Page images
PDF
EPUB

sent to the Pacific in the bark "Endeavor," where he perished subsequently by the hands of savages at one of the Sandwich islands. Observations upon these transits furnish data for important astronomical calculations.

119. In consequence of the earth's annual revolution around the sun, he appears to travel eastward, through all the signs of the zodiac, every 365 days. It is this eastward motion of the sun that causes the stars to rise and set earlier and earlier every night.

SUN'S APPARENT MOTION AROUND THE ECLIPTIO.

[ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors]

Let a person walk around a tree, for instance, at a short distance from it, and it will appear to sweep around the horizon in an opposite direction. So as the earth revolves annually about the sun, the sun appears to traverse the circle of the heavens ir the opposite direction. Suppose the earth is at A on the 20th of March: the sun will appear to be at B in the opposite side of the ecliptic. As the earth moves on in her or1⁄4't from A to C, the sun will appear to move from B to D: and will seem thus to traverse the whole circle of the heavens every 365 days, or as often as the earth revolves around him. The time of the sun's apparent entrance into the different constellations as he journeys eastward, is usually laid down in almanacs. Thus: "Sun enters ? (A es) 20th of March, &c.;" at which time the earth would enter the sign (Libra), and the un would seem to enter the opposite sign Aries.

119. What said of sun's apparent motion? Cause? Time of revolution! Effect upon the stars? (Illustration from tree? By diagram. What is meant by the sun's entering Aries? When? Where earth ther!

[ocr errors]
[ocr errors][merged small][merged small][merged small]

120. BESIDES the revolution around the sun, the planets all revolve rapidly about their respective aves, as they perform their celestial journeys. This is called their diurnal revolution.

The evidences of the earth's revolution have already been considered on pages 18 and 14. That most of the other planets revolve has been ascertained by carefully observing the motions of spots, as they seemed to pass periodically over their disks.

121. The axis of the earth is inclined to the plane of the ecliptic 23° 28'. It is always parallel to itself—that is, it always inclines the same way, and to the same

amount.

INCI.INATION OF THE EARTH'S AXIS TO THE PLANE OF THE ECLIPTIC.

[blocks in formation]

1. The inclination of the earth's axis, and its parallelism to itself, are exhibited in the above cut, as also in the cuts, pages 50, 51, and 64, to which the student will do well to

turn.

2. The author is aware that the poles of the earth have a slow motion around the pole of the ecliptic, requiring 25,000 years for a single revolution, but prefers to consider this point hereafter, in connection with the precession of the equinoxes.

122. The axes of all the planets are inclined more or less to the planes of their respective orbits. This inclination, so far as known, is as follows:

[merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors]

120. What revolution have the planets besides around the sun? What called? (What proof of the earth's revolution? Of the other planets?) 121. What said of the axis of the earth? Of the stability of its inclination? (Is there no variation ?)

122. Are the axes of the other planets inclined? To what extent, respect ively? (Substance of note 1? Illustrate by diagram. Note 2?)

1. The student will bear in mind that the above inclination is not to the ecliptic, or plane of the earth's orbit, but to the plane of the orbits of the several planets respectIvely. Take the case of Venus, for instance:

PLANE OF VENUS' ORBIT

PLANE OF THE ECLIPTIC

The orbit of Venus departs from the ecliptic 340, as stated at 108, while her axis is inclined to the plane of her orbit 750, as shown in the above figures. This distinction should be kept definitely in view by the student.

2. The inclination of the axes of the several planets, each to the plane of its own or bit, is represented in the following cut:

INCLINATION OF THE AXES OF THE SEVERAL PLANETS TO THE PLANES OF THEIR ORBITS

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed]

123. The inclination of the earth's axis to the plane of the ecliptic causes the equinoctial to depart 23° 28′ from the ecliptic. This angle made by the equinoctial and the ecliptic is called the Obliquity of the Ecliptic.

[blocks in formation]

Let the line A A represent the axis of the earth, and BB the poles or axis of the eclipic. Now if the line A A inclines toward the plane of the ecliptic, or, in other words, departs from the line B B to the amount of 280 28', it is obvious that the plane of the

123. What effect has the inclination of the earth's axis upon the equinoc tial? What is the obliquity of the ecliptic? (Illustrate by diagram.)

equator, or equinoctial, will depart from the ecliptic to the same amount. This depart. ure, shown by the angles CC, constitute the obliquity of the ecliptic.

