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A Description of the Astronomical Machinery serv.
ing to explain and illustrate the foregoing Part of this Treatise.
HE ORRERY. This machine shews the Fronting 397.
motions of the Sun, Mercury, Venus, page. The Earth, and Moon; and occasionally, the superior ORRERY.
planets, Mars, Jupiter, and Saturn, may be put on; Jupiter's four satellites are moved round him in their proper times by a small winch; and Saturn has his five satellites, and his ring, which keeps its parallelism round the Sun; and by a lamp put in the Sun's place, the ring shews all the phases de scribed in the 204th article.
In the centre, No. 1. represents the Sun, supported by its axis inclining almost 8 degrees from the axis of the ecliptic; and turning round in 254 days on its axis, of which the north pole inclines
toward the 8th degree of Pisces in the great ecliptic The eclip- (No. II.), whereon the months and days are en
graven over the signs and degrees in which the Sun appears, as seen from the Earth, on the different days of the
year. Mercury The nearest planet (No. 2.) to the Sun is Mer.
cury, which goes round him in 87 days 23 hours, „or 87. diurnal rotations of the Earth; but has no motion round its axis in the machine, because the time of its diurnal motion in the heavens is not
known to us. Venus. The next planet in order is Venus (No. 3.) which
performs her annual course in 224 days 17 hours; and turns round her axis in 24 days 8 hours, or in 24 diurnal rotations of the Earth. Her axis inclines 75 degrees from the axis of the ecliptic, and her north pole inclines toward the 20th degree of Aquarius, according to the observations of
Bianchini. She shews all the phenomena described from the 30th to the 44th article in chap. I.
Next without the orbit of Venus is the Earth, TheEarth. (No. 4.) which turns round its axis, to any fixed point at a great distance, in 23 hours 56 minutes 4 seconds, of mean solar time ($ 221, & seq.), but from the sun to the Sun again in 24 hours of the same time. No. 6. is a sidereal dial-plate under the Earth; and No. 7. a solar dial-plate on the cover of the machine. The index of the former shews sidereal, and of the latter, solar time; and hence, the former index gains one entire revolution on the latter every year, as 365 solar or natural days contain 366 sidereal days, or apparent revolutions of the stars. In the time that the Earth makes 3651 diurnal rotations on its axis, it goes once round the Sun in the plane of the ecliptic; and always keeps opposite to a moving index (No. 10.), which shews the Sun's apparent daily change of place, and also the days of the months.
The Earth is half covered with a black cap, to divide the apparently.enlightened half next the Sun from the other half, which when turned away from him is in the dark. The edge of the cap represents the circle bounding light and darkness, and shews at what time the Sun rises and sets to all places throughout the year. The Earth's axis inclines 234 degrees from the axis of the ecliptic, the north pole inclines toward the beginning of Cancer, and keeps its parallelism throughout its annual course, j 48, 202; so that in summer the northern parts of the Earth inclines toward the Sun, and in winter declines from him : by which means the different lengths of days and nights, and the cause of the various seasons, are demonstrated to sight.
There is a broad horizon, to the upper side of which is fixed a meridian-semicircle in the north and south points, graduated on both sides from the horizon to 90° in the zenith, or vertical point. The edge
of the horizon is graduated from the east and west to the south and north points, and within these divisions are the points of the compass. From the lower side of this thin horizon-plate, stand out four small wires, to which is fixed a twilight-circle 18 degrees from the graduated side of the horizon all round. This hori. zon may be put upon the Earth (when the cap is taken away), and rectified to the latitude of any place: and then, by a small wire called the solar ray, which may be put on so as to proceed directly from the Sun's centre toward the Earth's, but to come no farther than almost to touch the horizon. The beginning of twi. light, time of sun-rising, with his amplitude, meridian-altitude, time of setting, amplitude then, and end of twilight, are shewn for every day of the year, at
that place to which the horizon is rectified. TheMoon
The Moon (No. 5.) goes round the Earth, from between it and any fixed point at a great distance, in 27 days 7 hours 43 minutes, or through all the signs and degrees of her orbit; which is called her periodical revolution: but she goes round from the Sun to the Sun again, or from change to change, in 29 days 12 hours 45 minutes, which is her synodical revolution; and in that time she exhibits all the phases already described, \ 255.
When the above-mentioned horizon is rectified to the latitude of any given place, the times of the Moon's rising and setting, together with her amplitude, are shewn to that place as well as the Sun's, and all the various phenomena of the harvest-moon, ý 273, &
seq. are made obvious to sight. The nodes. The Moon's orbit (No. 9.) is inclined to the
ecliptic (No. 11.), one half being above, and the other below it. The nodes, or points at 0 and 0, lie in the plane of the ecliptic, as described ( 317, 318, and shift backward through all its signs and degrees in 18 years. The degrees of the Moon's latitude, to the highest at NL (north latitude), and lowest at S L (south latitude), are engraven both ways from her nodes at 0 and 0); and as the Moon rises and falls in her orbit according to its inclination, her latitude and distance from her nodes are shewn for every day; having first rectified her orbit so as to set the nodes to their proper places in the ecliptic: and then, as they come about at different, and almost opposite, tinies of the year, $ 319, and point twice toward the Sun; all the eclipses may be shewn for hundreds of years (without any new rectification) by turning the machinery backward for time past, or forward for time to come.
At 17 degrees distance from each node, on both sides, is engraven a small sun; and at 12 degrees distance, a small moon; which shew the limits of solar and lunar eclipses, \ 317: and when, at any change, the moon falls between either of these suns and the node, the Sun will be eclipsed on the day pointed to by the annual index (No. 10.), and as the Moon has then north or south latitude, one may easily judge whether that eclipse will be visible in the northern ‘or southern hemisphere; especially as the Earth's axis inclines toward the Sun or declines from him at that time. And when at any full, the Moon falls between either of the little moons and node, she will be eclipsed, and the annual index shews the day of that eclipse. There is a circle of 294 equal parts (No. 8.) on the cover of the machine, on which an index shews the days of the Moon's age.
A semi-ellipsis and semicircle are fixed to an el- Plate IX. liptical ring, which being put like a cap upon the Fig. X. Earth, and the forked part F upon the Moon, shews the tides as the Earth turns round within them, and they are led round it by the Moon. When the different places come to the semi-ellipsis AaEbB, they have tides of food : and when they come to the semicircle CED, they have tides of ebb, $ 304, 305 ;