(1) Had the moon an atmosphere, it would be indicated at the time of a solar eclipse, when the moon passes over the disk of the sun. If the atmosphere were of any considerable density, it would absorb a part of the sun's rays, so as to produce a dusky border in front of the moon's disk, as shown in Fig. 127. In reality no such dusky border is ever seen; but the limb of the moon appears sharp, and clearly defined, as in Fig. 128. [Illustration: Fig. 127.] [Illustration: Fig. 128.] If the atmosphere were not dense enough to produce this dusky border, its refraction would be sufficient to distort the delicate cusps of the sun's crescent in the manner shown at the top of Fig. 125; but no such distortion is ever observed. The cusps always appear clear and sharp, as shown at the bottom of the figure: hence it would seem that there can be no atmosphere of appreciable density at the moon. (2) The absence of an atmosphere from the moon is also shown by the absence of twilight and of diffused daylight. Upon the earth, twilight continues until the sun is eighteen degrees below the horizon; that is, day and night are separated by a belt twelve hundred miles in breadth, in which the transition from light to darkness is gradual. We have seen (66) that this twilight results from the refraction and reflection of light by our atmosphere; and, if the moon had an atmosphere, we should notice a similar gradual transition from the bright to the dark portions of her surface. Such, however, is not the case. The boundary between the light and darkness, though irregular, is sharply defined. Close to this boundary the unillumined portion of the moon appears just as dark as at any distance from it. The shadows on the moon are also pitchy black, without a trace of diffused daylight. [Illustration: Fig. 129.] (3) The absence of an atmosphere is also proved by the absence of refraction when the moon passes between us and the stars. Let _AB_ (Fig. 129) represent the disk of the moon, and _CD_ an atmosphere supposed to surround it. Let _SAE_ represent a straight line from the earth, touching the moon at _A_, and let _S_ be a star situated in the direction of this line. If the moon had no atmosphere, this star would appear to touch the edge of the moon at _A_; but, if the moon had an atmosphere, a star behind the edge of the moon, at _S'_, would be visible at the earth; for the ray _S'A_ would be bent by the atmosphere into the direction _AE'_. So, also, on the opposite side of the moon, a star might be seen at the earth, although really behind the edge of the moon: hence, if the moon had an atmosphere, the time during which a star would be concealed by the moon would be less than if it had no atmosphere, and the amount of this effect must be proportional to the density of the atmosphere.