ASTRONOMY.

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those times, it stands most erect above the horizon, and is there-fore clearest from the thick vapours of the twilight. Besides thedii'lerence of real extension of this light in length and breadth atdifferent times, it is diminished by the nearness of any other lightin the sky ; not to mention that the extent of it will be differentlydetermined by ditferent spectators, according to the goodness oftheir eyes.

(123.) From a fixed point at or near the centre of the Sun thestars would appear stationary ; but would have the same relativesituations as when viewed from the Earth . The planets would allappear to advance constantly from west to east, performing theirrevolutions in different times, the satellites attending their prima-ries during the whole period, f rom the surface oi the Sun , onaccount of its motion on his axis, the stars would apparently re-volve from east to west in 25 days, 14 hours, 8 minutes; but asseen from the planets, because they are moving from west to east,he will appear to he longer in performing the revolution.

(124.) The solar parallax being 8| seconds, his mean distancefrom the Earth is consequently 93513794 English miles; his meanapparent diameter, as seen from the Earth , is 32' 3", the true dia-meter is 886473 English miles; hence his magnitude or solidcontents amounts to 1384462 times the number of cubic miles, inthe whole Earth . These particulars are determined by methodsafterwards explained.

Sect.VI. Or the inferior Planets, Mercury and Venus .

(125.) Mercury is a small planet, but emits a very clear whitelight; he is so near the Sun , and moves so quick, that he is seldomseen. When viewed through telescopes magnifying about 200 or300 times, he appeal's equally luminous throughout his whole sur-face, without the least dark spot. He has the same phases withthe Moon , being sometimes horned, sometimes gibbous, and same-times shining with a round face, though never seen in this last phase,because his enlightened side is never turned directly towardsus, butwhen he is in his superior conjunction. lie constantly appears verywell defined without any ragged edge, and perfectly bright.

(126.) To an eye placed in Mercury, the Sun will appear al-mo-t three times broader than we see him from the Earth , becausethat planet is almost three times nearer to the Sun than the Earth .Whence also the solar disk, seen from Mercury , is seven timesgreater than the disk as it appears to us, and Mercury has seventimes more light, caeteris paribus, than the Earth . But his lightis much more or less intense, according to the different distancesof Mercury from the Sun ; for his orbit is much more eccentric,than the orbit of any other planet. The accelerating gravity, to-wards the Sun , is also seven times greater there than here. It hasnot yet been found by observation, whether Mercury turns uponhis own axis, and therefore it cannot be certainly defined,whetherto an eye, placed on his surface, all the things without will appearto revolve with a contrary motion ; that is, whether it has thevicissitude of day and night, much less what is the space of hisnychthemeron, or natural day: but we may slafely enough con-jecture, that he revolves about his axis, since the. other planetsdo. But the v, ar of Mercury is scarcely equal to a quarter of ouryear, though' it is uncertain whether lie has different seasons, be-cause they depend upon the inclination of tire axis of his rotation,which is unknown, to the plane of the orbit which he describesabout the Sun. The eye placed in Mercury, looking at the Sun ,will see the Sun ’s spots (when he has any) sometimes in a right linecrossing his disk from east to west, at another time their way willappear elliptical, and curved towards one side or the other, and allthe variety of this plr.enomenon will happen in one year, in whichthe track or way of the spots will appear twice rectilineal. Butthe way of the solar spots, seen Irom Mercury , will be always al-most straight; because Mercury never declines much from theplane of the Sun ’s equator, and therefore not sensibly from theplanes of the circles, which the spots describe by their rotation.And as the plane of the orbit of the Earth is most of all inclinedto the said plane of the Sun ’s equator, the way of the spots, seenfrom the Earth, appears more curve than if the Sun was seen fromany oilier planet. This curvature will be something less, seen fromHerschel, Jupiter, or Mars ; and yet less from Saturn , less still fromVenus, and least of all from Mercury , as^e have just observed.

(127.) The other planets are superior to Mercury , thereforetl-.eir pha’nomena are, to an eye placed in Mercury , as the phaeno-jnena of Mars, Jupiter, Saturn, and Ilerjichel, seen from the Earth .

