Pupil. I perceive, as I move round, that the same side of the ball is turned towards me whilst every part is turned to the candle. Is it so with the moon?
Tutor. It is: and as every part of the moon is turned to the sun, she makes one revolution on her axis whilst she makes one in her orbit.
Pupil. This is very singular. If the same side of the moon be always turned to the earth, the opposite side of course can never see it.
Tutor. And they must likewise be deprived of the earth as a moon.
Pupil. True. But how is it known that the same side of the moon is always opposed to the earth?
Tutor. The moon, like our earth, consists of mountains and valleys, which, when seen through a good telescope, are very beautiful. The mountainous parts appear as lucid spots and bright streaks of light: and as the same spots, &c. are constantly turned to the earth, she must keep the same side to the earth.
Pupil. It is very clear. Are there no seas?
Tutor. It was formerly imagined that the dark parts were seas, but later observations prove that they are hollow places or caverns, which do not reflect the light of the sun. Besides, if there were seas there would consequently be exhalations, and if exhalations, clouds and vapours, and an atmosphere to support them. That there are no clouds is evident, because when our atmosphere is clear, and the moon above our horizon in the night-time, all her parts appear constantly with the same clear, serene, and calm aspect.
Pupil. Has the moon then no atmosphere?
Tutor. If she has it is imperceptible to us: for, when she approaches any star, we cannot discover with our best telescopes any change of colour or diminution of lustre in the star till the instant it is lost behind her: whence it is clear, that she can have no such gross medium as our atmosphere to surround her.