 1623 miles from the North Pole. It is the southernmost limit of
the region where the sun disappears in winter, under the horizon, for
one day.

At the North Pole on the 22nd of September the sun descends to the
horizon and then disappears till the 20th of March, when it reappears
and remains in sight above the horizon until the 22nd of September. So
at the pole the year is made of one day and one night. On the 22nd day
of December it disappears at the Arctic Circle for one day only. The
space between the Arctic Circle and the pole is therefore called the
Arctic region, or the Frigid Zone. Consequently, the further one
advances to the north, the longer the duration of the night.

I will tell you the causes of this phenomenon of the Long Night. The
earth revolves about the sun once every year, and rotates on its axis
once in twenty-four hours, which makes what we call a day.

Rotate means to move round a centre; thus the daily turning of the earth
on its axis is a rotation. Its annual course round the sun is called a
revolution.

The axis about which the daily rotation takes place is an imaginary
straight line passing through the centre of the earth, and its
extremities are called poles, hence the names of the North and the South
pole. The diurnal movement is from West to East and takes place in
twenty-four hours.

The earth's orbit, or the path described by it in its annual revolution
about the sun, is, so to speak, a flattened circle, somewhat elongated,
called an ellipse. The axis of the earth is not perpendicular to the
plane of the orbit, which is an imaginary flat surface enclosed by the
line of the earth's revolution, but is inclined to it at an angle of 23 deg.
28', which angle is called the obliquity of the ecliptic. The ecliptic
is the path or way among the fixed stars which the earth in its orbit
appears to describe to an eye placed in the sun, for the sun is the
fixed centre and not the earth. The earth, therefore, in moving about
the sun, is not upright, but inclined, so that in different parts of its
course it always presents a half, but always a different half, of its
surface to the sun.

Twice in the year, 21st of March and 21st of September, the exact half
of the earth along its axis is illuminated. On these dates, therefore,
any point on the earth's surface is, during the rotation of t