« ForrigeFortsett »
DIFFERENCE OF TIME.
The hour of the day at any particular place being given, to find the hour in any other part of the world, and where it any other proposed hour.
Learn the Trigonometrical Canon given in K, on p. 16, and on the page succeeding.
RULE 1.-Bring the place, the time of which is given, to the brass meridian, and set the index of the hour circle to the time; then bring the other place to the brass meridian, and the index will point out the time required.
2. If it be required to find where it is any other proposed time; by turning the Globe until the index points to that proposed time, the places having that time will be found coinciding with the brass meridian; but due regard must be paid to the note appended to Rule of Prob. XII.
St. Helena..6h. 40m. A.M.
1 A. M... Where Noon.
22 P. M... Where Noon and where
8h. 20m. A. M.
Noon... Where 6 A. M.
Where 1 P. M.
Midnt... Where 5 A. M. and where 21 P. M.
EQUINOCTIAL AND ECLIPTIC.
B The pleasantness of the pupil's progress will much depend upon his just comprehension of the Equinoctial and Ecliptic. We hope he has already obtained correct ideas of a Plane by means of this portion of his knowledge the ecliptic and equinoctial may easily be understood by him.
We set out (page 3) by illustrating the point called the centre, within the needle-hole; and explained that the cir cumference of this minute circle, and the surface of the paper in which it had been pierced, the surface of the table on which the paper was laid, and the surfaces of the distant tables in any rooms, on the same floor, which were exactly of the same height, were in the same plane with this circle and its invisible centre.
C Now, the earth, the surface of which we, and one thousand millions of our species, inhabit, is but a speck compared with the space in which it is situated; yet it has a centre, and we can very readily understand, that this centre, being the middle of its axis, is in the same plane with the equator; (see def. 13,) and that, as the earth is surrounded by stars, being situated in the midst of myriads of them, there must be a circle of these stars, or of points of space amongst them, exactly opposite to the several points all around this equator; that is, there is a circular line amongst the stars in the same plane with our equator and its centre:—that line is the equinoctial. We see it is only an imaginary line, drawn over the several points in the sky which are exactly opposite our equator. A man living at the equator, as the earth turns on its axis every 23 hours, 56 minutes, 4 seconds, will therefore direct his head to every part of this equinoctial line.
D But, whilst the earth is turning thus on its axis 366 times, it is wheeling round the sun, its primary, in a nearly circular path; which, although vast compared with the earth's own dimensions, is proved to be insignificantly little when contrasted with the distance of the nearest fixed star : (G on page 4). So that, we may say any one part of this
EQUINOCTIAL AND ECLIPTIC.
circular path, or orbit, of the earth, is as distant from any fixed stars as other part of it, both being at an incalculable distance from them. Still, as with regard to the equator and the marking out of its plane in the heavens, called the equinoctial, so with regard to this circular path or orbit of the earth around the sun; there are stars, and points of space amongst them, in the same plane with this circular path; and these, being traced out amongst the stars, form the line called the ecliptic, which, like the equinoctial, is marked out on our celestial globe.
F The sun, around which we are thus circling, must, of course, be in the same plane with every point of this our path; and looking across the circle towards him, we should, were it not for his brilliant light, day after day perceive the stars so in the plane of our orbit, appearing, in consequence of our own motion in this orbit, to pass behind him in the opposite way to that in which we are going; in other words, forgetting the fact of our own movement, it would appear as if he were passing over them in the same way. This is the sun's motion in the ecliptic; not a real motion, but a seeming one, caused by our daily progress around him. If we were inhabitants of the sun to day, and had eyes, as now, capable of viewing so small a speck as the earth at so great a distance, we should see our planetary habitation also in the ecliptic line, but in the opposite portion of it; that is, as a bright speck appearing amongst those zodiacal stars which will be on our meridian this midnight, and making real progress in the same order as this apparent progress of the sun.
OBLIQUITY OF THE ECLIPTIC-EQUINOCTIAL POINTS-RIGHT ASCENSION-DECLINATION.
We shall still have recourse to very homely expedients, in the use of a round tea-table, and the earliest home-made toy of our infancy,- -a teetotum formed of a mould button and a wooden peg. Only, since our purpose is now a philosophical one, we will take care that our wooden peg (P) be exactly equal to the diameter of our horn button (B); and that the button be so placed on the peg as to be at equal distances from its two ends.
The peg, so placed, will now be the earth's axis.
The round flat button will be a fit representation of the plane of the equator. (The young reader will allow, that if the space above and below this equatorial plane were filled with cork, so as to make up the form of a globe, it would spin just as well; but we should not then see the position of the plane of the equator.)
The surface of our table is the plane of the ecliptic; and,
OBLIQUITY OF THE ECLIPTIC.
if we suppose a groove or track to be made near its edge, it will represent the earth's orbit.
The sun will be represented by the ball we have placed in the middle of the table.
Now, let our teetotum be spun towards the left, and turn round 366 times whilst it traverses the whole groove, passing A, in front of the pupil, towards B. If the peg or axis be perpendicular to the plane of its orbit, that is, if it spin in an upright position, as E in our figure above, the two planes, the button surface and the surface of the table, (the planes of the equator and of the orbit), will be parallel; and the portions of the distant wall, (supposed to be that of a large circular room), which we have marked as, severally, in the same planes with the button and with the table, will represent the equinoctial and the ecliptic; and, as they are so nearly together, being apart only to the extent of half the length of the peg, on a distant wall they would appear as one line.
H We remark then :-If the earth were to revolve in its orbit, with its axis, or line about which it spins, perpendicular to the plane of that orbit, the equinoctial and ecliptic would be one and the same line on our celestial globe.
But it is not so: the earth's axis is inclined towards the plane of its orbit, 23° 28'; and therefore, the plane of the equator is so inclined and intersects it.
If our teetotum could be made to spin, leaning exactly to this angle, of 23° 28', it would more fitly represent the earth whilst moving around the sun.