properly so called?
18. (Pg. 93) How do meridian lines extend, and what is meant by the
meridian of a place?
19. (Pg. 93) What is said of the meridian to which the sun is opposite,
and where is it then midnight?
20. (Pg. 94) What hour is it then, at places exactly half way between
these meridians?
21. (Pg. 94) How are greater and lesser circles distinguished?
22. (Pg. 94) What part of a circle is a degree, and how are these
further divided?
23. (Pg. 94) What is the diameter, and what the circumference of a
circle, and what proportion do they bear to each other?
24. (Pg. 94) What part of a circle is a meridian?
25. (Pg. 94) How many degrees are there between the equator and the
poles?
26. (Pg. 94) Into what parts, besides degrees, is the ecliptic divided?
27. (Pg. 94) How are degrees of latitude measured, and to what number do
they extend?
28. (Pg. 94) On what circles are degrees of longitude measured, and to
what number do they extend?
29. (Pg. 94) What is a parallel of latitude?
30. (Pg. 95) Degrees of longitude vary in length; what is the cause of
this?
31. (Pg. 95) What is the length of a degree of latitude, and why do not
these vary?
32. (Pg. 95) What causes the equator to be somewhat larger than a great
circle passing through the poles, and what effect has this on degrees of
longitude measured on the equator?
33. (Pg. 95) What is the cause of this form being given to the earth?
34. (Pg. 96) What would have been a consequence of the centrifugal
force, had the earth been a perfect sphere?
35. (Pg. 96) A body situated at the poles, is attracted more forcibly
than if placed at the equator, what is the reason?
36. (Pg. 97) What effect would be produced upon the gravity of a body,
were it placed beneath the surface of the earth, and what supposing it
at its centre?
37. (Pg. 97) What two circumstances combine, to lessen the weight of a
body on the equator?
38. (Pg. 97) Why could not this be proved by weighing a body at the
poles, and at the equator?
39. (Pg. 98) What is a pendulum?
40. (Pg. 98) What causes it to vibrate?
41. (Pg. 98) Why are not its vibrations perpetual?
42. (Pg. 98) Two pendulums of the same length, will not, in different
latitudes, perform their vibrations in equal times, what is the cause of
this?
43. (Pg. 98) To what use has this property of the pendulum been applied?
44. (Pg. 99) What change must be made in pendulums situated at the
equator and at the poles, to render their vibrations equal?
45. (Pg. 99) What do the vibrations of a pendulum resemble, and why will
it vibrate more rapidly if shortened?
46. (Pg. 99) In the revolution of the earth round the sun, what is the
position of its axis?
47. (Pg. 99) How much is the axis of the earth inclined, and with what
line does it form this angle?
48. (Pg. 99) What is represented by fig. 2, plate 9?
49. (Pg. 100) How is the north pole inclined in the middle of our
summer, and what effect has this on the north frigid zone?
50. (Pg. 100) In what direction does the north pole always point?
51. (Pg. 100) What is shown by the position of the earth at B, in the
figure?
52. (Pg. 100) How does the sun then shine at the poles, and what is the
effect on the days and nights?
53. (Pg. 101) When the earth has passed the autumnal equinox, what
changes take place at the poles, and also in the whole northern and
southern hemispheres?
54. (Pg. 101) Why is the heat greatest within the torrid zone?
55. (Pg. 101) How does the sun appear at the poles, during the period of
day there?
56. (Pg. 101) In what will the days and nights differ in the temperate
zone, from those at the poles, and at the equator?
57. (Pg. 102) Trace the earth from the winter solstice to the vernal
equinox, and inform me what changes take place.
58. (Pg. 102) What takes place at the time of the vernal equinox, and
what is meant by the term?
59. (Pg. 102) In proceeding from the vernal equinox to the summer
solstice, what changes take place?
60. (Pg. 103) From what cause arises the superior heat of the equatorial
regions?
61. (Pg. 103) Why should oblique rays afford less heat than those which
are perpendicular?
62. (Pg. 103) How is this explained by fig. 1. plate 10?
63. (Pg. 103) How do you account for the superior heat of summer, and
how is this exemplified in fig. 2 and 3, plate 10?
64. (Pg. 104) What other cause lessens the intensity of oblique rays?
65. (Pg. 104) How is this explained by fig. 4?
