308

ASTRONOMY.

past the meridian, takes 12 hours longer to come to London bridge,so that when it is high-water there, a new tide is already come to its1-,eight in the ocean, and in some intermediate place, it must below water at the same time. At several places, it Is high-wa erthree hours before the Moon comes to her meridian; but thattide, which the Moon pushes as it were before her, is only thetide opposite to that which was raised by her, when she was ninehours p..$t the opposite meridian. There are no sensible tides inthe Baltic, the Mediterranean, or the Black Seas ; for they com-municate with the ocean, by such narrow inlets, and are of so im-mense an extent, that they cannot speedily receive and empty wa-ter enough to raise or depress their surfaces sensibly. In the Cas-,pian sea, and in lakes, &c. the Moon ’s attraction is nearly thesame upon all parts of their surface, so that no sensible swellingcan take place in their waters.

(162.) We may al-.o conclude, that by reason of tbe fluidity ofthe atmosphere, it must have tides similar to those of the ocean ;and hence, there will be a general current from east to west,both of the waters ot the ocean, and of the air ; but the changesproduced in the state of the atmosphere, from chemical causes, willso much affect the general current, as to prevent it from beingperceived.

Sect. XI. Of the Harvest Moon*.

(163.) Tt is remarkable, that the Moon , during the week inwhich she is full about the time of harvest, rises soon alter sun-vetting, for several evenings together, than she does in any otherTull Moon week throughout the yean By this means, she affordsan immediate supply of light after sun-set, which is very beneficialfor those employed in the liarvest, and gathering in the fruits ofthe Earth. Hence this Full Moon is distinguished from all othersin the year, by calling it the Harvest Moon. To conceive thereason of this phenomenon, it may first bz considered, that theMoon is always opposite to the Suit, when she is full, and there-fore, in the harvest-months, she is full in Pisces and Aries, whichare opposite to Virgo and Libra, the signs occupied by the Sun about the same season. Now, the signs Pisces and Aries in anoblique sphere where the north pole is above the horizon rise in ashorter space of time than the others, as is easily shewn and illustratedby a celestial globe ; and the same thing may be conceived fromthis circumstance, that in northern latitudes, the smallest angle,made by the ecliptic and horizon, is when Aries rises, at whichtime Libra sets ; and it is obvious, that the smaller the angle con-tained by the ecliptic and horizon, the greater portion of the eclip-tic will rise by the Earth ’s rotation in a given time. Consequently,when the Moon is full in harvest, she rises with less difference oftime, or more immediately after suv-set, than at any other seasonof the year. In our winter, the Moon is in Pisce3 and Aries,-about the time of her first quarter, when she rises about noon,and therefore, her rising is not then noticed. In spring, the Moon is in Pisces and Aries about the time of her change, but as shethen gives no light, and rises with the Sun , her rising cannot beperceived. In summer, the Moon is in Pisces and Aries at thetime of the last quarter, and then as she does not rise till midnight,her rising usually passes unobserved. But in autumn, the Moon is in Pisces and Aries, at the time of her full, and rises soon aftersun-set, for several evenings successively ; which makes her re-gular risings very conspicuous, at that time of the year. All thiswould happen, even if the Moon ’s orbit lay in the ecliptic ; buther orbit makes with the ecliptic an angle of about 3° 18', andcrosses it in two points, called her nodes ; so that her rising whenin P.sces and Aries, will sometimes not differ above an hour and40 m nutes, through a whole week ; and at other times, in thesame.two signs she will differ in a week 3i hours, in the time oftier rising, according to the different positions of the nodes, withrespect to these signs; which positions are always changing, be-cause the nodes go backward through the ecliptic, in 18 years,225 day -. This revolution of the nodes causes the harvest-moonsto go through a whole course of the most advantageous and leastbeneficial states, with respect to the harvest, every 19 years. Theyv, ere least beneficial in 1796, and 1797; after which they were mostbeneficial for a period of 9} years, that is till 1806, then for a likeperiod they become again least beneficial, and so on.

Sect. XII. Of the Horizontal Sun and Moon.

