Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/524

 440 ASTRO N O M Y. When Venus was little more than half emerged into furement of the heft maps) contain 160 million 522 thoathe fun’s dilk. Mr Ma&elyne faw her whole circum- fand and 26 fquare miles; the inhabited parts 38 milference conlpieated, by means of a vivid, but narrow lion 990 thoufand 569; Europe 4 million 456 thoufand and ill defined border of light, which illuminated that and 65; Afia, 10 million 768 thoufand 823; Africa, part of her circumference which was off the fun, and o- 9 million 654 thoufand 807; America, 14 million no therwife not viable. They all obferved the black pro- thoufand 874. In all, 199 million 512 thoufand 59s; tuberance in the.internal contact. They likewife, after which is the number of fqiutre miles on the -whole furthe internal contadt, faw a luminous ring round the bo- face of our globe. dy of Venus, about the thicknefs of half her femi-dia- The Moon is not a planet, but only a fatellite or atnieter; it was brightefl: towards Venus’s body, and gra- tendant of the earth; going round the earth from change dually diminilhed in fplendor at greater diftance, but the to change in 29 days 12 hours and 44 minutes; and round the fun with it every year. The moon’s diameter is 2180 whole was e'xceifive white and faint. Venus may have a fatellite or moon, although it miles; and her diftance from the earth’s centre 240 be undifeovered by us : which will not appear very fur- thoufand. She goes round her orbit in 27 days yhouis prifing;, if we confider how inconveniently we are placed 43 minutes, moving about 2290 miles every hour; and from feeing it. For its enlightened fide can never be turns round her axis exadtly in the fame time that flic fully turned towards us, but when Venus is beyond the goes round the earth, which is the reafon of her keepfun; and then, as Venus appears little bigger than an ing always the fame fide towards us, and that her day ordinary liar, her moon may be too fmall to be percei- and nigkr, taken together, is as long as our lunar month. ved, at fuch .a diftance. Whenf fhe is between us and the The moon is an opaque globe like the earth, and fhines fun, her full moon has its dark fide towards us; and only by refledting the light of the fun : Therefore whilft then we cannot fee it any more than we can our own that half of her which is toward the fun is enlightened, moon at the time of change. When Venus is at her the other half muft be dark and invifible. Hence, fhe greateft elongation, we have but cne half of the enlighten- difappears when ihe comes between us and the fun; beed fide of her full moon towards us; and even then it caufe her dark fide is then towards us. When fhe is gone a little way forward, we fee a little of her enlightmay be too far diftant to be feen by us. The Earth is the -next planet above Venus in the ened fide; which ftill encreafes to our view, as fhe adfyfteto. It is 82 millions of milts from the fun, and vances forward, until fire comes to be oppofite the fun; goes round him (as in the circle in 365 days 5 hours and then her whole enlightened fide is towards the earth, 49 minutes, from any equinox or folftice to the fame and fhe appears with a round, illumined orb, which wc again; but from any fixed ftar to the fame again, as call the yh// moon; her dark fide being then turned afeen from the fun, in 365 days 6 hours and 9 minutes; way from the earth. From the full fhe feems to decreafe tbe. former being the length of the tropical year, and the gradually as fhe goes through the other half of her latter the length of the fyderial. It travels the rate of courfe; (hewing us lefs- and lefs of her enlightened fide 58 thoufand miles every hour; which motion, though every day, till her next change or conjundhon with the 420 times fwifter than that of a cannon-ball, is little fun, and then fhe difappears as before. more than half as fwift as Mercury’s motion in his or- This continual change of the moon’s phafes demonbit. The earth’s diameter is 79,70 miles; and by turn- ftrates that fhe fliines not by any light of her own; for ing round its axis every 24 hours from weft to eaft, it if (he did, being globular, we fhould always fee her with Ciufes an apparent diurnal motion of all the heavenly bo- a round full orb like the fun. Her orbit is reprefented dies from eaft to weft. By this rapid motion of the in the fcheme by the little circle w, upon the earth’s orearth on its axis, the inhabitants about the equator are bit ©j Plate XXXIX. fig. t.; but it is drawn fifty times, carried 1042 miles every hour, whilft thofe on the pa- too large in proportion to the earth’s; and yet is alrallel of London are carried only about 580, befides the moft too fmall to be feen in the diagram. 58 thoufand miles by the annual motion above mention- The moon has fcarce any difference of feafons; her ed, which is common to-all places whatever. axis being almoft perpendicular to the ecliptic. What The earth’s axis makes an angle of 234 degrees with is very lingular, one half of her has no darknefs at all; the axis of its orbit, and keeps always the fame oblique the earth conftantly affording it a ftrong light in die fun’s diredtion, inclining towards the fame fixed ftars through- abfence; while the other half has a fortnight’s darknefs, out its annual courfe, which caufes the returns of fpring, light by turns. furamer, autumn, and winter; as will be explained af- andOura fortnight’s earth is a moon to the moon, waxing and waning terwards. ., regularly, but appearing thirteen times as big, and afThe earth is round like a globe; as appears, 1. By fording her thirteen times as much light as fhe does to its (hadow in eclipfes of the moon, which fhadow is al- us. When (lie changes to us, the earth appears full to ways bounded by a circular ring. 2. By our feeing the her; and when fhe is in her firft quarter to us, the earth mafts of a fhip whiift the hull is hid by the convexity of is in its third quarter to her; .and vice verfa. the water. 3. By its having been failed round by many But from one half of the moon, the earth is never navigators. The hills take off no more from the round- feen at all; from the middle of the other half, it is alnefs of the earth in comparifon, than grains of duft do ways feen over head; turning round almoft thirty times from the roundnefs of a common globe. as quick as the moon does. From the circle which liThe feas and unknown parts of the earth (by a mea- mits our view of the moon, only one half of the earth’s fide