66. (Pg. 104) What causes conspire to lessen the solar heat in the
morning and evening?
67. (Pg. 104) The greatest heat of summer is after the solstice, and the
greatest heat of the day, after 12 o'clock, although the sun's rays are
then most direct, how is this accounted for?
68. (Pg. 105) Is there any change of seasons in the other planets?
69. (Pg. 105) What is said respecting the axes of Jupiter, of Mars, and
of Saturn?
70. (Pg. 105) In 365 days, how many times does the earth revolve on its
axis?
71. (Pg. 105) How is this accounted for?
72. (Pg. 105) Do the fixed stars require the same time as the sun, to
return to the same meridian?
73. (Pg. 106) How is this accounted for?
74. (Pg. 106) What is meant by the solar and the sidereal day?
75. (Pg. 106) What is the difference in time between them?
76. (Pg. 106) What is the length of the tropical year?
77. (Pg. 107) The solar year is completed before the earth has made a
complete revolution in its orbit, by what is this caused?
78. (Pg. 107) What is this called, and what is represented respecting it
by fig. 1, plate 11?
79. (Pg. 107) By what means can we ascertain the period of a complete
revolution of the earth in its orbit, as illustrated by the fixed star
E, in fig. 1?
80. (Pg. 107) What difference is there in the length of the solar and
sidereal year?
81. (Pg. 107) Why can we not always ascertain the true time by the
apparent place of the sun?
82. (Pg. 108) What would be the greatest difference between solar, and
true time, as indicated by a perfect clock?
CONVERSATION IX.
ON THE MOON.
OF THE MOON'S MOTION. PHASES OF THE MOON. ECLIPSES OF THE MOON. ECLIPSES
OF JUPITER'S MOONS. OF LATITUDE AND LONGITUDE. OF THE TRANSITS OF THE
INFERIOR PLANETS. OF THE TIDES.
MRS. B.
We shall, to-day, confine our attention to the moon, which offers many
interesting phenomena.
The moon revolves round the earth in the space of about twenty-nine days
and a half; in an orbit, the plane of which is inclined upwards of five
degrees to that of the earth; she accompanies us in our revolution round
the sun.
_Emily._ Her motion then must be of a complicated nature; for as the
earth is not stationary, but advances in her orbit, whilst the moon goes
round her, the moon, in passing round the sun, must proceed in a sort of
scolloped circle.
_Mrs. B._ That is true; and there are also other circumstances which
interfere with the simplicity, and regularity of the moon's motion, but
which are too intricate for you to understand at present.
The moon always presents the same face to us, by which it is evident
that she turns but once upon her axis, while she performs a revolution
round the earth; so that the inhabitants of the moon have but one day,
and one night, in the course of a lunar month.
_Caroline._ We afford them, however, the advantage of a magnificent moon
to enlighten their long nights.
_Mrs. B._ That advantage is put partial; for since we always see the
same hemisphere of the moon, the inhabitants of that hemisphere alone,
can perceive us.
_Caroline._ One half of the moon then enjoys our light, while the other
half has constantly nights of darkness. If there are any astronomers in
those regions, they would doubtless be tempted to visit the other
hemisphere, in order to behold so grand a luminary as we must appear to
them. But, pray, do they see the earth under all the changes, which the
moon exhibits to us?
_Mrs. B._ Exactly so. These changes are called the phases of the moon,
and require some explanation. In fig. 2, plate 11, let us say, that S
represents the sun, E the earth, and A B C D E F G H, the moon, in
different parts of her orbit. When the moon is at A, her dark side being
turned towards the earth, we shall not see her as at _a_; but her
disappearance is of very short duration, and as she advances in her
orbit, we perceive her under the form of a new moon: when she has gone
through one eighth of her orbit at B, one quarter of her enlightened
hemisphere will be turned towards the earth, and she will then appear
horned as at _b_; when she has performed one quarter of her orbit, she
shows us one half of her enlightened side, as at _c_, and this is called
her first quarter; at _d_ she is said to be gibbous, and at _e_ the
whole of the enlightened side appears to us, and the moon is at full. As
she proceeds in her orbit, she becomes again gibbous, and her
enlightened hemisphere turns gradually away from us, until she arrives
at G, which is her third quarter; proceeding thence she completes her
orbit and disappears, and then again resumes her form of a new moon, and
passes successively, through the same changes.
