MOONLIGHT. 521 



is below the horizon longer than she is above the horizon. Now, clearly, 

 when the length of what we may call the moon's diurnal path (meaning 

 her path above the horizon), is lengthening most, the time of her rising on 

 successive nights must change least. She comes to the south later and 

 later each successive night by about 50^ minutes, because she is always 

 traveling towards the east at such a rate as to complete one circuit in about 

 four weeks; and losing thus one day in 28, she loses about 50J minutes per 

 day. If the interval between her rising and arriving to the south were al- 

 ways the same, she would rise 50^ minutes later night after night. But if 

 the interval is lengthening, say 10 minutes per night, she would of course 

 rise only 40| minutes later; if the interval is lengthening 20 minutes per 

 night she v^^ould rise only 30^ minutes later, and so forth. But the lunar 

 diurnal arc is lengthening all the time she is passing from her position 

 farthest south of the equator to her position farther north, just in the same 

 way as the solar day is lengthening from midwinter to midsummer, only to 

 a much greater degree. And as the solar day lengthens fastest at spring 

 when the sun crosses the equator from south to north, so the time the moon 

 is above the horizon lengthens most, day by day, when the moon is crossing 

 the equator from south to north. It lengthens, then, from an hour to an 

 hour and 20 minutes in one day, that is the interval between moonrise and 

 moonsetting increases from 30 to 40 minutes. At this time, then, whenever 

 it happens in each lunar month, the moon's time of rising changes least — 

 instead of the moon rising night after night 50-| minutes later, the actual 

 difference varies only from 10 to 20 minutes. 



Now when this happens at a time when the moon is not nearly full, it is 

 not 8j)ecial]y noticed, because the moon's light is not then specially useful. 

 But if it happens when the moon is nearly full, it is noticed, because her 

 light is then so useful. A moon nearly full, afterwards quite full, and then 

 for a day or two still nearly full, rising night after night at nearly the same 

 time, remaining also night after night longer above the horizon, manifestly 

 iserves man for the time being in the most convenient way possible. But it 

 is clear that as the full moon is opposite the sun, and as to fulfill the condi- 

 tion described we have seen that she must be crossing the equator from 

 south to north, the sun, opposite to her, must be at the part of his path 

 where he crosses the equator from north to south. In other words, the time 

 of year must be the autumnal equinox. Thus the moon which comes to 

 •"full" nearest September 22 or 23 will behave in the convenient way de- 

 scribed. At this time, moreover, when she rises night after night at nearly 

 the same time, the nights are lengthening the fastest while the time the 

 the moon above the horizon is lengthening still more — and, therefore, 

 dn all respects the moon is then doing her best, so to speak, to illuminate 

 the nights. At this season the moon is called the harvest moon, from the 

 assistance she sometimes renders to harvesters. 



The moon which is full nearest to September 22-23, may precede or fol- 

 low that date. In the former case only can it properly be called a harvest 



