DIAMETER, 
bodies from each other, as saad from the fame table, in 
which the parallax i is likew ife piv 
fer ent 
aftronomers, of A a - ‘he fun, it will oe nae oe 
they have a tenden nifh as they sl tairatiae 
telefcopes, which render the apparent image of the fun fome- 
a {maller. 
he difference between the greateft and leaft diameters 
of dh fun is 64.”6. 
emidiameter of the fun, divided by the correfponding 
parang, {table T.) gives a conftant quantity, whic 
€s the ratio of the diameters of the fun and the earth. 
hus, 22 
8 7 
A to De !a Lande, who affumes the fun’s hori- 
zontal "paral, 8”. bs 3 the fun’s diameter is to that of the 
ear ats 
it appears, that if the centre of the fun be 
foppoted ee in the fame point as the centre of the earth, 
e globe of the fun wou'd then extend to a diftance near ly 
ea as ee as the orbit of the moon. 
Tn calculations of eclipfes, it is ufual to diminifh the 
diameter of the fun 5” or 6”, allow for the irradiation, 
which makes the a difk a the fun appear greater than 
it otherwife would de. 
e os see diameter of the moon varies from 29! 22” to 
33/34”: its iameter one nearly equal to the leaft 
sa Sie aes of the 
The variations in. a diameter of the moon are much 
diftance. 
conjunctions apogee, and greateft ia the fyz‘gies perigee. 
f igns, the di- 
h 
the fame quantity, though its diftance from the apogee 
fhould, in both cafes, be the fame. In the fame manrer 
has te to the argument of the variation, when that is 
I figna, the diameter the mcon increafes 14” or 
oe and when the argumentis III o figns; it de- 
creafes by the fame aed though a the fame aience 
from hi apogee. See Ev wand Var 
e. IATION. 
expreffion for the se of the moon for any given 
Geet is aie this: 
31. 7."3 — 1.42."3 cof. anomal + 5."4 cof. 2 atom: 
+ 13."7 cof. 2 dit. )© — 20.2 cof. dift. 
a he 
ee el Burg, publithed by the Bureau des Pend, 
at 
moon approaches the zenith, her diftance becomes 
oe apparent diameter isincreafed. Let I (fg. 
oe centre of the earth, O the place of an obferver on the 
fits, Z the moon fuppofed to be in the zenith. The dif- 
ytance Z O is about {% part lefs than the diftance ZT ; its 
pupil therefore, feen from O, will be greater than if 
feen in the fame proportion 
If the moon be at L, its zenith_ diftance being the angle 
@ OZ, the diftance LO _ be evidently lels than the 
diflance L T, When the moon is at the horizon at H, 
the augmentation will be nearly infenfible, for which reafon 
we confider the horizontal femi-diameter asequal to that which 
would be feen from pa peak of the ce ut this is not 
quite correét, for t be w the hori zon, by 
a quantity equal to half | her parallax, ie ee See 
y to be really equal O; that is when the triangle LOT i 
{celes. 
When the a diameter of the moon is known, it 
is eafy to compute the angmented diameter, fiace they are 
to each other as LO Ais Te 
the triangle L O T, the angle O is the fupplement 
to the apparent ” zenith diftance ; the angle L TO is the 
true zenith diftance, as feen fron the centre of the een 
: fine OTL : fin. LOT or LOZ. 
Therefore, the honcoural diameter is to the applet di- 
ameter, as the fine of the true zenith diltance of the moon, 
as feen from the centre of ke earth, is to the apparent dif- 
tance, as feen from the poin e augmented diameter 
of the moon may be found, ee by this proportion 
of true altitude is to co 
fo is the horiz i : 
n’s diameters a table of wie been 
iven under DECLINATION. 
In the above computation, if the moon is very near the 
zenith, her diftance from the centre and furface of the earth 
fhould be employed initead of the zenith diftances 
The diameter of the moon may be meafured in ‘the fame 
manner as that of the fun, or it may be inferred with great 
accuracy, from the time ae ase ean the immer- 
fion and emerfion of a fixe who aes uted 
it by this latter method for his | junar le ound no dif- 
ference from the refult, by anobje&t glafs micrometer of 
Dollon 
The apparent diareters of the planets, at leaft of thofe 
near to us, vary much more than the diameters of the fun 
and moon. 
The diameter of a planet, when its diftance is equal to 
the mean diflance of the fun, being divided by the cif. 
tance of the planet from the earth, gives its atual or appa- 
e earth; and the diftance is 
3 
the eecenic latitude, is the diftance of the 
planet na the earth, 
moft favourable bi aloe ed of obferving the dis 
ameter of } Mercury is w paflcs over the dife of the fun, 
to a telefcope of 18 ii t, — 8 found i it 
the diftance of Merc he earth, was to the mean dif- 
tance of the fun to ae fet as 55674 to T1007. 
Therefore, 1010 : 557 2: 11."3 2: 6." 
Hence, 6."5 is the diameter of Mereiry at a diftance 
equal to the mean diftance of the 
Dr. Bradley, by a prea! ere on a or of 120 
feet, in 1723, found the diameter on the s dife 107, 
which gives 7.3 for cieiean diftance. By ee cies it from 
the time it took to quit the difc, De Ja Lande found 5."9. 
The diameter of Aaagy is found in the fame manner, and 
with ex for every fecond of the diameter 
of Venus employs 19” to quit the difc of fun. And as it i 
not eafy to clk 5" in the (aged of time the planet takes 
to 
