Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/583

 ASTRO and that part of the ecliptic FG, in which the moon is then new, touches the tropic of Cancer T" at Z>. The north pole N zx. that time^inclining 23! degrees towards the fun, falls fo many degrees within the earth’s enlightened dilk, becaufe the fun is then vertical to D, 23-r degrees north offeem the equator the equator with all its parallels elliptic curves and bending downward, or towards the fouth pole, as feen from the fun ; which pole, together with 234 degrees ail round it, is hid behind the dilk in the dark hemifphere of the earth. At the autumnal equinox, the earth’s axis has the polition NOS, lying to the left hand as feen from the fun or new moon, which are then vertical to 0, where the ecliptic cuts the equator Both poles now lie in the circumference of the dilk, the north pole juft going to difappear behind it, and the fouth pole juft entering into it ; and the equator, with all its parallels, feem to be ftraight lines, becaufe their planes pafs through the obferver’s eye, as feen from the fun, and very nearly fo .as feen from the moon. At the winter iolftice, the earth’s axis has the pofition NNS; when its fouth polee S inclining 234 degrees toward the fun, falls 234 d ' grees within the enlightened dilk, as feen from the fun or new moon, which are then vertical to the tropic of Capricorn t, 234 degrees fouth of the equator and the equator, with all its parallels, feem elliptic curves bending upward ; the north pole being as far hid behind the dike in the dark heipifphere, as tfie fouth pole is come into the light. T he nearer that any time of the year is to the equinoxes or folftices, the more it partakes of the phenomena relating to them. Thus it appears, that from the vernal equinox to the autumnal, the north pole is enlightened ; and the equa-i tor, and all its parallels, appear elliptical as feen from the fun, more or lefs curved as the time is nearer to, or farther from, the fummer folftice ; and bending downwards, or towards the fouth pole ; the reverfe of which happens from the autumnal equinox to the vernal. A little confideration will be fuffreient to convince the reader, that the earth’s axis inclines towards the fun at the fummer folftice ;. from the fun at the winter folftice ; and lidewife to the fun at the equinoxes ; but towards the right hand, as feen from the fun at the vernal equinox ; and towards the left hand at the autumnal. From the winter to the fummer folftice, the earth’s axis inclines more or lefs to the right hand, as feen from the fun; and the contrary from the fummer to the winter; folftice. The different pofitions of the earth’s axis,- as feew from the fun at different times of the year, affedi folar eclipfes greatly ^ith regard to particular places ; yea, fo far as would make central eclipfes which fall at one time of the year invifible if they fell at another, even thong}* the moon fhould always change in the nodes, and at the fame hour of the day; of which indefinitely various affedtions, we fhall only give examples- for the times of the equinoxes and folftices. In the fame diagram, (Plate XLVJI. fig. 2.), let FG be part of the ecliptic, and IK, ik, ik, ik, part of the moon’s orbit; both feen edgewife, and therefore projedted into right lines j and let the interfedlions NODE.

N O M Y. be one and the fame node at the above times, when the; earth has the forementioned different pofitions ; and let the fpaces included by the circles Pppp be the penumbra at thefe times, as its centre is paffing over the centre of the earth’s difk. At the winter folftice, when the earthV axis has the pofition NNS, the centre of the penumbra P touches the tropic of Capricorn / in Ad at the middle of the general eclipfe ; but no part of the penumbra touches the tropic of Cancer T. At the fummer folftice, when the earth’s axis has the pofition NDS (iDk being then part of the moon’s orbit, whofe node- is at D) the penumbra p has its centre at D, on the tropic of Cancer T, at the middle of the general eclipfe, and then no part of it touches the tropic of Capricorn t. At the autumnal equinox, the earth’s axis has the pofition NOS, [iOk being then part of the moon’s orbit),, and the penumbra equally includes part of both tropics T and t at the middle of the general eclipfe : at the verml equinox it does the fame, becaufe the earth’s axis has the pofition NFS ; but, in the former of thefe two laft cafes, the penumbra enters the earth at A, north of the tropic of Cancer T, and leaves it at m, fouth of the tropic of Capricorn t; having gone over the earth obliquely fouthward, as its centre deferibed the line AOm : whereas, in the latter cafe, the penumbra touches the earth at/?, fouch of the .equator and deferibing the line-;w.Ej', (fimvlar to the former line AOm in open, fpace), goes obliquely northward over, the earth, and leaves it at q, north of the equator. In all thefe circumftances, the moon has been fuppofed to change at noon in her defeending node: Had Ills' changed in her afeending node, the phenomena would have been as various the contrary way, with refpedt to; the penumbra’s going northward or fouthward over the earth.. But becaufe the moon changes at all hours, asoften in. one node as in the other, and at all. diftances from them both at different times as it happens, the variety of the phafes of eclipfes are almoft innumerable, evenatthe fame places; confidering alfo how varioufly the fame places are.fituated on the enlightened difk of the earth, with refpedt to the penumbra’s motion, at the different hours when eclipfes -happen. When the moon changes 17 degrees fhort of her defcending..node, .the penumbra P 18 juft touches the northern part of the earths difk, near the north pole W; and, as feen from that place, the moon appears to touch the fun, but hides no, part of him from fight. Had the change been as far fhort of the afeending node, the penumbra would have touched the fouthern part of the dike near the fouth pole 5. When the moon changes 12 degrees fhort of the defeending node, more than a third part of the penumbra.Pi2 falls on the. northern parts of the. earth at the middle of the general eclipfe: Had ihe changed as far paft the fame node, as much of the other fide of the penumbra .about P would have fallen on the. fouthern part of the earth ; all the reft in the expanfum, or open fpace. When the moon.,changes 6 degrees from the node, almoft the whole penumbra P6 falls on the earth at the.middle of the general eclipfe. And laftly, whenthe moon changes in the node at N, the penumbra PN takes the longed courfe poffible on the earth’s dilk ; its centre