ThereforeVenus and theEarth,\vhen they are in opposition with theSun, will shine u ith a full orb, reflecting the Sun ’s rays very direct-ly upon Mercury all night. Mercury has no inferior planetknown to us, consequently sees no celestial body falcated or horn-ed ; and therefore a spectator there will want the argument takenfrom such phases of the planets, to establish the true system of theworld. But though we do not see any planets inferior to Mer-cury , it does by no means follow that there are none : for we sel-dom see Mercury himself, and a planet much nearer the Sun could never be seen from the Earth .

(128.) Venus , the most beautiful star in the heavens, is easilydistinguished by her brightness and whiteness, which exceeds thatof all the other planets, and which is so considerable, that in adusky place she projects a sensible shadow. She constantly at-tends the Sun , and never departs from him more than about 47°When she goes before the Sun , that is, rises before him, she is calledphosphorus, or lucifer, or the morning-star; and when she followshim, that is, sets after him, hesperus, or vesper, or the evening-star.The eye in Venus will see only one planet below her, which isMercury: when our Earth is in opposition to the Sun , it will appearat Venus (in the night) to shine with a full orb, and be very bright.The Moon will appear always to accompany the Earth , and neverto be seen from her above half a degree. Mercury will neverappear to be above 38° distant from the Sun . October 14, 1666,N. S. Cassini observed several spots in the body of this planet, bywhose motion he judged (though he was not certain) that she movedeither by a circulation, or a kind of iibration, round her axis, inabout 23 hours. Mr. Schroeter has lately determined the rotationot Venus to be performed in 23h.21m. {Phil. Trans. 1795.) A. D.1672, and 1686, Cassini,with atelescope of 34feet, believes hesawasatellite moving round this planet, and distant from it about three-fifths of Venus’s diameter. It had the same phases w ith Venus ,but was without any well-defined form, and its diameter scarcelyexceeded one-fourth of that of Venus . Dr. Gregory thinks itmore than probable, that this was a satellite ; and supposes thereason why it is not usually seen, to be the unfitness of its surfaceto reflect the rays of the Sim’s light; as is the case with the spotsin the Moon, of which, if the whole disk of the Moon were com-posed, he thinks that the planet could not be seen in Venus . M.de la Lande and M. Wargentin has clearly proved, that the pre-tended satellite of Venus is merely an optic illusion. See thethird volume of La Eande’s Astronomy , and the third volume ofthe New Series of the Memoirs of Upsal. Venus , when viewedthrough a telescope, is rarely seen to shine with a full face, butlias phases just like those of the Moon , being now gibbous, nowhorned, &c. and her illuminated part constantly turned towardsthe Sun , i. e. looks towards the east, when phosphorus, and to-wards the west,when hesperus. M. De La Hire, in 1700, through atelescope of 16 feet, discovered mountains inVenus,which he foundto be larger than those in the Moon . These observations have beenlately confirmed by Mr. Schroeter. See Phil. Trans. 1793, 1795.

Sect. VII. Or the Transits of Venus and Mercury .

(129.) These two planets which turn round the Sun at a less distancefrom him than the Earth is, must consequently come between usand the Sun once in every synodic, revolution; and if the geo-centric latitude of these planets at the time of their inferior con-junction be less than the solar semidiameter, we may see a roundblack spot upon the disk of the Sun , whose diameter will appearto occupy about the thirtieth part of that of the Sun , if the planetbe Venus, but only the 150th part, if it be Mercury . Thesetransits are of great importance in Astronomy , as they* furnish ameans of determining exactly the place of the node of Mercury or \ enus, independently of the parallax of the great orb. Blitthe transits of Venus have this singular advantage of giving us toknow exactly the solar parallax, whence depend the distances of allthe planets from the Sun. In the transit of Venus there are threethings which stamp worth on these kind of observations : 1. 'l'hegreat precision with which the contact of these small very blackbodies may be observed, when upon the luminous disk of tlxSun. 2. The known relation of the parallax of Venus with thatof the Sun and all the other planets; and 3. The quantity of thisparallax, which amounts to 21", and produces the difle’mice ofmore than a quarter of an hour in the duration of a transit, whenobserved from two points of the Earth , wjiich will give the greatest,result, Kepler was the first who, in 1627, after having corrected

his