When the moon is new, she is said to be in conjunction with the sun, as
they are then both in the same direction from the earth; at the time of
full moon, she is said to be in opposition, because she and the sun, are
at opposite sides of the earth; at the time of her first and third
quarters, she is said to be in her quadratures, because she is then
one-fourth of a circle, or 90 deg., from her conjunction, or the period
of new moon.
_Emily._ Are not the eclipses of the sun produced by the moon passing
between the sun and the earth?
_Mrs. B._ Yes; when the moon passes between the sun and the earth, she
intercepts his rays, or, in other words, casts a shadow on the earth,
then the sun is eclipsed, and daylight gives place to darkness, while
the moon's shadow is passing over us.
When, on the contrary, the earth is between the sun and the moon, it is
we who intercept the sun's rays, and cast a shadow on the moon; she is
then said to be eclipsed, and disappears from our view.
_Emily._ But as the moon goes round the earth every month, she must be,
once during that time, between the earth and the sun; and the earth must
likewise be once between the sun and the moon, and yet we have not a
solar and a lunar eclipse every month?
_Mrs. B._ I have already informed you, that the orbits of the earth and
moon are not in the same plane, but cross or intersect each other; and
the moon generally passes either above or below that of the earth, when
she is in conjunction with the sun, and does not therefore intercept its
rays, and produce an eclipse; for this can take place only when the moon
is in, or near her nodes, which is the name given to those two points in
which her orbit crosses that of the earth; eclipses cannot happen at any
other time, because it is then only, that they are both in a right line
with the sun.
_Emily._ And a partial eclipse of the moon takes place, I suppose, when,
in passing by the earth, she is not sufficiently above or below the
shadow, to escape it entirely?
_Mrs. B._ Yes, one edge of her disk then dips into the shadow, and is
eclipsed; but as the earth is larger than the moon, when eclipses happen
precisely at the nodes, they are not only total, but last for upwards of
three hours.
[Illustration: PLATE XII.]
A total eclipse of the sun rarely occurs, and when it happens, the total
darkness is confined to one particular part of the earth, the diameter
of the shadow not exceeding 180 miles; evidently showing that the moon
is smaller than the sun, since she cannot entirely hide it from the
earth. In fig. 1, plate 12, you will find a solar eclipse described; S
is the sun, M the moon, and E the earth; and the moon's shadow, you see,
is not large enough to cover the earth. The lunar eclipses, on the
contrary, are visible from every part of the earth, where the moon is
above the horizon; and we discover, by the length of time which the moon
is passing through the earth's shadow, that it would be sufficient to
eclipse her totally, were she many times her actual size; it follows,
therefore, that the earth is much larger than the moon.
In fig. 2, S represents the sun, which pours forth rays of light in
straight lines, in every direction. E is the earth, and M the moon. Now
a ray of light coming from one extremity of the sun's disk, in the
direction A B, will meet another, coming from the opposite extremity, in
the direction C B; the shadow of the earth cannot therefore extend
beyond B; as the sun is larger than the earth, the shadow of the latter
is conical, or in the figure of a sugar loaf; it gradually diminishes,
and is much smaller than the earth where the moon passes through it, and
yet we find the moon to be, not only totally eclipsed, but to remain for
a considerable length of time in darkness, and hence we are enabled to
ascertain its real dimensions.
_Emily._ When the moon eclipses the sun to us, we must be eclipsed to
the moon?
_Mrs. B._ Certainly; for if the moon intercepts the sun's rays, and
casts a shadow on us, we must necessarily disappear to the moon, but
only partially, as in fig. 1.
_Caroline._ There must be a great number of eclipses in the distant
planets, which have so many moons?
_Mrs. B._ Yes, few days pass without an eclipse taking place; for among
the number of satellites, one or the other of them are continually
passing either between their primary and the sun; or between the planet,
and each other. Astronomers are so well acquainted with the motion of
the planets, and their satellites, that they have calculated not only
the eclipses of our moon, but those of Jupiter, with such perfect
accuracy, that it has afforded a means of ascertaining the longitude.
_Caroline._ But is it not very easy to find both the latitude and
longitude of any place by a map or globe?
_Mrs. B._ If you know where you are situated, there is no difficulty in
ascertaining the latitude or longitude of the place, by referring to a
map; but supposing that you had been a length of time at sea,
interrupted in your course by storms, a map would afford you very little
assistance in discovering where you were.
