ASTRONOMY. 
parts of its year to be accelerated and retard- 
ed, it would almost amount to a demonstration 
of its monsoons, and their periodical changes. 
The ecliptic and equator of Jupiter are nearly 
parallel to each other, that is, the axis of the 
! planet is nearly perpendicular to its orbit, and 
on that account its inhabitants experience no 
! sensible change of seasons. This is a wise pro- 
vision, for if tiie axis of J upiter were inclined any 
! considerable number of degrees, just so many 
degrees round each pole would, by turns, be 
I almost six years in darkness. Jupiter is sur- 
[ rounded by four moons of different sizes, 
I which move about it in different times. These 
I moons are sometimes eclipsed by the shadow 
of Jupiter falling upon them. The eclipses 
I have been found of great use in determining 
t the longitudes of places on the earth. 
Saturn can hardly be seen by the naked 
j eye. When examined by a telescope, it ex- 
| hibits a very remarkable appearance. It is 
| surrounded by a thin, flat, broad luminous 
; ring, which surrounds the body of the planet, 
; but does not touch it. This ring casts a 
j strong shadow upon the planet ; and appears 
to be divided into two, by a distinct line in the 
| middle ot its breadth. This ring is circular, 
i but appears elliptical from its being viewed ob- 
liquely. Besides this ring, Saturn has seven 
moons of different sizes ; and its body is sur- 
rounded also by belts, like those of Jupiter. 
The Herschel, with its six satellites, have 
been entirely discovered by Dr. Herschel. It 
i cannot be seen without a telescope, but it does 
not require a powerful one. The satellites 
; cannot be seen without the most powerful te- 
: lescopes. 
Having given a brief account of the Sun 
; and planets, we shall shew by what means we 
| can ascertain their apparent and real diame- 
ters. 
On the apparent and real Diameters of the 
[ Sun and Planets. — It is obvious from the prin- 
; ciples of optics, that the determination of the 
real diameters of the heavenly bodies will de- 
r pend conjointly on a knowledge of their ap- 
parent diameters, and their real distances 
from the earth. For, suppose APB (fig. 8) 
a section of such body made by a plane pass- 
ing through the place O of the eye, and the 
centre C of the body : then A O B will be the 
' angle, which will measure tiie apparent diame- 
ter of the body; this being known, A O C, its 
[half, will be known. And A O being a tan- 
gent to the surface, the angle C A O will be a 
right angle ; whence, co-sine AOC: sine 
A O C : : A O : AC, the semidiameter of 
the body. Now AC or O C, may be found 
by observation ; or, when the body is in op- 
position or conjunction with the Sun, by taking 
the difference or sum of its distance from the 
Sun, and the earth’s distance from that lumi- 
nary, according to the respective cases; taking 
care to attend to the different operations re- 
quired for a superior and inferior planet. And 
as to the apparent diameters, it may be worth 
\vhile to point out a few methods of ascertain- 
ing them. 
The Sun’s Vertical or perpendicular diameter 
may be found by two observers taking, the 
one the height of the upper edge of the disc, 
the other that of the lower, at the same 
instant. This is most conveniently done 
when the Sun is at or near the meridian ; be- 
cause there is then no sensible change in his 
altitude during the space of two or three mi- 
nutes. The height of each edge must be 
corrected, by allowing for parallax and re- 
fraction ; and the apparent diameter will be 
equal to the difference of the corrected alti- 
tudes of the upper and lower edge. This 
method is very simple, and gives the apparent 
diameter with exactness proportional to the 
accuracy of the instruments made use of. 
Another method of determining the Sun’s 
apparent diameter, is to observe by a good clock 
the time in which the sun’s disc passes over the 
plane of the meridian, or some other hour 
circle. At, or very near, one of the equi- 
noxes, when the sun’s apparent diurnal mo- 
tion is in the equator, or a parallel very near it, 
say, as the time between the Sun’s leaving the 
meridian and returning to it again : 360° : : 
the time in which he passes over the meridian : 
his apparent semidiameter. At any other 
time oi the year, when the Sun is in a parallel 
at some distance from tiie equator, his diame- 
ter measures a greater number of minutes and 
seconds in that parallel, than it would do in a 
great circle, and takes up proportionally more 
time in passing over the meridian ; we may 
then use this analogy, As radius : co-sine of the 
Sun’s declination : : the time in which the Sun 
passes the meridian, converted into motion-, 
at the rate of tour minutes in time to 1° : the 
arc of the great circle which measures the 
Sun’s apparent horizontal diameter. This 
method may be easily put in practice by two 
observers ; or indeed by one, if he have an 
half-second pendulum placed near enough for 
him to hear the beats of it, whilst he observes 
the transit. But the diameters of the planets 
are best taken by the micrometer, an instru- 
ment so contrived that two parallel wires being 
placed in the focus of a telescope, one fixed 
and the other moveable, or both moveable, 
they may be made to approach or recede 
one from the other till they appear to touch 
exactly two opposite points in the disc of the 
planet, and then the index shews the appa- 
rent diameter in minutes and seconds. The 
apparent diameters of the planets when at 
about their respective mean distances from the 
earth, are as we have seen': Mercury, ]0 // ; 
Venus, 58"; Mars, 27" ; Jupiter, 39" ; Saturn, 
1 8" ; Georgium Sidus, 3" 54'".. And from 
these apparent diameters, and the respective 
distances from the earth, the diameters of the 
Sun and planets have been determined in 
English miles as here stated : Mercury, 3224 ; 
Venus, 7867; Mars, 4189; Jupiter, 89170 ; 
Saturn, 79042 ; the Herschel 35112; the Sun, 
883246. Observations upon the planets Hers- 
chel, Saturn, Jupiter, and Mars, prove that 
there is a sensible difference between their 
equatorial and polar diameters ; and it is pro- 
bable that there is a like difference between 
the diameters of the other planets, but this has 
not yet been determined by observation. 
