EMERSIONS. 1st Satellite, 3d day, at 18 m. after 5 evening 48 Other Phenomena. 5 Venus will be stationary on the 6th of this month; and in her inferior, conjunction at 3 in the afternoon of the 26th. Mercury and Jupiter will be in conjunction on the 21st, when Mercury will be 79' south of Jupiter. Mercury will also attain his greatest elongation on the 22d. Uranus will be in conjunction on the 12th, at a quarter before 10 in the evening; and the Moon will be in conjunction with in Scorpio at 35 m. after 6 in the evening of the 25th of this month, On the MOTION and ABERRATION of LIGHT. [Concluded from p. 293.] The velocity of light having been discovered by Roemer, as stated under the head of Astronomical Occurrences in last month, it was afterwards very satisfactorily confirmed by Dr. Bradley, at the same time that he discovered its aberration, which is one of the most curious and delicate discoveries noticed in the history of the celestial phenomena. Aberration is a certain apparent motion of the heavenly bodies arising from the progressive motion of light, and the annual motion of the Earth in her orbit. This apparent motion of the heavenly bodies is so very minute, that it could never have been discovered except by the most accurate observations: it was not even so much as suggested by theory. The history of human knowledge presents numerous instances in which the means that have led to the discovery of important facts are not less singular than the facts themselves. A fortunate accident frequently leads to truths which the most abstruse speculations could never have developed. The absurd attempts that were made to find the philosopher's stone gave rise to some of the noblest discoveries in chemistry; and in the present case, those observations that were intended to ascer tain the parallax of the stars, terminated in confirming the motion, and making known the aberration of light. Before the time of Roemer, it was generally conceived that the motion of light was instantaneous; but from observations made on the eclipses of Jupiter's satellites and the apparent change of place of the fixed stars, this is now known to be incorrect. Nor is this removal of error the only advantage that resulted from the discovery; since it necessarily proves the motion of the Earth in its annual orbit, and consequently affords a new confirmation of the truth of the Copernican system. Dr. Bradley related the history of this discovery in No. 406 of the Philosophical Transactions; which exhibits the knowledge, sagacity and perseverance of that eminent astronomer in a strong light; but our limits oblige us to refer to the doctor's own account for the particulars, and only permit us to make the following remarks on this subject. The annual motion of the Earth had been much doubted, and was the subject of a warm contest among the philosophers of that time. Those who defended that motion conceived the idea of deriving an incontestible proof from the annual parallax of the fixed stars, if these should be within such a distance from the Earth as to render this parallax sensible, and instruments could be obtained and observations made that were capable of ascertaining it. Before M. Picard made his observations on the pole star, in 1672, it was thought that the stars did not change their positions; but he conceived that he had discovered, from a series of observations continued for about 10 years, that this star had an apparent motion of about 40" a year. In 1674, Dr. Hook published an account of his observations on the star y Draconis, which he thought was 23′ more northerly in July than it was in October. Flamsteed found that the declination of the pole star, in 1689, was 40" less in July than in December. In this state of uncertainty with respect to the apparent motion of the fixed stars, and the cause from which it sprung, Dr. Bradley and the Honourable Samuel Molineux undertook a series of observations, in 1725, for the purpose of verifying those which Dr. Hook had made about half a century before. The instrument with which they made their ob servations was much more accurate than those which had been previously used for the same purpose. The first observation was made on the 3d of December 1725, and the situation of the bright star in the head of Draco carefully marked. In the beginning of the following March, this star was found to be 20" more southerly than it was at the time of the first observation. From that time the star continued to move northward till the following September, when it was found to be about 20" more towards the north than it was when first observed, and more than 39" to the north of the point it had occupied in the preceding March. From September it again appeared to return towards the south, till in December it arrived at the situation in which it was first observed, allowing for the difference of declination occasioned by the precession of the equinox. Having ascertained this fact, Dr. Bradley continued his observations till he had not only fully satisfied himself of the real existence of these phenomena, but also of the general laws which they follow. He then began to search for the cause by which they were produced, and which their regularity had indu ced him to believe was permanent. The first idea that suggested itself was the nutation of the Earth's axis; but this he soon found was insufficient to account for the phenomena as given by observation. The next suggestion was an alternation in the direction of the plumb line, by which the instrument was adjusted; but this he soon found to be insufficient also. The effects of refraction were then applied, but without affording any satisfaction on the subject. At last his natural sagacity, aided by continued reflection, enabled him to perceive that the progressive motion of light, combined with the annual motion of the Earth in her orbit, would fully explain all the phenomena. This arose from conceiving that, if the motion of light was not instantaneous, the apparent place of an object would not be the same when the eye was in motion as when it was at rest, except the motion was in a line passing through the object and the eye; and that, if the eye were in motion in different directions, the apparent place of the object would of course be different. Hence the motion of the Earth about the Sun being elliptical, the apparent motion of the star ought to participate of the same nature; which was agreeable to the observations. The following illustration and the annexed diagram will render the aberration of light more familiar to such of our readers as are not already conversant with the subject. D h s Let AB represent a part of the Earth's orbit, and CB a ray of light proceeding from a star perpendicular to the line AB. Then if the eye were at rest at B, the object would appear in the direction BC, whether the transmission of the light was instantaneous or progressive; but if the eye was in motion from A towards B, and the motion of light was such that its velocity was to the velocity of the eye as BC to BA, that particle of it by which the object would be discerned when the eye comes to B will be at C when the eye is at A. The star will therefore appear to be in the direction AC; and as the Earth moves through the equal parts of its orbit denoted by Aa, ab, bc, &c., the light which proceeds from the star and reaches the eye of the observer at B, will appear B successively in the directions ae, bf, cg, &c., which are parallel to the former AC; so that, when the eye of the observer arrives at B, the object will appear in the direction BD. Though the real place of an object is in a line perpendicular to that in which the eye of the observer moves, yet the apparent place of the same object will be in the direction in which the ray of light meets the eye; and the difference between the true and apparent places will therefore be greater or less according to the difference between the velocity of light and that of the eye. From numerous observations, made during a space of three years, Dr. Bradley found that the apparent places of the stars differed from their true places by about 20"; by which it is proved that the velocity of light is about 10313 times as great as the velocity of the earth in her orbit; from which he found that light passed from the Sun to the earth in 8' 7", which is very nearly the same time as had been deduced by Roemer, from very different phenomena, These, therefore, mutually confirm each other, and establish the progressive motion of light beyond a doubt. The time, however, which light actually occupies in passing from the Sun to the earth, as determined by the most accurate observations of more modern astronomers, is 8′ 13′′. From the rules which were given by Dr. Bradley, and demonstrated by subsequent mathematicians, the following results have been obtained. Each of the stars appears to describe, in the space of a year, by means of its aberration, a small ellipse, having its transverse axis equal to 40", and the conjugate axis, which is perpendicular to the ecliptic, equal to the transverse multiplied by the sine of the star's latitude, the radius being 1. The greatest aberration in longitude is equal to 20" divided by the cosine of the star's latitude; and the aberration in longitude for any time is equal to 20" multiplied by the cosine of the elongation of the star Dd |