_Caroline._ Under such circumstances, I confess I should be equally at a
loss to discover either latitude, or longitude.
_Mrs. B._ The latitude is usually found by taking the altitude of the
sun at mid-day; that is to say, the number of degrees that it is
elevated above the horizon, for the sun appears more elevated as we
approach the equator, and less as we recede from it.
_Caroline._ But unless you can see the sun, how can you take its
altitude?
_Mrs. B._ When it is too cloudy to see the sun, the latitude is
sometimes found at night, by the polar star; the north pole of the
earth, points constantly towards one particular part of the heavens, in
which a star is situated, called the Polar star: this star is visible on
clear nights, from every part of the northern hemisphere; the altitude
of the polar star, is therefore the same number of degrees, as that of
the pole; the latitude may also be determined by observations made on
any of the fixed stars: the situation therefore of a vessel at sea, with
regard to north and south, is easily ascertained. The difficulty is,
respecting east and west, that is to say, its longitude. As we have no
eastern poles from which we can reckon our distance, some particular
spot, or line, must be fixed upon for that purpose. The English, reckon
from the meridian of Greenwich, where the royal observatory is situated;
in French maps, you will find that the longitude is reckoned from the
meridian of Paris.
The rotation of the earth on its axis in 24 hours from west to east,
occasions, you know, an apparent motion of the sun and stars in a
contrary direction, and the sun appears to go round the earth in the
space of 24 hours, passing over fifteen degrees, or a twenty-fourth part
of the earth's circumference every hour; therefore, when it is twelve
o'clock in London, it is one o'clock in any place situated fifteen
degrees to the east of London, as the sun must have passed the meridian
of that place, an hour before he reaches that of London. For the same
reason it is eleven o'clock in any place situated fifteen degrees to the
west of London, as the sun will not come to that meridian till an hour
later.
If then the captain of a vessel at sea, could know precisely what was
the hour at London, he could, by looking at his watch, and comparing it
with the hour at the spot in which he was, ascertain the longitude.
_Emily._ But if he had not altered his watch, since he sailed from
London, it would indicate the hour it then was in London.
_Mrs. B._ True; but in order to know the hour of the day at the spot in
which he is, the captain of a vessel regulates his watch by the sun when
it reaches the meridian.
_Emily._ Then if he had two watches, he might keep one regulated daily,
and leave the other unaltered; the former would indicate the hour of the
place in which he was situated, and the latter the hour at London; and
by comparing them together, he would be able to calculate his longitude.
_Mrs. B._ You have discovered, Emily, a mode of finding the longitude,
which I have the pleasure to tell you, is universally adopted: watches
of a superior construction, called chronometers, or time-keepers, are
used for this purpose, and are now made with such accuracy, as not to
vary more than four or five seconds in a whole year; but the best
watches are liable to imperfections, and should the time-keeper go too
fast or too slow, there would be no means of ascertaining the error;
implicit reliance, cannot consequently be placed upon them.
Recourse, therefore, is sometimes had to the eclipses of Jupiter's
satellites. A table is made, of the precise time at which the several
moons are eclipsed to a spectator at London; when they appear eclipsed
to a spectator in any other spot, he may, by consulting the table, know
what is the hour at London; for the eclipse is visible at the same
moment, from whatever place on the earth it is seen. He has then only to
look at his watch, which he regulates by the sun, and which therefore
points out the hour of the place in which he is, and by observing the
difference of time there, and at London, he may immediately determine
his longitude.
Let us suppose, that a certain moon of Jupiter is always eclipsed at six
o'clock in the evening; and that a man at sea consults his watch, and
finds that it is ten o'clock at night, where he is situated, at the
moment the eclipse takes place, what will be his longitude?
_Emily._ That is four hours later than in London: four times fifteen
degrees, make 60; he would, therefore, be sixty degrees east of London,
for the sun must have passed his meridian before it reaches that of
London.
_Mrs. B._ For this reason the hour is always later than in London, when
the place is east longitude, and earlier when it is west longitude. Thus
the longitude can be ascertained whenever the eclipses of Jupiter's
moons are visible.
_Caroline._ But do not the primary planets, sometimes eclipse the sun
from each other, as they pass round in their orbits?
