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Earth for our Moon, to see her always on our sky, never rising, never
setting, never undergoing any change except from New Earth to Last
Quarter! Would not it be fun to trace the shape of our great Oceans and
Continents, and to say: 'there is the Mediterranean! there is China!
there is the gulf of Mexico! there is the white line of the Rocky
Mountains where old Marston is watching for us with his big telescope!'
Then we should see every line, and brightness, and shadow fade away by
degrees, as she came nearer and nearer to the Sun, until at last she sat
completely lost in his dazzling rays! But--by the way--Barbican, are
there any eclipses in the Moon?"

"O yes; solar eclipses" replied Barbican, "must always occur whenever
the centres of the three heavenly bodies are in the same line, the Earth
occupying the middle place. However, such eclipses must always be
annular, as the Earth, projected like a screen on the solar disc, allows
more than half of the Sun to be still visible."

"How is that?" asked M'Nicholl, "no total eclipses in the Moon? Surely
the cone of the Earth's shadow must extend far enough to envelop her
surface?"

"It does reach her, in one sense," replied Barbican, "but it does not in
another. Remember the great refraction of the solar rays that must be
produced by the Earth's atmosphere. It is easy to show that this
refraction prevents the Sun from ever being totally invisible. See
here!" he continued, pulling out his tablets, "Let _a_ represent the
horizontal parallax, and _b_ the half of the Sun's apparent diameter--"

"Ouch!" cried the Frenchman, making a wry face, "here comes Mr. _x_
square riding to the mischief on a pair of double zeros again! Talk
English, or Yankee, or Dutch, or Greek, and I'm your man! Even a little
Arabic I can digest! But hang me, if I can endure your Algebra!"

"Well then, talking Yankee," replied Barbican with a smile, "the mean
distance of the Moon from the Earth being sixty terrestrial radii, the
length of the conic shadow, in consequence of atmospheric refraction, is
reduced to less than forty-two radii. Consequently, at the moment of an
eclipse, the Moon is far beyond the reach of the real shadow, so that
she can see not only the border rays of the Sun, but even those
proceeding from his very centre."

"Oh then," cried Ardan with a loud laugh, "we have an eclipse of the Sun
at the moment when the Sun is quite visible! Isn't that very like a
bull, Mr. Philosopher Barbican?"

"Yet it is perfectly true notwithstanding," answered Barbican. "At such
a moment the Sun is not eclipsed, because we can see him: and then again
he is eclipsed because we see him only by means of a few of his rays,
and even these have lost nearly all their brightness in their passage
through the terrestrial atmosphere!"

"Barbican is right, friend Michael," observed the Captain slowly: "the
same phenomenon occurs on earth every morning at sunrise, when

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