i873-] Condition of the Moon's Surface. 487 



down again towards the horizon, the shadows, though they 

 increase again in length, are yet thrown in quite a 

 different direction from that in which they fell in the earlier 

 part of the day. The effect of such changes will depend 

 partly on the nature of the surface ; but all parts of the 

 moon's surface where one would look for changes due to 

 volcanic action, or to the effects of expansion and con- 

 traction, would be certainly very much affected by changes 

 of illumination. Thus it is found that the whole aspect of 

 a lunar region at morning time differs from its noon aspect, 

 and its noon aspect again from the aspect it presents when 

 its evening is in progress. We can take the diurnal changes 

 into account in successive lunations, because (weather per- 

 mitting) we can observe any given lunar region repeatedly 

 at about the same hour of the lunar day. But we cannot 

 do this with perfect exactness ; for the lunar day, that is 

 the lunation, is not commensurable with our day. Since one 

 lunation in fact contains approximately 29*53 of our mean 

 days, we see that if any lunar feature is observed in a given 

 part of our sky, at a given lunar hour in one lunation, then, 

 in the next lunation, that part of the lunar day will 

 correspond to a time when the moon is nearly 12 hours of 

 diurnal rotation from that part of the sky. For instance, 

 if true full moon occurs at midnight in one lunation, so 

 that a place on the moon's central meridian is observed at its 

 noon and at our midnight, then, in the next lunation, the 

 noon of that place will occur nearly at our mid-day, and 

 the moon was on the meridian about half a day of our 

 time before, or will be on the meridian about half a day of 

 our time after the time of true moon for places on the 

 central meridian of the moon ; in half a day of our time 

 a place on the moon undergoes a considerable change of 

 illumination. Since two lunations amount to 59*06 days, 

 that is to 59 days and nearly an hour and a half, we 

 see, that in the next lunation but one, there will be a much 

 smaller difference of illumination if any lunar region is 

 observed at almost the same hour of terrestrial time ; for 

 an hour and a half of our time corresponds to only about 

 three minutes of lunar time,* and as we know the sun's 



* Since the lunar day contains 29*53 °f our days, it follows that the lunar 

 hour, or the 24th part of the day, corresponds to 1/23 terrestrial days, or 29-53 

 terrestrial hours. Again, one terrestrial day corresponds to 1 -•- 29^53 of a 

 lunar day, or to rather less than 48m. 46s. of lunar time supposed to be 

 divided as ours (that is, the day into 24 equal parts, to be called lunar hours, 

 the hour into 60 minutes, and the minute into 60 seconds). These two 

 relations are sufficient for the ready conversion of terrestrial into lunar time, 

 and vice versa. 



