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times when the observer is certain that he does see the satellite, previous to immersion, and certain that he does not see it, after immersion, the mean of which is to be taken for the true time. A similar method must be adopted for emersion. With common care, Mr. L. thinks that these observations should give the longitude within fifteen or twenty miles; and those made by himself, on board the Conqueror, 74, for six months, at St. Helena, and during the passage home, came much nearer to the truth.
Saturn in conjunction with o, in Leo, on the 25th, difference of latitude 10'.
FORM OF SATURN'S RING.
Semi-conjugate axis — 1.20. A body which weighs one pound at the equator of the earth, would, if removed to the equator of Saturn, weigh 1.01 pounds.
Uranus in opposition to the sun at 5 in the morning of the 9th; the apparent diameter at this time, though at its bearest to the earth, is scarcely 4". It is remarkable that this planet was observed as far back as the year 1690. It was seen three times by Flamstead, once by Bradley, once by Mayer, and eleven times by Lemonnier : not one of whom suspected it to be a planet. This brilliant discovery was reserved for Herschel.
For the positions of the comet of Biela in this month, see “ Cometary Astronomy,” in the Ast, Occ. for October.
Sphere of the Fixed Stars. Altitudes and positions of the fixed stars on the 1st of the month, at 10 in the evening.
Cassiopeia, 45° N.E. Auriga, 10° N.N.E. Perseus, 21° N.E. by N. Andromeda, 30° E.N.E. Aries, 8° E.N.E. Pisces, East. Pegasus, 30° E. by S. Cygnus, 73° E. by S. Aquarius, 25° S.E. Delphinus, 48° S.E. by S. Capricornus, 15° S.S.E. Aquila et Antinous, 40° S. by E. Sagittarius, South. Lyra, 76° S.S.W. Ophiucus, 40° S.W. by S. Scorpio, S.W. by S. early in the horizon. Hercules, 60° S.W. by S. Libra, S.W. by W. Corona, 50° W. by S. Boötes, 36° W. Virgo in the western horizon. Caput Draconis, 76° W.N.W. Coma Berenices, 15° N.W. by W. Ursa Major, 30° N.W. by N. Ursa Minor, 56° N.N.W. Lynx, 15° North.
TELESCOPIC OBJECTS. Lyra. Near Vega, (a in Lyra) is a very faint, and small star. Between y and ß is a round mottled nebula. B is a variable star, maximum and minimum brightness, third and fifth magnitudes; a series of observations have lately been completed with this star, which determined its periodical variation to be 6 days, 10 brs. 40 min. instead of 6 days, 9 hrs. as was formerly supposed ; there is, however, a probability, that the deviation of its maximum, as well as of its minimum brightness, is somewhat irregular. & in Lyra, with a telescope of low power, appears only double; with a higher power, each star is seen to be double; under peculiarly favourable circumstances, a fifth star is visible-constituting ε in Lyra, a quintuple star. (See plate 1, p. 16.)
'Tis midnight-deep silence reigns around,
Contemplative soul to view the starry
Rev. W. Munsey.
COMETARY ASTRONOMY, Motions of Comets. The motions of comets through the system may be divided into apparent and true, being compounded of the earth's motion, and the motion of the comet, each in its respective orbit. When the two bodies are proceeding in different directions, the apparent motion of the comet will exceed its true motion : when moving the same way, its apparent motion will be less than its absolute motion. The apparent velocity has, in some instances, been very great, the comet of 1770 described an arc of 50° in 24 hours; the apparent
swiftness will also be augmented in proportion to the proximity of a comet to the earth, for the arc through which it moves, in any given time, like all other objects, appears larger the nearer it is to the eye.
With respect to the true motions of comets, it may be said, in general, that when at their perihelion, or nearest the sun, they move swiftest, and when at their aphelion, or farthest from the sun, they move slowest. The velocity of some, when at their perihelion, is almost inconceivably great: the comet of 1680, which approached within a very short space of the solar orb, so very near, indeed, that it may be said that its place of perihelion was in the globe of the sun,—this comet went half round the sun in ten hours !
A striking illustration is afforded of the inequality of the motions of cometary bodies, when at these opposite points of their orbits, by the comet of 1811: the period of which, as computed by Lemaur, is 4237 years; the time taken up in traversing the half of its ellipse nearer the sun is only 775 years; the other 3462 years of its revolution being employed in traversing the most remote half.
The velocity of the comet of 1680 has been referred to: its middle distance from the sun is more than 5000 millions of miles, and its greatest distance may be stated as twice as much, (for its nearest is only a 20,000th part of its most remote distance,) so that in its whole revolution it is subject to the greatest extremes. It sees the sun's orb as a vast globe filling the heavens, and, in a lapse of 287 years, it beholds it as dwindled to a point! It bathes its glowing globe in the full effulgence of the solar light, and gradually retreats, till that light almost sinks into the subdued brilliancy of surrounding suns! It wbirls round the sun with tremendous rapidity, and, by degrees, lags in its pace, and, comparatively, creeps in its path! The centrifugal and the centripetal forces are, at one period of the revolution, so intense, that it may be conceived to be possible that one principle would be sacrificed to the other, and the comet either rush to the sun, or fly off never to return ! At the further extremity of its long travel, these forces are so exhausted, that the wanderer glides slowly along its course with dimmed splendor, destitute of its brilliant train, and seems abandoned, in the vast fields of ether, to the random attraction of any neighbouring sun, yet these forces are so adjusted, (though a cannon ball would not reach the aphelion of the comet of 1630 in less than 2260 years,) that the comet can neither rush to the sun when near him, nor leave him when most remote, but is bound to bis glorious chariot, and reined in ultimately, to return in its appointed period of time.
To ascertain the orbit of a comet is the most difficult problem in astronomy; chiefly because no comet is visible through the whole of its course, and rarely seen in those points of its orbit, which enable us, as with the planets, to determine, with accuracy, the figure of the path described. Comets are seldom seen either in opposition, conjunction, or the plane of the ecliptic, which latter being the place of the node, is the most important particular for calculating the elements.
According to the relation which the centrifugal and centripetal forces bear to each other, will be the nature of the path of a moving body, a circle, an ellipse, a parabola, or a hyperbola ; if the velocity of the earth were