124. The permanent inclination of the earth's axis, and her revolution around the sun, cause first one pole to be enlightened and then the other, thus producing the seasons. The same inclination and revolution cause the sun to appear to oscillate fron north to south, crossing the equator twice every year. This is called the sun's decli nation. (See page 26.)

This subject of the seasons will be sufficiently understood by examining the cuts on pages 64 and 65.

125. The equinoctial points in the earth's orbit are two points in opposite sides of the ecliptic, at which the sun is exactly in the equinoctial; or, in other words, the plane of the equinoctial exactly cuts the sun's center. The first of these is passed on the 20th of March (the sun beginning then to decline northward), on account of which it is called the vernal equinox; and the other on the 23d of September, on account of which it is called the autumnal equinox. (See the earth at A and B, in the cut, page 64.)

If the sun is vertical at the equator, he will, of course, shine to both poles, as reprerented in the cut, and the days and nights will be equal all over the world. Hence the name equinoctial, from the Latin æquus, equal, and nʊx, night.

126. The solstitial points are those points in the earth's orbit where the sun ceases to decline from the equinoctial, and begins again to return toward it. They are respectively 90° from the equinoctial points.

The Summer Solstice is reached on the 21st of June, when the sun has the greatest northern declination, and it is summer in the northern hemisphere.

The Winter Solstice is reached on the 23d of December, when the sun has the greatest southern declination, and it is summer in the southern hemisphere, and winter in the northern. (See the earth at E F, cut, page 64.)

124. What other effects from the inclination of the earth's axis? Sun's declination?

125. What are the equinoctial points? How distinguished, and why! When passed? (Substance of note ?)

126. The solstitial points? How far from the equinoctial points? How distinguished? When passed?

127. The amount of the sun's declination north and south of the equinoctial is 23° 28'; answering to the inclination of the earth's axis, by which it is caused, and marking the limits of the tropics upon the earth's surface.

SHADOWS AT THE EQUATOR

1. On the 21st of June the sun reaches his greatest northern declination or Summer Solstice, and is vertica on the Tropic of Cancer. From this time he approaches the equator of the heavens till the 20th of September, when he crosses it, and begins to decline southward. In the 23d of December he has reached his greatest southern declination, or Winter Solstice, and begins to return toward the equinoctial, which he passes on the 20th of March, and reaches his Summer Solstice again on the 21st of June. In this manner he conti. vies to deck. e, drst north and then south of the equato from year to var. But it should not he forgotter tha the sun does not really move, first north and then south, but that the apparent motion is caused simply by the inclination of the earth's axis and her revolution around the sun.

DEC.23.

23°28'

23°28'

B

2. The sun's declination may be easily measured by the shadow of&uitable obje upon the earth's surface. Suppose the flag staff n the cut to stand perpendicularly, and exactly or the equator. On the 23d of December the shadow vould be thrown northward to A, or 230 28-just as far as the sun has declined south. At 12 o'clock on the 20th of March, and the 23d of September, there would be no shadow; and on the 21st of June, it would extend southward 230 28′ to C. Thus, at the equator, the shadow falls first north and then south of all perpendicular objects, for six months alternately.

[blocks in formation]

3. This cut shows how the student may measure the sun's declination wherever he may be located north of the equator. The shadows are such as are cast by objects during the year, about 450 north of the equator. On the 23d of December, when the sun has his greatest declination, the shadow of the flag-staff extends north at 12 o'clock to the point C, where two boys are seen, having just driven down a stake. From this time to June 21st the shadow gradually shortens, till on that day it reaches the point B, where another stake is driven. It then begins to elongate, and in six months is extended to Cagain. The point A is just half-way from B to C in angular measurement, though the distances on the plain in the picture are very different. When the sun is on the equator, March 21st and September 23d, the shadow will reach only to A; and the angle A B and the top of the staff shows the northern, and A C and the top of the staff the southern declination. It will be found to be 23° 28' each way, as marked in the figure.

127. To what extent does the sun decline from the equinoctial north and south? Why not more? (Substance of note 1? Note 2, and explain vy diagram. Note 3, and diagram. What is a gnomon ?)

« PreviousContinue »