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retrograde when the earth is in the nearer part of its or. · bit with respect to them. Satellites, or secondary planets, or moons, those smaller
bodies in the solar system which regard the primaries as
their centres of motion. Saturn, a primary planet, the 6th in order from the sun, at
the distance of about 900 millions of miles. It is furnish
ed with a stupendous double ring and 7 satellites. Second, the 60th part of a minute, whether of time or of a
degree. Sexiile, that aspect where the difference of longitude of the two bodies
60%, Sign of the ecliptic, an arch of 309. or the 12th part of the
whole circle. Signs of the zodiac, twelve constellations, distributed through
the zodiac, and nearly at equal distances. The vernal equinoctial point was formerly in the constellation Aries, but owing to the precession of the equinoxes it is now in the constellation Pisces; yet the artificial signs continue to be called by their former names. The equinoctial points being still denominated Aries and Libra, and the solstitial.
points, Cancer and Capricorn. Solar system, comprehends the sun, the centre of the sys
tem, the primary planets, the secondary planets, and the
comets. Solar cycle. See Cycle. Solstices, the times when the sun enters the two solstitial
points of the ecliptic, viz. the 21st of June, the time of the northern solstice, and the 22d of December, that of the southern solstice. These with relation to the northern hemisphere, are frequently denominated the summer,
and winter, solstices, respectively. Solstitial points of the ecliptic, those opposite points in which
the sun has the greatest declination, viz. the beginning of the sign Cancer in the northern hemisphere, and the be
ginning of the sign Capricorn, in the southern. Sphere, in a geometrical sense, is a solid contained under a
uniformly-curved surface, every point of which is equally distant from a certain point within the same, called the centre. This term is applied to the several celestial bodies, as they are probably all nearly of this figure. It is also applied to the apparent concave surface of the heavens, and is then called the celestial sphere.
The sphere, in geography and astronomy, is frequently distinguished by the epithets right, oblique, or parallel, according to the position of the equator and horizon:
i right sphere, is that in which the equator cuts the horizon at-right angles, and such is the case to an inhabitant at the equator. In this sphere the lengths of the days and nights are always equal.
An oblique sphere, is that in which the equator cuts the horizon at oblique angles; and such is the case to any inhabitant north or south of the equator. In this sphere the lengths of the days and nights are always varyingthe variation being greater, the greater the latitude.
Aparallel sphere, is that in which the equator is parallel, or rather coincident, with the horizon; and such is the case to an inhabitant at either pole. In this sphere, the sun will be six months successively visible, and six in
visible. Spheroid, a solid which may be conceived as generated by
the rotation of an ellipsis round its tranverse or conjugate diameter. In the former case, the spheroid is said to be prolate, and in the latter, oblate. The figure of the earth, and perhaps that of most of the other planets, is nearly that of an oblate spheroid. This arises from their rotatory motion round their axes, by which, the attraction at the surface is continually diminished from the poles to the equator, by the continued increase of the centrifugal force; and thus, the equatorial diameter becomes greater than the polar. It follows from this figure, that the length of the degrees of latitude gradually increase from the equator to the poles. To this figure of the earth we are to ascribe many of the apparent irregularities in the motions of the celestial bodies: as, the precession of the equinoxes,
the nutation of the stars, &c. Stars, or fixed stars, luminous bodies, at an immense dis
tance, appearing in all parts of the heavens. They all probably resemble the sun in matter and in magnitude, and are each the centre of a system, similar to the solar system. They are said to be fixed because they constantly preserve, very nearly, the same relative position to each other. Besides the small apparent motion of the stars arising from aberration, and nutation, and the precession of the equinoxes; in some of them there has been discovered a very slow (indeed) proper motion. Whence it is conjectured that not only the bodies belonging to the innumerable systems of stars are in motion round their respective centres, but that all the systems of bodies in the universe are themselves in motion round some com. mon centre and that thus they are prevented from approaching each other, which, from their mutual attracrions, they must otherwise do.
Stationary. This term is applied to a planet, when, for some
time, it appears to a spectator to occupy the same place in the zodiac. To a spectator in the sun, the planets' motions would always appear direct; and that they ever appear otherwise to a spectator on the earth, is owing to its own motion, and being placed out of the centre of the system. To such a spectator, Mercury and Venus will appear stationary when at their greatest elongation; and all the other planets will appear stationary when the earth
is at its greatest elongation with respect to them. Style, the particular manner of counting time. It is distinguished into old and new.
Old style, is that which follows the Julian calendar.
New style, is that which follows the Gregorian calendar. See Calendar. In the year 1800 the latter was 12 days ahead of the former, and in every centurial year not divi
sible by 4, the difference will be increased 1 day. Systems of the Universe. Of these there are 3 noted ones
in the history of astronomy, viz. the Ptolemean system, advocated by many of the ancient philosophers. According to this, the earth occupies the centre of the universe, and is at rest; while all the celestial bodies revolve round it from east to west, every 24 hours. The Tychonean system, invented by Tycho Brahe, a noted Danish Astrononomer, born A. D. 1546. According to this system, the earth, as in the Ptolemean system, is placed in the centre of the universe, the moon revolving round the earth as her proper centre, while the sun, with all the other planets moving round him as satellites, revolve also round the earth.
