«6 ASTRONOMY. 
Of MERCURY. 
Mercury is the fmalleft of the inferior planets, and the 
neardt to the Sun, about which it is carried with a very 
rapid'motion. Hence it was, that the Greeks called this 
planet after the name of the nimble meffenger of the gods, 
and represented it by the figure of a youth with wings at 
his head and feet; from whence is derived the charac¬ 
ter in prefent ufe for this planet. It is from his proximity 
to the Sun, that he is fo feldom within the fphere of our 
obfervation, being loft in the fplendour of the folar bright¬ 
ness ; yet he emits a very bright white light. This planet 
is oftener feen in thofe parts of the world which are more 
Southward than that which we inhabit; and oftener to us 
than thofe which live nearer the north pole ; for, the more 
oblique the fphere is, the leis is the planet’s elevation above 
the horizon. 
The mean distance of Mercury from the Sun, is to that 
of the Earth from the Sun, as 387 to 1000, and therefore 
his distance is about thirty-Six millions of miles, or little 
more than one-third of the Earth’s distance, from the Sun. 
Hence the Sun’s diameter will appear at Mercury near 
three times as large as at the Earth ; and hence alfo the 
Sun’s light and heat received there is about feven times 
thofe at the Earth; a degree of heat Sufficient to make wa¬ 
ter boil. Such a degree of heat therefore muft render 
Mercury not habitable to creatures of our constitution: 
and if bodies on its Surface be not inflamed, and Set on 
fire, it muft be becaufe their degree of denfity is propor- 
tionably greater than that of fuch bodies is with us. The 
diameter of Mercury is alio nearly one-third of the dia¬ 
meter of the Earth, or about 2600 miles. Hence the fur- 
face of Mercury is nearly one-ninth, and his magnitude or 
bulk 1-27th, of that of the Earth. 
The inclination of his orbit to the plane of the ecliptic, 
is 6° 34'; his period of revolution round the Sun, is eigh¬ 
ty-feven days twenty-three hours, which consequently is 
the length of his year; the year of each planet being that 
Space of time which it occupies in going round the Sun. 
His greateft elongation from the Sun is 28°; the eccentri¬ 
city of his orbit one-fifth of his mean distance, which is 
far greater than that of any of the other planets ; and he 
moves in his-, orbit about the Sun at the amazing rate of 
95,000 miles an hour. 
The place of his aphelion is in Sagittarius 23 0 8'; place 
of afeending node is in Taurus 14° 43', and consequently 
that of the descending node is in Scorpio 14 0 43'. His 
length of day or rotation on his axis, inclination of axis to 
his orbit, gravity on his furface, denfity, and quantity of 
matter, are all unknown. 
Mercury changes his phafes, like the Moon, according 
to his various politicos with regard to the Earth and Sun; 
except only, that he never appears quite full, becaufe his 
enlightened fide is never turned directly towards 11s, unlefs 
when he is fo“ near the Sun as to be loft to our fight in his 
beams. And, as his enlightened ilde is always towards the 
Sun, it is plain that he fliines not by any light of his own ; 
for, if he did, he would conftantly appear round. 
