Part 5

Feb. 11: 1 h.—There were two satellites on the east, and one on the west (Fig. 42). The western satellite was at a distance of 4´ from Jupiter. The satellite on the east, nearest to the planet, was likewise 4´ from Jupiter; but the satellite further to the east was at a distance from this one of 8´; they were fairly clear to view, and in the same straight line; but at the third hour the fourth satellite was visible near to Jupiter on the east, less in magnitude than the rest, separated from Jupiter by a distance of 0´ 30´´, and slightly to the north out of the straight line drawn through the rest (Fig. 43). They were all very bright and extremely distinct, but at 5 h. 30 m. the satellite on the east nearest to Jupiter had moved further away from the planet, and was occupying a position midway between the planet and the neighbouring satellite further to the east. They were all in the same straight line exactly, and of the same magnitude, as may be seen in the accompanying diagram (Fig. 44).

Feb. 12: 0 h. 40 m.—A pair of satellites on the east, a pair likewise on the west, were near the planet (Fig. 45). The satellite on the east furthest removed from Jupiter was at a distance of 10´, and the further of the satellites on the west was 8´ off. They were both fairly distinct. The other two were very near to Jupiter, and very small, especially the satellite to the east, which was at a distance of 0´ 40´´ from Jupiter. The distance of the western satellite was 1´. But at the fourth hour the satellite which was nearest to Jupiter on the east was visible no longer.

Feb. 13: 0 h. 30 m.—Two satellites were visible in the east, two also in the west (Fig. 46). The satellite on the east near Jupiter was fairly distinct; its distance from the planet was 2´. The satellite further to the east was less noticeable; it was distant 4´ from the other. Of the satellites on the west, the one furthest from Jupiter, which was very distinct, was parted from the planet 4´. Between this satellite and Jupiter intervened a small satellite close to the most westerly satellite, being not more than 0´ 3´´ off. They were all in the same straight line, corresponding exactly to the direction of the ecliptic.

Feb. 15 (for on the 14th the sky was covered with clouds), at the first hour, the position of the satellites was thus (Fig. 47); that is, there were three satellites on the east, but none were visible on the west. The satellite on the east nearest to Jupiter was at a distance of 0´ 50´´ from the planet; the next in order was 0´ 20´´ from this satellite, and the furthest to the east was 2´ from the second satellite, and it was larger than the others, for those nearer to Jupiter were very small. But about the fifth hour only one of the satellites which had been near to Jupiter was to be seen; its distance from Jupiter was 0´ 30´´. The distance of the satellite furthest to the east from Jupiter had increased, for it was then 4´ (Fig. 48). But at the sixth hour, besides the two situated as just described on the east, one satellite was visible towards the west, very small, at a distance of 2´ from Jupiter (Fig. 49).

Feb. 16: 6 h.—Their places were arranged as follows (Fig. 50); that is, the satellite on the east was 7´ from Jupiter, Jupiter 5´ from the next satellite on the west, and this 3´ from the remaining satellite still further to the west. They were all of the same magnitude nearly, rather bright, and in the same straight line, corresponding accurately to the direction of the Zodiac.

Feb. 17: 1 h.—Two satellites were in view, one on the east, distant 3´ from Jupiter; the other on the west, distant 10´ (Fig. 51). The latter was somewhat less than the satellite on the east; but at the sixth hour the eastern satellite was nearer to Jupiter, being at a distance of 0´ 50´´, and the western satellite was further off, namely 12´. At both observations they were in the same straight line with Jupiter, and were both rather small, especially the eastern satellite in the second observation.

Feb. 18: 1 h.—Three satellites were in view, of which two were on the west and one on the east; the distance of the eastern satellite from Jupiter was 3´, and of the nearest satellite on the west 2´; the remaining satellite, still further to the west, was 8´ from the middle satellite (Fig. 52). They were all in the same straight line exactly, and of about the same magnitude. But at the second hour the satellites nearest to the planet were at equal distances from Jupiter, for the western satellite was now also 3´ from the planet. But at the sixth hour the fourth satellite was visible between the satellite on the east and Jupiter, in the following configuration (Fig. 53). The satellite furthest to the east was at a distance of 3´ from the next in order; this one was at a distance of 1´ 50´´ from Jupiter; Jupiter was at a distance of 3´ from the next satellite on the west, and this 7´ from the satellite still further to the west. These were nearly equal in magnitude, only the satellite on the east nearest to Jupiter was a little smaller than the rest, and they were all in the same straight line parallel to the ecliptic.

Feb. 19: 0 h. 40 m.—Two satellites only were in view, west of Jupiter, rather large, and arranged exactly in the same straight line with Jupiter, in the direction of the ecliptic (Fig. 54). The nearer satellite was at a distance of 7´ from Jupiter and of 6´ from the satellite further to the west.

Feb. 20.—The sky was cloudy.

Feb. 21: 1 h. 30 m.—Three satellites, rather small, were in view, placed thus (Fig. 55). The satellite to the east was 2´ from Jupiter; Jupiter was 3´ from the next, which was on the west; and this one was 7´ from the satellite further to the west. They were exactly in the same straight line parallel to the ecliptic.

Feb. 25: 1 h. 30 m. (for on the three previous nights the sky was overcast).—Three satellites appeared, two on the east, which were at a distance of 4´ apart, the same as the distance of the nearer satellite from Jupiter; on the west there was one satellite at a distance of 2´ from Jupiter. They were exactly in the same straight line in the direction of the ecliptic (Fig. 56).

