39*5 A S T R 
the axis of the Earth’s fhadow will terminate In the fird. 
point of Libra. It is clear, therefore, that there can be 
no eclipfe of the Moon but when the Earth is interpofed 
between it and the Sun, that is, at the time of its oppofition , 
or when it is full; for, unlefs it is oppofite to the Sun, it 
never can be in the Earth’s (hadow : and, if the Moon al¬ 
ways moved in the plane of the ecliptic, (lie would every 
full Moon pafs through the body of the (hadow, and there 
would be a total eclipfe of the Moon. 
We have already obferved, that the Moon’s orbit is in¬ 
filled to the plane of the ecliptic, and only coincides with 
it in two places, which are the nodes. It may therefore 
be full Moon without her being in the plane of the eclip¬ 
tic ; (he may either be on the north or the fouth fide of it j 
in either ot thefe cafes ihe will not enter into the (hadow, 
but be above it in the one cafe, and below it in the other. 
To illuftrate this, let HG, f.g. 5, in the Adronomical- 
Plate V, reprefent the orbit of the Moon, EF, the plane 
of the ecliptic, in which the centre of the Earth's fhadow 
always moves, and N the node of the Moon’s orbit; ABCD 
tour places of the (hadow of the Earth in the ecliptic. 
When the fhadow is at A, and the Moon at I, there will 
be no ecliple : when the full Moon is nearer the node, as 
at K, only part of her globe pa lies through the fhadow, 
and, that part becoming dark, it is called a partial eclipfe; 
and it is faid to be of fo many digits as there are twelfth 
parts of the Moon’s diameter darkened. When the full 
Moon is at M, (he enters into the (hadow C ; and, parting 
through it, becomes wholly darkened at L, and leaves the 
(hadow at O : as the whole body of the Moon is here im- 
merfed in the (hade, this is called a total eclipfe : but 
when the Moon’s centre partes through that of the (hadow, 
w hich can only happen when (he is in the node at N, it is 
called a total and central eclipfe. There will always be 
fuch eclipfes, when the centre of the Moon and axis of 
the fhadow meet in the nodes. 
The duration of a central eclipfe is (o long, as to let 
the Moon go the length of three of its diameters totally 
eclipfed, which (fay in the Earth’s fhadow is computed to 
be 3I1. 57'. 6". from the beginning to the end, if the .Moon 
be at her greateftdiftance from the Earth ; and 3b. 37'. 26". 
if (lie be at her lead dirtance. The reafon of this diffe¬ 
rence is, that when the Moon is fartheft from the Earth 
fhe moves (lowed, and when neared to it, qiticked.. In 
the beginning of an eclipfe, the Moon enters the wedern 
part of the diailow with the eadern part of her limb ; and, 
in the end-of it, (lie leaves the eadern part of the (hadow 
with the wedern part of her limb. All the intermediate 
time, from her entrance to her quitting the fhadow, is reck¬ 
oned into the eclipfe ; but only fo much into the total im- 
merfion, as partes while the Moon is altogether obfenred. 
From the magnitude of the Sun, the ftze of the Earth, 
their dirtance from each other, the refraction of the atmo- 
fphere, and the didance of the Moon from the Earth, it 
it has been calculated that the fhadow of the Earth termi¬ 
nates in a point, which does not reach fo fnr.as the Moon’s 
orbit. The Moon is not, therefore, eclipfed by the (ha¬ 
dow of the Earth alone. The atmofphere, by refracting 
Come of the rays of the Sun, and reflecting others, cads 
a fhadow, though not fo dark a one as that which arifes 
from an opaque body : when, therefore, we fay the Moon 
is eclipfed, by parting into the (hadow of the Earth, it is 
to be underrtood of the (hadow of the Earth together with 
its atmofphere. Hence it is that the Moon is vifible in 
eclipfes, the (hadow caff by the atmofphere not being fo 
dark as that cad by the Earth. The cone of this (hadow 
is larger than the cone of the Earth’s fhadow, the bafe 
thereof broader, and the axis longer. There have been 
eclipfes of the Moon, in which the Moon has entirely 
difappeared : Hevelius mentions one of this kind, which 
happened in Augud 1647, when he was not able to dif- 
tinguifh (he place of the Moon, even with a good teie- 
fcope, although the (ky was fufficiently clear for'him to 
fee the (tars of the fifth magnitude. 
rW opaque bodies, when illuminated by the rays of the 
O N O M Y. 