_Mrs. B._ They must of course sometimes pass between each other and the
sun, but as their shadows never reach each other, they hide so little of
his light, that the term eclipse is not in this case used; this
phenomenon is called a transit. The primary planets do not any of them
revolve in the same plane, and the times of their revolution round the
sun is considerable, it therefore but rarely happens that they are at
the same time, in conjunction with the sun, and in their nodes. It is
evident also, that a planet must be inferior (that is within the orbit
of another) in order to its apparently passing over the disk of the sun.
Mercury, and Venus, have sometimes passed in a right line between us,
and the sun, but being at so great a distance from us, their shadows did
not extend so far as the earth; no darkness was therefore produced on
any part of our globe; but the planet appeared like a small black spot,
passing across the sun's disk.
It was by the last transit of Venus, that astronomers were enabled to
calculate, with some degree of accuracy, the distance of the earth from
the sun, and the dimensions of the latter.
_Emily._ I have heard that the tides are affected by the moon, but I
cannot conceive what influence it can have on them.
_Mrs. B._ They are produced by the moon's attraction, which draws up the
waters of that part of the ocean over which the moon passes, so as to
cause it to stand considerably higher than the surrounding parts.
_Caroline._ Does attraction act on water more powerfully than on land? I
should have thought it would have been just the contrary, for land is
certainly a more dense body than water?
_Mrs B._ Tides do not arise from water being more strongly attracted
than land, for this certainly is not the case; but the cohesion of
fluids, being much less than that of solid bodies, they more easily
yield to the power of gravity; in consequence of which, the waters
immediately below the moon, are drawn up by it, producing a full tide,
or what is commonly called, high water, at the spot where it happens. So
far, the theory of the tides is not difficult to understand.
_Caroline._ On the contrary, nothing can be more simple; the waters, in
order to rise up under the moon, must draw the waters from the opposite
side of the globe, and occasion ebb-tide, or low water, in those parts.
_Mrs. B._ You draw your conclusion rather too hastily, my dear; for
according to your theory, we should have full tide only once in about
twenty-four hours, that is, every time that we were below the moon,
while we find that in this time we have two tides, and that it is high
water with us, and with our antipodes, at the same time.
_Caroline._ Yet it must be impossible for the moon to attract the sea in
opposite parts of the globe, and in opposite directions, at the same
time.
_Mrs. B._ This opposite tide, is rather more difficult to explain, than
that which is immediately beneath the moon; with a little attention,
however, I hope I shall be able to make you understand the explanation
which has been given of it, by astronomers. It must be confessed,
however, that the theory upon this subject, is attended with some
difficulties. You recollect that the earth and the moon mutually attract
each other, but do you suppose that every part of the earth is equally
attracted by the moon?
_Emily._ Certainly not; you have taught us that the force of attraction
decreases, with the increase of distance, and therefore that part of the
earth which is farthest from the moon, must be attracted less
powerfully, than that to which she is nearest.
_Mrs. B._ This fact will aid us in the explanation which I am about to
give to you.
In order to render the question more simple, let us suppose the earth to
be every where covered by the ocean, as represented in (fig. 3. pl. 12.)
M is the moon, A B C D the earth. Now the waters on the surface of the
earth, about A, being more strongly attracted than any other part, will
be elevated: the attraction of the moon at B and C being less, and at D
least of all. The high tide at A, is accounted for from the direct
attraction of the moon; to produce this the waters are drawn from B and
C, where it will consequently be low water. At D, the attraction of the
moon being considerably decreased, the waters are left relatively high,
which height is increased, by the centrifugal force of the earth being
greater at D than at A, in consequence of its greater distance from the
common centre of gravity X, between the earth and the moon.
_Emily._ The tide A, then, is produced by the moon's attraction, and the
tide D, is produced by the centrifugal force, and increased by the
feebleness of the moon's attraction, in those parts.
_Caroline._ And when it is high water at A and D, it is low water at B
and C: now I think I comprehend the nature of the tides, though I
confess it is not quite so easy as I at first thought.
But, Mrs. B., why does not the sun produce tides, as well as the moon;
for its attraction is greater than that of the moon?
_Mrs. B._ It would be at an equal distance, but our vicinity to the
moon, makes her influence more powerful. The sun has, however, a
considerable effect on the tides, and increases or diminishes them as it
acts in conjunction with, or in opposition to the moon.
_Emily._ I do not quite understand that.