hundreds of miles of air they have become tinged
with a ruddy or copper-like hue. Nor is this property of our atmosphere
an unfamiliar one. The sun both at sunrise and at sunset glows with a
light which is much more ruddy than the beams it dispenses at noonday.
But at sunset or at sunrise the rays which reach our eyes have much more
of our atmosphere to penetrate than they have at noon, and accordingly
the atmosphere imparts to them that ruddy colour so characteristic and
often so lovely. If the spectrum of the sun when close to the horizon is
examined it is seen to be filled with numerous dark lines and bands
situated chiefly towards the blue and violet end. These are caused by
the increased absorption which the light suffers in the atmosphere, and
give rise to the preponderating red light on the sun under such
conditions. In the case of the eclipsed moon, the sunbeams have to take
an atmospheric journey more than double as long as that at sunrise or
sunset, and hence the ruddy glow of the eclipsed moon may be accounted
for.

The almanacs give the full particulars of each eclipse that happens in
the corresponding year. These predictions are reliable, because
astronomers have been carefully observing the moon for ages, and have
learned from these observations not only how the moon moves at present,
but also how it will move for ages to come. The actual calculations are
so complicated that we cannot here discuss them. There is, however, one
leading principle about eclipses which is so simple that we must refer
to it. The eclipses occurring this year have no very obvious relation
to the eclipses that occurred last year, or to those that will occur
next year. Yet, when we take a more extended view of the sequence of
these phenomena, a very definite principle becomes manifest. If we
observe all the eclipses in a period of eighteen years, or nineteen
years, then we can predict, with at least an approximation to the truth,
all the future eclipses for many years. It is only necessary to
recollect that in 6,585-1/3 days after one eclipse a nearly similar
eclipse follows. For instance, a beautiful eclipse of the moon occurred
on the 5th of December, 1881. If we count back 6,585 days from that
date, or, that is, eighteen years and eleven days, we come to November
24th, 1863, and a similar eclipse of the moon took place then. Again,
there were four eclipses in the year 1881. If we add 6,585-1/3 days to
the date of each eclipse, it