The belt obfervations of this planet are thofe made when 
it is feen on the Sun’s diik, called its tranjit ; for, in its low¬ 
er conjunction, it lometimes paffes before the Sun like a 
little (pot, eclipfing a fmall part of the Sun’s body, only 
obfervable with a telefcope. That node from which Mer¬ 
cury afeends northward above the ecliptic, is in the fif¬ 
teenth degree of Taurus, and the oppolite in the fifteenth 
degree of Scorpio. The Earth is in thofe parts on the 6th 
of November, and 4th of May, new ftyle; and, when 
Mercury comes to either of his nodes at his inferior con¬ 
junction about thefe times, lie will appear in this manner 
to pals over the dilk of the Sun. But, in all other parts 
of his orbit, his conjunctions are invilible, becaufe he goes 
either above or below the Sun. As we neither know the 
time in which this planet revolves round its axis, nor the 
inclination of that axis to the plane of its orbit, we are 
necefiarily ignorant of the length of its day and night, or 
the variety of fcaions it may be liable to, 
The periodic time of Mercury may be very accurately 
determined from its tranfits over the Sun’s dilk; for, as 
they have frequently been obferved, we have an oppor¬ 
tunity of choofing fuch as will give us a very accurate con- 
clufion. From the obfervations of the conjunction of Mer¬ 
cury on November 6, 1631, Calfini found the time of the 
conjunction to be at 19I1. 50', and the true place of Mer¬ 
cury if. 14 0 41' 35''. On November 9, 1.723, at 5I1. 29', 
the conjunction was in if. 16 0 47' 20", only 2 0 5' 45" be¬ 
yond the place at the firft obfervation. Now, according to 
the Tables of Calfini, this difference is juft equal to "the 
motion of the aphelion of Mars in the fame time; confe- 
quently Mercury was in the fame place in its orbit at each 
time, and therefore the equation was the fame. Alfo, the 
conjunctions happening very nearly at the fame time of the 
year, the equation of time was very nearly the fame, and 
therefore the difference of the apparent times is the fame 
as of the true. Hence, in the interval of ninety-two years, 
(of which twenty-two were biflextiles,) and 2d. 9I1. 39', 
Mercury (from firft finding nearly the time of a revolution 
by two conjunctions near each othCr) is found to have 
made 382 revolutions 2 0 5' 45"; hence, by proportion, the 
time of a tropical revolution is 8yd. 23b. 14' 2o"'9; and 
the mean annual motion comes out if. 23°43' n" 39"',, 
Calfini, in his Tables, makes it if. 23 0 43'11". Dr. Hal¬ 
ley makes it if. 23 0 43' 2", and DelaLande if. 23°43'3". 
Of VENUS. 
Venus is one of the inferior planets, but the blighted, 
and to appearance the largeft, of all the planets. She is 
eafily diftinguilhed by her whitenefs and brightnefs, in 
which (lie exceeds all the reft, even Jupiter himlelf, and 
which is lo confiderable, that in a dulky place Ihe caufes 
an objeCt to projeCt a fenfible fliadow, and (lie is often vi- 
fible in the day-time. Her place in the fyftem is the fe- 
cond from the Sun, and in magnitude Ihe is about equal 
to the Earth, or rather a little larger, according to Dr. 
Herfchel’s obfervations. As Venus moves round the Sun, 
in a circle beneath that of the Earth, (lie is never feen in 
oppofltion to him, nor indeed very far from him; but feems 
to move backward and forward, palling him from fide to 
fide, to the diftance of about forty-feven or forty-eight 
degrees, both ways, which is her greateft elongation. 
When Ihe appears weft of the Sun, which is from her in¬ 
ferior conjunction to her fuperior, fhe rifes before him, or 
is a morning ftar, and is called pho/pkorus, lucifer, or the 
morningfiar ; and when Ihe is eaftwards from the fun, which 
is from her fuperior conjunCIion to her inferior, Ihe fets af¬ 
ter him, or is an evening ftar, and is called hefperus, vcfper , 
or the evening jlar: being each of thofe in its turn for 290 
days. The inhabitants of Venus fee the planet Mercury 
always accompanying the Sun; and he is to them, by turns, 
an evening or a morning ftar, as Venus is to us. To the 
fame inhabitants the Sun will appear almoft twice as large 
as he does to us. 
The real diameter of Venus is nearly equal to that of the 
Earth, being about 7900 miles ; her apparent mean diame¬ 
ter feen from the Earth 59", feen from the Sun, or her ho¬ 
rizontal parallax, 30"; but as feen from the Earth i 8"‘79, 
according to Dr. Herlchel : her diftance from the Sun fe- 
venty millions of miles; her eccentricity -j-g^yths of the 
fame, or 490,000 miles; the inclination of her orbit to the 
plane of the ecliptic 3 0 23'; the points of their int.erfeftion 
or nodes are i4°of Gemini and Sagittarius; the place of 
her aphelion is in Aquarius, 4 0 20'; her axis inclined to her 
orbit 75 0 o'; her periodical courfe round the fun is per¬ 
formed in 224 days 17 hours, moving at the rate of 80,995 
per hour; the diurnal rotation round her axis is very un? 
certain, being according to Calfini only twenty-three hours, 
but according to the obfervations of Bianchini it is in twen¬ 
ty-four days eight hours; though Dr. He rich el thinks it 
cannot be fo much. 
Venus, when viewed through a telefcope, is rarely feen 
to fhine with a full face, but has phafes and changes juft 
like thofe of the Moon, being increafing, decreafing, horn¬ 
ed, 
