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motion round the
and consequently the circle described in the sphere the same
points of of the stars by the axis of the Earth, produced, if the heaviewed from the earth, must appear but as a point ; vens, not
withstan. that is, its diameter appears too little to be measur
ding the ed by observation: for Dr. BRADLEY has assured Earth's us, that if it had amounted to a single second, or two at most, he should have perceived it in the number Sun. of observations he has made, especially upon op Dra. conis; and that it seemed to him very probable that the annual parallax of this star is not so great as a single second: and consequently, that it is above 400 thousand times farther from us than the Sun. Hence the celestial poles seem to continue in the same points of the heavens throughout the year; which by no means disproves the Earth's annual motion, but plainly proves the distance of the stars to be exceeding great.
197. The small apparent motion of the stars, $ 113, discovered by that great astronomer, he found to be no ways owing to their annual parallax, (for it came out contrary thereto,) but to the aberration of their light, which can result from no known cause, besides that of the Earth's annual motion; and as it agrees so exactly therewith, it proves beyond dispute, that the Earth has such a motion; for this aberration completes all its various phenomena every year; and The amaproves that the velocity of star-light is such as car-zing velories it through a space equal to the Sun's distance light. from us in 8 minutes 13 seconds of time. Hence the velocity of light is * 10 thousand 210 times as great as the Earth's velocity in its orbit; which velocity, (from what we know already of the Earth's distance from the Sun) may be asserted to be at least between 57 and 58 thousand miles every hour : and supposing it to be 58000, this number multiplied by 10210, gives 592 million 180 thousand miles for the hourly motion of light : which last number divided by 3600, the number of seconds in an hour,
* SMITH's Optic's $ 1197.
Plate IV. shews that light flies at the rate of more than 164
thousand miles every second of time, or swing of a common clock pendulum.
The Circles of the Globe described. The different
Lengths of Days and Nights, and the Vicissitudes
F the reader be hitherto unacquainted with Circles of 198.
the principal circles of the globe, he should sphere. now learn to know them; which he may do suffi
ciently for his present purpose in a quarter of an
hour, if he sets the ball of a terrestrial globe before Fig. 11. him, or looks at the figure of it, wherein these cir.
cles are drawn and named. The equator is that Equator, tropics,
great circle which divides the northern half of the polar cir. Earth from the southern. The tropics are lesser cles, and poles.
circles parallel to the equator; each of them being 23) degrees from it; a degree in this sense being the 360th part of any great circle; or that which divides the Earth into two equal parts. The tropic of Cancer lies on the north side of the equator, and the
tropic of Capricorn on the south. The Arctic cir- . Fig. II. cle' has the North pole for its centre, and is just as
far from it as the tropics are from the equator ; and the Antarctic circle, (hid by the supposed convexity of the figure) is just as far from the south pole every way round it. These poles are the very north and south points of the globe: and all other places are denominated northward or southward, according
to the side of the equator they lie on, and the pole to Earth's which they are nearest. The Earth's axis is a straight
line passing through the centre of the Earth, perpendicular to the equator, and terminating in the poles at its surface. This, in the real Earth and planets, is only an imaginary line ; but in artificial globes or planets it is a wire by which they are supported, and
turned round in Orreries, or such like machines, by Plate IV. wheel-work. The circles 12. 1. 2. 3. 4. &c. are meridians to all places they pass through; and we Meridimust suppose thousands more to be drawn, because ans. every place, that is ever so little to the east or west of any other place, has a different meridian from that other place. All the meridians meet in the poles; and whenever the Sun's centre is passing over any meridian in his apparent motion round the Earth, it is mid-day or noon to all places on that meridian.
199. The broad space lying between the tropics, like a girdle surrounding the globe, is called the torrid zone, of which the equator is in the middle all Zones. round. The space between the tropic of Cancer and Arctic circle, is called the north temperate zone; that between the tropic of Capricorn and the An. tarctic circle, the south temperate zone; and the two circular spaces bounded by the polar circles, are the two frigid zones; denominated north or south, from that pole which is in the centre of the one or the other of them.
200. Having acquired this easy branch of knowledge, the learner may proceed to make the following experiment with his terrestrial ball; which will give him a plain idea of the diurnal and annual motions of the Earth, together with the different lengths of days, nights, and all the beautiful variety of seasons, depending on those motions.
Take about seven feet of strong wire, and bend Fig. III. it into a circular form, as abcd, which being viewed Apleas
ing expeobliquely, appears elliptical, as in the figure. Place riment a lighted candle on a table, and having fixed one end
the differ. of a silk thread K, to the north pole of a small terres- ent trial globe H, about three inches diameter, cause lengths of another person to hold the wire-circle, so that it may nights
, be parallel to the table, and as high as the flame of and the the candle I, which should be in or near the variety
centre. Then, having twisted the thread as toward the left hand, that by untwisting it may turn the globe round eastward, or contrary to the way that the hands of a watch move, hang the globe by the thread within this circle, almost contiguous to it; and as the thread untwists, the globe (which is enlightened half round by the candle, as the Earth is by the Sun) will turn round its axis, and the different places upon it will be car. ried through the light and dark hemispheres, and have the appearance of a regular succession of days and nights, as our Earth has in reality by such a motion. As the globe turns, move your hand slowly, so as to carry the globe round the candle according to the order of the letters abcd, keeping its centre even with the wire-circle; and you will perceive, that the candle, being still perpendicular to the equator, will enlighten the globe from pole to pole in its whole motion round the circle; and that every place on the globe goes equally through the light and the dark, as it turns round by the untwisting of the thread, and therefore has a perpetual equinox. The globe thus turning round represents the Earth turning round its axis; and the motion of the globe round the candle represents the Earth's annual motion round the Sun, and shews, that if the Earth's orbit had no inclination to its axis, all the days and nights of the year would be equally long, and there would be no different seasons. But now, desire the person who holds the wire to hold it obliquely in the position ABCD, raising the side I just as much as he depresses the side vs, that the fame may be still in the plane of the circle; and twisting the thread as before, that the globe may turn round its axis the same way as you carry it round the candle, that is, from west to east, let the globe down into the lowermost part of the wire circle at 18, and if the circle be properly inclined, the candle will shine perpendicular
ularly on the tropic of Cancer, and the frigid zone, Summer lying within the Arctic or north polar circle, will be solstice. all in the light, as in the figure; and will keep in the light, let the globe turn round its axis ever so often. From the equator to the north polar circle all the places have longer days and shorter nights; but from the equator to the south polar circle just the reverse. The Sun does not set to any part of the north frigid zone, as shewn by the candle's shining on it, so that the motion of the globe can carry no place of that zone into the dark : and at the same time the south frigid zone is involved in darkness, and the turning of the globe brings none of its places into the light. If the Earth were to continue in the like part of its orbit, the Sun would never set to the inhabitants of the north frigid zone, nor rise to those of the south. At the equator it would be always equal day and night; and as places are gradually more and more distant from the equator, toward the Arctic circle, they would have longer days and shorter nights; while those on the south side of the equator would have their nights longer than their days. In this case there would be continual summer on the north side of the equator, and continual winter on the south side of it.
But as the globe turns round its axis, move your hand slowly forward, so as to carry the globe from H toward E, and the boundary of light and darkness will approach toward the north pole, and recede from the south pok; the northern places will go through less and less of the light, and the southern places through more and more of it; shewing how the northern days decrease in length, and the southern days increase, while the globe proceeds from H to Ē. When the globe is at E, it is at a mean state between the lowest and highest parts of autumnal its orbit; the candle is directly over the equator, the equinox. boundary of light and darkness just reaches to both