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led to infer that each of the satellites rotated on its axis in the same time that it made a sidereal revolution round its primary thus presenting an analogy to the case of our Moon. The immediate reason which led to this conclusion was a belief that the variation in their brilliancy always recurred in nearly the same positions of the satellites with respect to Jupiter and the Sun, which supposition had previously presented itself to the mind of Cassiniy. But modern observations do not harmonise with these statements; that is to say, we are not entitled to affirm now that peculiarities in the appearances of the satellites correspond with definite orbital positions. On the contrary, the peculiarities observed are not governed by any known law of time or place.

Arago thus summed up Sir W. Herschel's photometric deductions. “The Ist satellite is at its maximum brightness when it attains the point of its orbit which is almost midway between the greatest Eastern Elongation and its Conjunction. The brightest side of the IInd satellite is also turned towards the Earth when that body is between the greatest Eastern Elongation and Conjunction. The brightness of the III'd satellite attains 2 maxima in the course of a revolution, namely at the 2 Elongations. The IVth shines with a bright light only a little before and a little after Opposition?."

Various observers have assigned colours, or rather tinges of colour, to the different satellites, but the results are not sufficiently of accord to be worth citing.

Eclipses as viewed on Jupiter take place on a grand scale; for in consequence of the small inclinations of the orbits of the satellites to the planet's equator and the small inclination of the latter to the ecliptic, all the satellites, the IVth excepted, are eclipsed some time in every revolution; so that a spectator on Jupiter might witness during the Jovian year 4500 eclipses of the Moon (Moons) and about the same number of the Sun.

Soon after their discovery it suggested itself to the reflecting mind of Galileo that eclipses of the satellites of Jupiter might be made useful for determining the longitude. Regarding eclipses as instantaneous phenomena visible at the same moment in every place which has the planet above its horizon, it is clear that a comparison of observations recorded in 2 local times would afford data for determining the difference of time (longitude) between the places to which the times belong. Eclipses accurately predicted for one meridian when observed under another one would afford a still more advanced means of ascertaining the difference of longitude between them. These eclipses could be predicted if sufficiently accurate tables of the satellites were in existence ; but at sea, where the problem has chiefly to be solved, they cannot be observed with the most refined accuracy, and on land some difficulties present themselves ; so that the method to some extent breaks down, and is only available where very rough approximations will suffice.

y Mém. Acad. des Sciences, vol. i. p. 266. < Pop. Ast., vol. ii. p. 549. Eng. ed.

It was to observations of one of the satellites of Jupiter, and Römer's discussion of them in 1675, that we owe the discovery that light is not propagated instantaneously through space". It was found that the calculated times of the eclipses did not correspond with the observed times, and that the difference was a quantity constantly affected by opposite signs of error according as Jupiter was in perigee or apogee. In the former case the eclipse always occurred before the calculated time; in the latter, always after it. The regularity with which these anomalies showed themselves led Römer to suspect that they had their origin in the variations which occurred in the distance of Jupiter from the Earth: that as this distance increased or diminished so a longer or a shorter period was requisite for light to traverse the space between the 2 planets. Assuming from the data in his possession that light travelled at the rate of 192,000 miles per second, and required 164m to traverse the diameter of the Earth's orbit, and applying this (as yet hypothetical) conclusion to the eclipses in the form of a trial-correction, Römer promptly obtained proofs of the accuracy of his reasoning; but it was Bradley's discovery

* Opere di Galileo, vol. ii. p. 33. Padua ed., 1744.

of aberration some half a century later which completely demonstrated the soundness of Römer's views and caused their general acceptance. The modern experiments of Fizeau have given for the velocity of light a result but slightly differing in amount from Römer's, namely, 194,000 miles per second b.

Like most new discoveries Römer's did not, when promulgated, find favour in the scientific world, and many years elapsed ere it was generally accepted.

