3*2 A S T R O 
'tre of the'Earfh, it thence follows, that the .lunar atmof- 
\phere is only one-third as denfe as that of the Earth, which 
is too little to produce any fenfible refraction of the (tar’s 
light. Other aftronomefs have contended, that fuch re- 
fraflion was fometimes very apparent. M. CatTini fays, 
that he often obferved that Saturn, Jupiter, and the fixed 
liars, had their circular figures changed into an elliptical 
one, when they approached either to the Moon’s dark or 
illuminated limb, though they own that, in other occul- 
tations, no fuch change could be obferved. And, with 
regard to the fixed (tars, it has been urged that, granting 
the Moon to have an atmofphere of the fame nature and 
quantity as ours, no fuch effect as a gradual diminution of 
light ought to take place; at lead none that we could be 
•capable of perceiving. At the height of forty-four miles, 
our atmofphere is fo rare as to be incapable of ref rafting 
the rays of light: this height is the iSoth part of the 
Earth’s diameter; but fince clouds are never obferved 
higher than four miles, it appears that the vaporous or ob- 
fcure part is only the 1980th part. The mean apparent 
diameter of the Moon is 31' 29", or 1889” : therefore the 
•obfcure parts of her atmofphere, when viewed from the 
Earth, muff fubtend an angle of lefs than one fecond; 
which fpace is palled over by the Moon in lefs than two 
feconds of time. It can therefore hardly be expected that 
obfervation fliould generally determine whether the fup- 
pofed obfcuration takes place or not. 
As to the third argument, it concludes nothing, becaufe 
it is not known that there is no water in the Moon; nor, tho 1 
this could be proved, would it follow, that the lunar at¬ 
mofphere anfwers no other purpofe than the railing of wa¬ 
ter into vapour. There is, however, a ftrong argument in 
favour of the exiltence of a lunar atmofphere, taken from 
the appearance of a luminous circle round the Moon in 
the time of total folar eclipfes; a circumftance that has 
been obferved by many aftronomers; efpecially in the to¬ 
tal eclipfe of the Sun which happened May 1, 1706. 
M. Schroeter likewife adduces a variety of proofs in 
fupport of a lunar atmofphere. He alfo makes a num¬ 
ber of obfervations on feveral of its relative properties, 
compared with the fame in our atmofphere; fuch as its 
greater drynefs, rarity, and clearnefs, which, however, 
do not prevent its refracting the folar rays, having point¬ 
ed out the circumftance, that the mountains in the dark 
hemifphere of the Moon, near its luminous border, which 
are of fufficient height to receive the light of the Sun, are 
the more feebly illuminated the more diffant they are from 
that border: from which proofs of a refracting atmof¬ 
phere, he alfo deduced the probability of the exiltence of 
a faint twilight, though his long feries of obfervations had 
not yet fully evinced it. He had, however, afeertained 
the exiltence of a twilight on Venus; and, as one fortu¬ 
nate difeovery often leads to another, he had no fooner fuc- 
•ceeded in his obfervations on that planet, than, he was 
induced to direct his attention, for a fimilar purpofe, to 
the Moon. In doing this, he applied the calculations and 
inferences he there made to fome appearances he had alrea¬ 
dy noticed on this fatellite. It occurred to him, that if 
in fact there were a twilight on the Moon, as there is on 
Venus and our Earth, it could not, contidering the greater 
rarity of its atmofphere, be fo conliderable: that the vefti- 
ges of it, allowing for the brightnefs of the luminous part 
of the Moon, the ftrong light that is thence thrown tipon 
the field of the telelcope, and in lome meafure the reflect¬ 
ed light of our Earth, could only be traced on the limb, 
particularly at the cufps; and even this only at tire time 
when our own twilight is not (trong, but the air very clear, 
and when the Moon, in one of its lead phafes, is in a high 
■altitude, either in the fpring, following the Sun two days 
after a new Moon; or in the autumn, preceding the Sun 
in the morning, with the fame afpect: in a word, that the 
projection of this twilight will be the greater and more 
perceptible the more falcated the phafe, and the higher 
the Moon above the horizon, and out of our own twilight. 
All the requifite circumftances do not often coincide. 
N O M Y. 
