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motions of Venus appear in the heavens, and the path she appears to us to describe among the fixed stars.
Let Venus be placed near her superior conjunction, and the instrument in motion, the wire will mark out the apparent motion of Venus in the ecliptic. Thus Venus will appear to move eastward in the ecliptic, till the wire becomes a tangent to the orbit of Venus, in which situation she will appear to us to be stationary, or not to advance at all among the fixed stars; a circumstance which is exceeding visible and clear upon the planetarium.
Continue turning, till Venus be in her superior conjunction, and you will find by the wire, or visual ray, that she now appears to move backward in the ecliptic, or from east to west, till she is arrived to that part where the visual ray again becomes a tangent to her orbit. In which position, Venus will again appear stationary for some time; after which she will commence anew her direct motion.
Hence, when Venus is in the superior part of her orbit, she is always seen to move directly, according to the order of the signs; but when she is in the inferior part, she appears to move in a contrary direction.
What has been said concerning the motions of Venus, is applicable to those of Mercury ; but the conjunctions of Mercury with the sun, as well as the times of his being direct, stationary, or retrograde, are more frequent than those of Venus.
often behind the sun as before it; we may, therefore, from hence conclude that this system is erroneous.
It is also apparent in the planetarium, that on this scheme these planets might be seen in conjunction with, or in opposition to, the sun, or at any distance from it. But this is contrary to experience, for they are never seen in opposition to the sun, or on the meridian of London; for instance, at midnight, nor ever recede from it beyond certain limits.
Again, on the Ptolemaic system all the planets would be at an equal distance from the earth, in all parts of their orbits, and would therefore necessarily appear always of the same magnitude, and moving with equal and uniform velocities in one direction; circumstances which are known to be repugnant to observation and experience,
To rectify the planetarium, or place the planets in their true situations, as seen from the sun.
The situations of the planets in the heavens are accurately calculated by astronomers, and published in almanacks appropriated to the purpose, as the • Nautical Almanack, White's Ephemeris, &c. An ephemeris is a diary or daily register of the motions and places of the heavenly bodies, shewing the situation of each planet at 12 o'clock each day. These situations it exhibits, both as seen from the sun, and from the earth ; but as for the former, or the heliccentric, is the only one of any use for this purpose, we shall here insert, and explain, so much of that part of Mr. White's ephemeris, as will enable the pupil to rectify his planetarium.
In the foregoing table, for May, 1790, you have the heliocentric places calculated to every six days of the month, which is sufficiently accurate for general purposes. Thus, on the i9th, you have Saturn in 28° 11' of Pisces, Jupiter 3° 37' of Virgo, Mars in 5° 20' of Libra, the Earth 28° 36'J of Virgo, Venus 7° 7' of Capricorn, and Mercury 4° i5' of Virgo; to which places on the ecliptic of the! planetarium the several planets are to be set, and they will then exhibit their real situations, both with respect to the sun and the earth, for that day,
To use the Instrument as a tellurian,plate 12,Jig. i.
The sun, the earth, and the moon, are bodies vrhich, from our connection with them, are so interesting to us, that it is necessary to enter into a, minute detail of their respective phenomena. To render this instrument a tellurian, all ihe planets are first to be taken oft", the piece of wheel-work, ABC, is to be placed on in their stead, in such a manner that the wheel c n:ay fall into the teeth that are cut upon the edge of the ecliptic. The milled nut, D, is then to be firmly screwed on, to keep the wheel-work firmly in its place. It is best to place this wheel-work in such a manner, that the index E may point to the 21 of June, and then to move the support of the globe so that the north pole may be turned towards the sun, or the ir of Cancer.
The instrument will then shew, in an accurate and clear manner, all the phenomena arising from the annual and diurnal motion of the earth; as the globe is of three inches diameter, all the continents, seas, kingdoms, &c. may be distinctly seen; the equator, the ecliptic, tropics, and other circles, are very visible; so that the problems relative to peculiar places may be satisfactorily solved. The axis of the earth is inclined to the ecliptic in an angle of 664 degrees, and preserves its parallehsm during the whole of its revolution. About the globe there is a circle, HI, to represent the terminator, or boun-r dary between light and darkness, dividing the enlightened from the dark hemisphere. At NO is an hour-circle, to determine the time of sun-rising or setting, lengths of days and nights, &c.
The brass index G represents a central solar ray; it serves to shew when it is noon, or when, the sun W upon the meridian at any given place; it also shews what sign and degree of the ecliptic on the globe the sun describes on any day, and the parallel it describes.
The plane of the terminator, III, passes through the centre of the earth, and is perpendicular to the central soUir ray. The index E points out the sun's place in the ecliptic circle for any given day in the year,
To explain the changes of seasons by the tellurian.
Before I shew how the seasons are explained by the instrument, it is necessary to assume two propositions: 1. That a globular luminous body, sending out parallel rays of light, will only Cik lighten one half of another globe, and that of course will be the hemisphere turned towards the luminous body. 2. That the earth moves round the sun in such a manner, that in all parts of its orbit its axi^ is parallel to itself, and has a certain inclination to the plane of the orbit. These being understood, the first thing to be done is to rectify the tellurian; or, in other words, to put the globe into a position similar to that of the earth, for any given day. Thus, to rectify the tellurian for the 21st of June, turn the handle till the annual index comes to the given day; then move the globe by the arm KL, so that the north pole may be turned towards the sun; and adjust the terminator, so that it may just touch the edge of the arctic circle. The globe