Since the apparent diameters of distant bo- 
dies vary inversely as their distances, we may, 
having the distances from the earth at which 
the respective planets subtended the above 
angles, and knowing their mean distances from 
the Sun, find the mean apparent diameters of 
all the planets,, as seen from the Sun ; they ha ve 
been thus given: Mercury, 20"; Venus, 30" ; 
Earth, 17" ; Mars 10"; Jupiter, 3 7" ; Saturn, 
16"; Georgium Sidus, 4". 
'To measure the quantity of matter in dis- 
tant bodies appears a problem of insuperable 
difficulty : but this has been effected to a con- 
siderable degree, by the principles of the 
Newtonian philosophy, Since the quantity 
173 
of matter in a globe is proportional to the 
mean density multiplied into the cube of the 
diameter, and the diameters of the planets are 
known, the mean densities are all that are 
required for the solution of the problem. 
Now, in homogeneous, unequal, spherical 
bodies, the gravities on their surfaces are as 
their diameters, when the densities are equal ; 
or the gravities are as the densities, when the 
bulks are equal: therefore, in spheres of un- 
equal magnitude and density, the gravity is 
in the compound ratio of the diameters and 
densities ; or the densities are as the gravities 
divided by the diameters. But the diameters 
are known, and the gravities at the surface 
are nearly found, either by means of the re- 
volutions of the satellites, or by calculations 
deduced from the effects the planets are found 
to produce upon each oilier; consequently 
the relation of the densities becomes known. 
The mean density of the earth was calculated 
by Dr. Hutton, from observations made by 
Dr. Maskelyne at the mountain Schehallien ; 
he made it to that of water as 9 to 2, and to 
common stone, as 9 to 5, on the supposition 
that the hill is only of the density of common 
stone. He also states the mean densities of 
the Sun and planets to that of water, thus : 
Sun, ]_ 2 _; Mercury, 9±; Venus, 5ii; Earth, 
4-| ; Mars, 3 1.; Jupiter, 1 ; Saturn, 0A-| ; 
and the Herschel OJtjL.. These densities 
are such as the bodies would have if they were 
homogeneal ; and may be admitted as a fair 
estimate of the whole, although the density of 
each planet may vary considerably at differ- 
ent distances from the surface. From the 
densities, as thus estimated, and the known 
diameters, we may readily find the propor- 
tions of the quantities of matter ; they are as 
under: the Sun, 333928 ; Mercury, 1654; 
Venus, *8899 ; the Earth, 1 ; Mars, ’0875 ; 
Jupiter, 312-1; Saturn, 97-76; Georgium 
Sidus, l6 - 84. 
Of the Satellites. 
We have observed that Jupiter, Saturn, and 
the Herschel planet, are attended by satellites ; 
of which Jupiter has four, Saturn seven, anti 
the Herschel six. In order to arrive at cer- 
tainty concerning the laws of these bodies, it 
is necessary to consider facts ; we therefore 
briefly describe the chief phenomena. — 1. 
These satellites are sometimes to the east- 
ward, sometimes to the westward, of their re- 
spective planets, moving successively from 
one side to the other : each at its greatest 
excursion, as observed from the earth, is 
nearly as far distant from the one side as it 
was from the other, and is found on the same 
side again in much about the same interval 
of time ; from which it is inferred that the 
orbits of the satellites are curves returning 
into themselves. 2. All the satellites (ex- 
cept those of the Herschel), in going from 
the western excursion to the- eastern, are 
often hid by the planet’s disc, and consequent- 
ly pass behind it : sometimes one or other 
of them passes above or below,, but never on, 
the planet’s disc. On the contrary, in going 
from the eastern excursion to the western, 
those which passed behind, now pass over the 
disc; and those which passed above, now 
pass below, and reciprocally ; which proves 
that they move with the primary as their 
centre. 3. The paths of the satellites being 
reduced to their respective planet’s centre,, 
sometimes appear in a right line passing 