.Sun, cad an umbra , or (hadow, from them, which is en. 
compalfed by a penumbra, or thinner diadow, which every 
where furrounds the former, growing larger and larger as 
we recede from the body : in other words, the penumbra 
is all that fpace furrounding the umbra, into which the 
rays of light can only come from fome part of that half 
of the globe of the Sun which is turned towards the pla¬ 
net, all the red being intercepted by the intervening body. 
To illudrate this, let S, fig. 6., of the forementioned plate,, 
be the Sun, E the planet, then the penumbral cone is 
FGH. The nearer any part of the penumbra is to the 
umbra or (hadow, the lefs light it receives from the Sun ; 
but, the farther it is, the more it is enlightened ; thus the 
parts of the penumbra near M are illuminated by thofe 
rays of light which come from that part of the Sun near 
to I, all the red being intercepted by the planet E. In 
like manner, the parts about N can only receive the light 
that comes from the part of the Sun near to L ; whereas 
the parts of the penumbra at P and Q^are enlightened in a 
much greater degree : for the planet intercepts from P 
only thofe rays Which come from the Sun near L, and 
hides from CLjoniy a finall part of the Sun near I. The 
Moon partes through the penumbra before (lie enters into 
the umbra or fnadow. This caufes her gradually to lofe 
her light, which is not fenlible at fird ; but, as (he goes 
into the darker part of the penumbra, (lie grows paler. 
The penumbra, where it is contiguous to the (hadow, is 
fo dark, that it is diflicult to didinguifli one from the other. 
If the atmofphere be ferene, every eclipfe of the Moon 
is vifible at the fame indant to all the inhabitants of that 
fide of the Eartli to which (he is oppofite. In a total 
eclipfe, the Moon generally appears of a dufky reddidi 
colour, efpecially towards the edges; but of a darker to¬ 
wards the middle of the fhadow. 
Since an eclipfe of the Moon arifes from its real depri¬ 
vation of light, and appears to begin at the lame indant 
of time to every place on that hemifphere, it affords a 
very ready method of finding the difference of longitudes 
of places upon the Earth. As the Moon enters the pe¬ 
numbra of the Earth before it comes to the umbra, it very 
gradually lofes its light; and the penumbra is fo dark juft 
at the umbra, that it is difficult to afeertain the exaCt time 
when the Moon’s limb touches the umbra, or when the 
eclipfe really begins. When the Moon lias entered int© 
the umbra, the diadow upon its di(k is generally fo well 
defined that we may determine to a confiderable degree of 
accuracy the time when any fpots enter into the umbra. 
Hence the beginning and end of a lunar eclipfe are not fo 
proper to determine the longitude from, as the times at 
which the umbra touches any of the fpots. 
Of Eclipses of the Sun. 
The Moon, when in conjunction, if near one of her 
nodes, will be interpofed between us and the Sun, and 
will consequently hide the Sun, or a part of him, from 
11s, and cad a diadow upon the Earth : this is called an 
eclipfe of the Sun ; it may be either partial or total. An 
ecliple of any lucid body is a deficiency or diminution of 
light, which would otherwife come from it to our eye, 
and is can led by the interpolition of fome opaque body. 
The eclipfes of the Sun and Moon, though exprerted by 
the fame word, are in nature very different; the Sun, in 
reality, lofes nothing of its native ludre in the greated 
eclipfes, but is all the while incelfantly (ending forth 
dreams of light every way around him, as copiourty as be¬ 
fore. Some of thele dreams are, however, intercepted in their 
way towards our Earth, by the Moon coming between the 
Earth and the Sun : and the Moon having no light of her 
own, and receiving none from the Sun on that half of the 
globe which is towards our eye, mud appear dark, and 
make fo much of the Sun’s diik appear fo, as is hid from 
us by her interpolition. What is called an eclipfe of the 
Sun, is therefore, in reality, an eclipfe of the Earth, which 
is deprivectof the Sun’s light, by the Moon’s coining be¬ 
tween, and cafting a (hadow upon it. The Earth being a 
globe, 