Copernican system, maintained by many of the ancients, particularly by Pythagoras, revived by Copernicus a na. tive of Thorn in Prussia (born 1473), and demonstrated by Sir Isaac Newton. According to this it is demonstrated that the sun is the centre of the planetary system; the primary planets revolving round him in their annual orbits, and the secondaries round their respective primaries. That the orbits both of the primary and secondary planets are all nearly circular, though in fact elliptical; the sun, or primary, being placed in one of the foci of the respective orbits. That they all lie nearly in the same plane. That all the planets revolve nearly in the same direction, the square of their periodical times being directly proportional to the cubes of their mean distances from the centre of motion. That the earth, and perhaps most, if not all the other primary planets,
perform a diurnal rotation round their axes; and that the moon, or satellite of the earth, as well as perhaps all the other satellites, constantly present the same face towards their primaries. That the inclination of the axis of rotation to the plane of the orbit is different in different planets; and that thus they experience a difference
in their diversity of seasons. Syzigy. This general term is applied both to signify the con.
junction and opposition of a planet with the Sun.----It is
however chiefly used in relation to the moon. Tides, a periodical alternate motion or flux and reflux of the waters of the sea.
These are caused chiefly by the attraction of the moon, though in part by that of the Sun also ; and accordingly there are two tides of flood (and consequently two of ebb) in the course of every lunar day. The apex of the tides of high water is immediately under, or rather about 45° eastward of, the moon; and the other, diametrically opposite. These are produced by the unequal attractions of the moon on the part of the earth nearest to her, on the centre of the earth, and on the part farthest from her (attraction decreasing inversely with the square of the distance.) One tide therefore is produced by a redundancy of attraction, drawing the waters up towards the moon, and the opposite tide, by a deficiency of attraction, leaving, as it were, the waters behind. When the sun and moon are in conjunction or opposition, the tides, being then produced by their joint influence, are higher than usual, and hence called spring-tides; but when these bodies are in quadrature, the tides, being produced by the difference of their influence, are lower than usual, and
hence called neap-tides. Time is measured by the apparent motion of the celestial bodies; and is variously distinguished : thus
Apparent solar time, is that measured by the apparent motion of the sun; and hence the apparent solar time from noon, is equal to the sun's horary angle reduced to time, at the rate of 15° to the hour.
Mean solar time, is that shewn by a true time-piece, going with an equable motion throughout the year.
Sidereal time, is that measured by the apparent equable motion of the stars.
Lunar time, that measured by the apparent motion of the moon. See Day.
Transit of an inferior planet (Mercury or Venus) over the sun's disc, is when the planet, at the time of an in. ferior conjunction, passes between the sun and the oh.
server. This will only happen when the planet, at the time of this conjunction, is in or near its node.
Trine, an aspect where the bodies are at the distance of of the ecliptic or 1200. Twilight. See Crepusculum. Venus, the second primary planet from the sun, at the
distance of about 68 millions of miles. Year, a period of time gencrally considered as compre
hending a complete revolution of the seasons. The year 18 variously distinguished, viz.
1. Tropical Solar year, the time in which the sun appears to perform a complete revolution through all the signs of the zodiac 365d. 5h. 48m. 488.
2. Sidereal year, the time in which the sun appears to revolve from any fixed star to the same again = 365d. 6h. 9m. 17s. The difference between the tropical and sidereal year (20m. 29 s.) is the time of the sun's apparent motion through 50''1, the arch of annual precession.
3. Lunar astronomical year, consists of 12 lunar synodical months = 354d. 8h. 48m. 38s, and therefore 10d. 21h.Om. 10s. less than the solar year--a difference which is the foundation of the epact.
4. The common lunar civil year, consists of 12 lunar civil months,
= 324 days. 5. The embolismic or intercalary lunar year, consists of 13 lunar civil months = 384 days.
6. The common civil year, contains 365 days, divided into 12 calendar months.
7. Bissextile or leap-year, containing 366 days. See calendar. Zenith, the upper pole of the horizon. Zenith-distance of a celestial body, its distance from the ze. nith, measured on the azimuth-circle passing through the body, and is equal to the complement of the altitude to 90°. Zodiac, a zone or broad circle in the heavens including all
the planets, and extending about 10°, on each side of the
ecliptic. Zodiacal light. a pyramidal lucid appearance, sometimes ob
served in the zodiac, resembling the galaxy, or milky way. It is most plainly observable after the evening twi. light about the latter end of February; and before the morning twilight about the beginning of October. For at these times it appears nearly perpendicular to the horizon. This appearance is generally supposed to be occasioned by the sun's atmosphere.