M. Schroeter, however, was fo fortunate as to be favoured 
with a combination of them on the 24th of February 1793 : 
and the cbfervation proved in every refpect fo complete, 
and the inferences deducible from it appeared to him fo 
new and intereding, that he could not with-hold the im¬ 
mediate communication of it from the public. His obfer¬ 
vations concerning both the Moon and Venus were accord¬ 
ingly detailed in a paper lent to tire Royal Society of Lon¬ 
don, and inferted in their TranfaCtions for 1792. 
Of the Harvest Moon. 
ft is remarkable that the Moon, during the week in 
which the is full about the time of harveft, riles fooner af¬ 
ter fun-fetting, than (he does in any other full-moon week 
in the year. By this means (lie affords an immediate fup- 
ply of light after fun-fet, which is very beneficial for the 
harveft, and for gathering in the fruits of the Earth ; and 
hence this full-moon is diftingnifhed from all others in the 
year, by calling it the harvejl-moon. 
To conceive the reafon of this phenomenon, it may firft 
be confidered, that the Moon is always oppolite to the Sim 
when file is full; that (he is full in the ligns Pifces and 
Aries in our harveft months, thole being the figns oppo- 
fite to Virgo and T.ibra, the figns occupied by the Sun 
about the fame feafon; and becaufe thofe parts of the 
ecliptic rife in a ftiorter fpace of time than others, as may 
he (hewn and illuftrated by the celeftial globe : confe- 
quently, when the Moon is about her full in harveft, (lie 
riles with lefs difference of time, or more immediately af¬ 
ter fun-fet, than when (lie is full at any other feafon of 
the year. 
The following Table (hews in what years the harveft- 
moons are leaft beneficial as to the times of their riling, and 
in what years they are mod beneficial, from the year 1790 to 
1861 : the column of years under the letter L, are thofe 
in which the harveff-moons are leaff of all beneficial, be¬ 
caufe they fall about the defeending node; and thofe un¬ 
der the letter M are the moft of all beneficial, becaufe they 
fall about the afeending node. 
Harveft-Moons. 
L 
M 
L 
M 
L 
M 
L 
M 
1790 
1798 
1S07 
1816 
1826 
*83.5 
1844 
1 8.53 
x 79 r 
1799 
1808 
1817 
1827 
1836 
1845 
>854 
1792 
1800 
1809 
1818 
1828 
1837 
1846 
1855 
x 793 
1801 
1810 
1819 
1829 
1838 
1847 
1856 
1794 
1802 
1S11 
1820 
1830 
1839 
1848 
x 8 j 7 
1795 
1803 
1812 
1821 
1831 
1840 
1849 
1858 
1796 
1804 
1813 
1822 
1832 
1841 
1850 
1 839 
x 797 
1805 
1814 
1823 
1833 
1842 
1851 
1860 
1806 
1815 
1824 
1825 
1834 
1843 
1852 
1S61 
I11 our winter 
•, the Moon 
is in Pifces and 
• Aries about 
the time of her fil'd quarter, when (lie riles about noon ; 
but her rifing is not then noticed, becaufe' the Sun is above 
the horizon. In fpring, the Moon is in Pifces and Aries 
about the time of her change ; at which time, as (lie gives 
no light, and rifes with the Sun, her riling cannot be per¬ 
ceived. In fummer, the Moon is in Pifces and Aries 
about the time of her laft quarter; and then, as (lie is on 
the decreafe, and lifes not till midnight, her rifing ufually 
piaffes unobferved. But, in autumn, the Moon is in Pifces 
and Aries at the time of her full, and rifes foon after fun- 
fet for feveral evenings fucceflively ; which makes her re¬ 
gular rifing very confpicuous at that time of the year. 
And this would always be the cafe, if the Moon’s orbit 
lay in the plane of the ecliptic. But, as her erbit makes 
an” angle of 5° 18' with the ecliptic, and erodes it only in 
the two oppolite points, or nodes, her riling when in Pilces 
and Aries will fometimes not differ above ill. 40'. through 
the whole of feven days; and at other times, in tire fame 
two figns, (he will differ three hours and a half in the time 
of her riling in a week, according to the different politions 
of the nodes with refpefft to tliefe figns; which politions 
r are 