(184.) Philosophers have been much at a loss to account for theapparent magnitudes of the §«n and Moot), being greater when

they are in the horizon, than when elevated above it. For ac-cording to the laws of vision, they should appear least, whennearest the horizon, because they are then farthest from the eye;and yet, it is found, llihj: the contrary is true, in fact. Thus, al-though the diameter of the Moon , when in the horizon, as mea-sured by an instrument, is not found to be greater than when mea-sured at her greatest elevation in the meridian, yet her apparentdiameter, when in the horizon, seems to the eye, 2 or 3 limesgreater than when she is considerab’y elevated above it. Accord-ing to Alhazen , one of the earliest writers on optics, the sight ap-prehends the surface of the heavens as fiat, and judges of the stars,as it would of ordinary objects extended upon a wide plane. The.eye sees them indeed under equal angles, but at the same timeperceives a difference in their distances, and (on account of theseinidiumeter of the Earth , which is interposed in the one case, butnot in the other) it is hence induced to judge those that appearmore remote to be greater. Des Carles, and from him Dr. Wal-lis, and most other authors, account for the appearance of a dif-ferent distance under the same angle, from the long series of objectsinterposed between the eye and the .extremity of the horizon,whic h makes us imagine it is more remote than when in the me-ridian, where the eye sees nothing in the way between itself andthe object. This idea of a great distance mates uij imagine the lu-minary larger; for an object being seen under any certain angle,and believed at the same time more remote, we naturally imagineit to be very large, to appear under such an angle at such a dis-tance, and thus a pure judgment of the mind makes us seethe Sun or the Moon larger in the horizon than in the meridian, notwith-standing their diameters, when measured, are really less in theformer situation than in the latter. 'Phis opinion, however, seemshardly tenable, although it be sanctioned by the authority of veryeminent men; for it is daily seen, that the Sun and Moon , whennear the horizon, very suddenly change their magnitude as theyascend and descend,, though all the intervening objects remainthe same as before; and the luminaries appear largest of all, whenfewest objects appear on the Earth , as in a thick fog or mist. Dr.Desaguliers lias illustrated the doctrine of the Horizontal Moon,upon the supposition that we imagine the visible heavens to beonly a small portion of a spherical surface, and consequently sup-pose the Moon to be farthe r from us in the horizon than near thezenith; and lie has demonstrated how liable w e are to such decep-tions. Upon this idea, Dr. Smith has determined, in his optics,that the centre of the apparent spherical segment of sky, lyingmuch below the eye, the apparent distance of its parts, near thehorizon, was about 3 or 4 times greater than the apparent distanceof its parts overhead; for which reason it is, he infers, thatlheMoon always appears larger as she is lower, and also that wealways think the height of a celestial object to be greater than itreally is.

Sect. XIII. Of the Phenomena of the Superior Planets.

(165.) Mars is the first of the superior planets in order from theSun ; he is of a red fiery colour, and always gives a much dullerlight than Venus , though he.sometimes equals her in his apparentmagnitude, lie sometimes appears'near the Sun , and rises withhim ; at other times, he appears at a great distance from the Sun ,and ever! opposite to him, and then rises when the Sun sets. Thisplanet, as well as the rest, borrows its light from the Sun , and hasits increase and decrease of light like the Moon ; it may be seenalmost bisected when in its quadratures w ith the Sun ; but is never■ corniculated or lalcated, as the inferior planets. Dr. Hook, inlG6o, observed several spots in this planet, which having a mo-tion, he concluded the planet to have a turbinated motion roundits centre. In 1666, M. Cassini observed several spots in the twohemispheres of Mars , which, by continuing his different observa-tions very diligently, lie found to move by little and little from eastto west, and to return in the space of 24 hours 40in. to I heir formersituation. Whence both the motion and period, or natural day ofthat planet, were determined. Mara always appears with a ruddytroubled light, whence we conclude that it is encompassed with athick cloudy atmosphere, which by disturbing the rays of light iutheir passage and re-qiassage through it, occasion that appearance :also when any of the fixed stars are seen near his body, they ap-pear somewhat obscured. A spectator in Mars will scarcely eversee Mercury, unless perhaps in the Sun at the time of conjunction,when Mercury passes over hi? disk, as liu sometimes appears to

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