Imágenes de página
PDF
ePub

and them. These stars are observable at night, because they are not intercepted from our sight by the sun's rays; and in this manner they appear during the whole winter, only they seem to get more westerly every night, as the earth moves gradually by them to the east. To make this still more clear, place the earth in the planetarium between the sun and any of the signs, that side towards the sun will be day, and that towards the sign night: it follows, that at night we are turned towards the stars, which in that sign (suppose, as before, the Pleiades in Taurus) will then be conspicuous to us; but as the spring approaches, the earth withdraws itself from between the sun and the Pleiades, till at length the earth, by its progressive motion, gets the sun between it and the stars, which then lie hid behind the solar rays; after the same manner, while the earth performs his annual tract, the sun, which always seems to move the contrary way, darkens, by his splendor, the other constellations successively; but the stars op; posite to those hid by the sun, are at night presented to our view.

GENERAL PHENOMENON OF THE PLANETS.

Let the tutor now place the Earth, Mars, and Venus, on the planetarium; and as each planet moves with a different degree of velocity, they are continually changing their relative positions. Thus, on turning the handle of the machine, he will find, first, that the Earth moves twice as fast as Mars, making

1

two revolutions while he makes one; and Venus, on the other hand, moves much faster than the earth. Secondly, that in each revolution of the earth these planets continually change their relative positions, corresponding sometimes with the same point of the ecllptic, but much oftener with different points.

1

To explain the conjunction, opposition, elongation,

and other phenomena of inferior planets.

I may now proceed to make some observations on the motions of Venus, as observed in the planetari um. If considered as viewed from the sun, we shall find that Venus would appear at one time nearer to the earth than at another ; that sometimes she would appear in the same part of the heavens, and at others in opposite parts thereof.

As the planets, when seen from the sun, change their position with respect to the earth, so do they also, when seen from the earth, change their positions with respect to the sun, being sometimes nearer to, at others farther from, and, at times, in conjunction with him.

But the conjunctions Venus or Mercury, seen from the earth, not only happen when they are seen together from the sun, but also when they appear to be in opposition to the solar spectator. To illustrate this, bring the Earth and Venus to the first point of Capricorn; then by applying a string from the sun over Venus and the Earth, you will find them to be in conjunction, or on the same point of the ecliptic.

CC

| Whiereas, if you turn the handle till the sun is between Venus and the Earth, a spectator in the sun will see Venus and the Earth in opposition ; but an inhabitant of the earth will see Venus not in opposition to the sun, but in conjunction with him.

In the first conjunction Venus is between the sun, and earth; this is called the inferior conjunction. In the second, the sun is situated between the earth and Venus; this is called the superior conjunction.

After either of these conjunctions, Venus will be seen to recede daily from the sun, but never departing beyond certain bounds, never appearing opposite to the sun; and when she is seen at the greatest distance from him, a line joining her centre with the centre of the earth, will be a tangent to the orbit of Venus.

To illustrate this, take off the sun from its support, and the ball of Venus from its supporting stemy; place the wire, plate ti, fig. 2, so that the part

P may be on the stem that supports the earth, and a similar socket, fig. 3, on the pin which supports the ball of Venus ; the wire F is to lie in a notch at the top of the socket, which has been put upon the supporting stem of Venus ; then will the wire represent a visual ray going from an inhabitant of the earth to Venus. By turning the handle, you will now find that the planet never departs further than certaie. limits from the sun, which are called its greatest elongations, when the wire becomes its tangents to the orbit; after which, it approaches the sun till

[ocr errors]
[ocr errors]

it arrives at either the inferior or superior conjunction.

It will also be evident from the instrument, that Venus, from her superior conjunction, when she is furthest from the earth, to the time of her inferior conjunction, when she is nearest, sets later than the sun, is seen after sun-set, and is, as it were, the forerunner of night and darkness. But from the inferior conjunction till she comes to the superior one, she is always seen westward of the sun, and must consequently set before him in the evening, and rise before him in the morning, foretelling that light and day are at hand.

Bring Venus and the earth to the beginning of Aries, when they will be in conjunction ; and turn the handle for nearly 225 days, and as Venus moves faster than the earth, she will be come to Aries, and have finished her course, but will not have overtaken the earth, who has moved on in the mean time; and Venus must go on for some time, in order to overtake her. Therefore, if Venus should be this day in conjunction with the sun, in the inferior part of her orbit, she will not come again to the same conjunction till after 1 year, 7 months, and 12 days.

It is also plain, by inspection of the planetarium, that though Venus does always keep nearly at the same distance from the sun, yet she is continually changing her distance from the earth; her distance is greatest when she is in her superior, and least when she is in her inferior conjunction.

To explain the phases, the retrograde, direct, and

stationary situations of the planets.

As Venus is an opake globe, and only shines by the light she receives from the sun, that face which is turned towards the sun will always be bright, while the opposite one will be in darkness ; consequently, if the situation of the earth be such, that the dark side of Venus be turned towards us, she will then he invisible, except she appears like a spot on the disc of the sun.

If her whole illuminated face is turned towards the earth, as it is in her superior conjunction, she appears of a circular form ; and according to the different positions of the earth and Venus, she will have different forms, and appear with different phases, undergoing the same changes of form as the moon, These different phases are seen very plain in this instrument, as the side of the planet, which is opposite to the sun, is blackened; so that in any position, a line drawn from the earth to the planet, will represent that part of her disc which is visible to us.

The irregularities in the apparent motions of the planets, is a subject that this instrument will fully elucidate ; and the pupil will find that they are only apparent, taking their rise from the situation and motion of the observer. To illustrate this, let us suppose the above-mentioned wire, when connected with Venus and the earth, to be the visual ray of an observer on the earth, it will then point out how the

« AnteriorContinuar »