The mass of Jupiter has never been a very doubtful quantity, all the values of it being much more in accord with one another than is usually the case. Laplace, from Pound's observations of the IVth satellite, placed it at Toot; Bouvard, from the perturbations of Saturn, at ioro; Nicolai, from the perturbations of Juno, at toiseyi; Encke, from the perturbations of Vesta, at Toso; and from perturbations of the Comet bearing his name, at 10'57; Santini at 105o; Bessel at Tomas7; Airy, from motions of the satellites, at oan; Krüger, from observations of Themis, at 10 :57; Jacob, from the motions of the satellites, at 101.57; and Möller, from the motions of Faye's Comet, at 1024.78; Schur, from heliometer measures of the satellites, at ooitty. Any one of the 4 last values may be taken to be substantially exact.

“ The most ancient observation of Jupiter which we are acquainted with is that reported by Ptolemy in Book X. chap. iii. of the Almagest, and considered by him free from all doubt. It is dated in the 83rd year after the death of Alexander the Great, on the 18th of the Egyptian month Epiphi, in the morning, when the planet eclipsed the star now known as 8 Cancri. This observation was made on Sept. 3, B.C. 240, about 18h on the meridian of Alexandria.”

This is a convenient place to mention the “Great Inequality" in the motion of Jupiter and Saturn, so far as the fact of its existence is concerned, for a particular account of it would be altogether foreign to the purposes of this work. The period of each of these planets is subject to a continuous change owing to the mutual influence exerted by each on the orbit of the other and the time required for this change to go through its various stages is the Period of the Great Inequality. It amounts to 918 years.

b In consequence of the increase in the received value of the Sun's parallax a reduction in the velocity of light by several thousands of miles per second must be assumed, and singularly enough some experiments of Foucault's made

before the parallax question came up for general discussion pointed to the same conclusion. The value for the velocity of light now generally accepted is about 186,660 miles per second.

The Tables of Jupiter used till recently were those of A. Bouvard, published in 1821, but the new and far superior Tables of Le Verrier have superseded thema. For the satellites, Damoiseau's Tables (published in 1836) are employed. As regards the satellites there is room for much improvement in the Tables at present employed. They fail to give results characterised by the precision which modern science demands.

+ See Sir J. Herschel's Outlines, p. 502.

These tables were employed for the

first time in England in the preparation of the Nautical Almanac for 1878.

CHAPTER XII.

SATURN. ħ

Period, &c.Figure and Colour of Saturn.-Belts and Spots.- Observations of

the Belts by Holden.-By Ranyard.--Bright spot recorded by Hall.-Probable atmosphere.- Observations of Galileo, and the perplexity they caused.-Logogriph sent by him to Kepler.-Huygens's discovery of the Ring.--His logogriph.The bisection of the Ring discovered by the brothers Ball.Sir W. Herschel's Doubts.-Historical epitome of the progress of discovery. The DuskyRing.Facts relating to the Rings.- Appearances presented by them under different circumstances.Rotation of the Ring.--Secchi's inquiries into this.The Ring not concentric with the Ball.-Measurements by W. Struve.Other measurements.--Miscellaneous particulars.Theory of the Ring being fluid.-Now Thought to consist of an aggregation of Satellites.-- The Beadedappearance of the Ring.-0. Struve's surmise about its contraction.--Irregularities in the appearances of the ans@.-Rings not bounded by plane surfaces.Molintains suspected on them.- An atmosphere suspected.-Physical obxerrations between 1872 and 1876 by Trourelot.- Observations by MM. Henry.By Keeler.- Brightness of Rings and Ball.-Bessels incestigations into the Mass of the Rings.-Saturn attended by 8 Satellites.Table of them.Physical data relating to each.-- Elements by Jacob.- Coincidences in the Rotation-periods of certain of them.Transits of Titan.- Celestial phenomena on Saturn.-Lockyer's summary of the appearancex presented by the Rings.Peculiarity relative to the illumination of Iapetus.Mass of Saturn.Ancient obserrations.-Saturnian Astronomy.

TNFERIOR in size to Jupiter only, Saturn may fairly be pro1 nounced to be the most interesting member of the Solar System. It revolves round the Sun in 10759•20 or 2945at a

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