Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 26

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...o...b..t, 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...o...b..t, as ill.u.s.trated 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 LAt.i.tUDE 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 s.p.a.ce 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 pa.s.sing round the sun, must proceed in a sort of scolloped circle.

_Mrs. B._ That is true; and there are also other circ.u.mstances 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 pa.s.ses 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, from her conjunction, or the period of new moon.

_Emily._ Are not the eclipses of the sun produced by the moon pa.s.sing between the sun and the earth?

_Mrs. B._ Yes; when the moon pa.s.ses 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 pa.s.sing 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 pa.s.ses 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 pa.s.sing 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.

[Ill.u.s.tration: 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 pa.s.sing 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.