Feb. 26: 0 h. 30 m.—A pair of satellites only were present, one on the east, distant 10´ from Jupiter; the other was on the west, at a distance of 6´ (Fig. 57). The eastern satellite was slightly smaller than the western. At the fifth hour three satellites were visible; for, besides the two already noticed, a third satellite was in view, on the west, near Jupiter, very small, which previously had been hidden behind Jupiter, and it was at a distance of 1´ from the planet (Fig. 58).

But the satellite on the east was seen to be further off than before, being at a distance of 11´ from Jupiter. On this night, for the first time, I determined to observe the motion of Jupiter and the adjacent planets (his satellites) along the zodiac, by reference to some fixed star; for there was a fixed star in view, eastwards of Jupiter, at a distance of 11´ from the satellite on the east, and a little to the south, in the following manner (Fig. 59).

Feb. 27: 1 h. 4 m.—The satellites appeared in the following configuration. The satellite furthest to the east was at a distance of 10´ from Jupiter; the next in order was near Jupiter, being at a distance of 0´ 30´´ from the planet. The next satellite was on the western side, at a distance of 2´ 30´´ from Jupiter; and the satellite further to the west was at a distance of 1´ from this. The two satellites near to Jupiter appeared small, especially the satellite on the east; but the satellites furthest off were very bright,

## particularly that on the west, and they made a straight line in the

direction of the ecliptic exactly. The motion of the planets towards the east was plainly seen by reference to the aforesaid fixed star, for Jupiter and his attendant satellites were nearer to it, as may be seen in the accompanying figure (Fig. 60). At the fifth hour the satellite on the east, near to Jupiter, was 1´ from the planet.

Feb. 28: 1 h.—Only two satellites were visible, one on the east, at a distance of 9´ from Jupiter, and another on the west, at a distance of 2´; they were both rather bright, and in the same straight line with Jupiter, and a straight line drawn from the fixed star perpendicular to this straight line fell upon the satellite on the east, as in the figure (Fig. 61). At the fifth hour a third satellite was seen at a distance of 2´ from Jupiter, on the east, in the position shown in the figure (Fig. 62).

March 1: 0 h. 40 m.—Four satellites, all on the east of the planet, were seen; the satellite nearest to Jupiter was 2´ from the planet; the next 1´ from this; the third was 0´ 20´´ from the second, and was brighter than the others; and the satellite still further to the east was at a distance of 4´ from it, and was smaller than the others (Fig. 63). They made a straight line very nearly, only the satellite third from Jupiter was slightly above the line. The fixed star formed with Jupiter and the most easterly satellite an equilateral triangle, as in the figure.

March 2: 0 h. 40 m.—Three satellites were in attendance, two on the east and one on the west, in the configuration shown in the diagram (Fig. 64). The satellite furthest to the east was 7´ from Jupiter; from this satellite the next was distant 0´ 30´´, and the satellite on the west was separated from Jupiter by an interval of 2´. The satellites furthest off were brighter and larger than the remaining satellite, which appeared very small. The satellite furthest to the east seemed to be raised a little towards the north, out of the straight line drawn through the other satellites and Jupiter.

The fixed star already noticed was at a distance of 8´ from the western satellite, that is, from the perpendicular drawn from that satellite to the straight line drawn through all the system, as shown in the figure given.

These determinations of the motion of Jupiter and the adjacent planets (his satellites) by reference to a fixed star, I have thought well to present to the notice of astronomers, in order that any one may be able to understand from them that the movements of these planets (Jupiter’s satellites) both in longitude and in latitude agree exactly with the motions [of Jupiter] which are extracted from tables.

These are my observations upon the four Medicean planets, recently discovered for the first time by me; and although it is not yet permitted me to deduce by calculation from these observations the orbits of these bodies, yet I may be allowed to make some statements, based upon them, well worthy of attention.

[Sidenote: Deductions from the previous observations concerning the orbits and periods of Jupiter’s satellites.]

And, in the first place, since they are sometimes behind, sometimes before Jupiter, at like distances, and withdraw from this planet towards the east and towards the west only within very narrow limits of divergence, and since they accompany this planet alike when its motion is retrograde and direct, it can be a matter of doubt to no one that they perform their revolutions about this planet, while at the same time they all accomplish together orbits of twelve years’ length about the centre of the world. Moreover, they revolve in unequal circles, which is evidently the conclusion to be drawn from the fact that I have never been permitted to see two satellites in conjunction when their distance from Jupiter was great, whereas near Jupiter two, three, and sometimes all (four), have been found closely packed together. Moreover, it may be detected that the revolutions of the satellites which describe the smallest circles round Jupiter are the most rapid, for the satellites nearest to Jupiter are often to be seen in the east, when the day before they have appeared in the west, and contrariwise. Also the satellite moving in the greatest orbit seems to me, after carefully weighing the occasions of its returning to positions previously noticed, to have a periodic time of half a month.[17] Besides, we have a notable and splendid argument to remove the scruples of those who can tolerate the revolution of the planets round the Sun in the Copernican system, yet are so disturbed by the motion of one Moon about the Earth, while both accomplish an orbit of a year’s length about the Sun, that they consider that this theory of the constitution of the universe must be upset as impossible; for now we have not one planet only revolving about another, while both traverse a vast orbit about the Sun, but our sense of sight presents to us four satellites circling about Jupiter, like the Moon about the Earth, while the whole system travels over a mighty orbit about the Sun in the space of twelve years.

[17] In the edition of Galileo’s works published at Florence, 1854, there are given the tables of the hourly movements of the satellites of Jupiter, from which Galileo determined their periods of revolution. In the beginning of his treatise on floating bodies, _Discorso intorno i Galleggianti_, 1611-12, Galileo gives the times of rotation as approximately, (i.) 1 d. 18-1/2 h.; (ii.) 3 d. 13-1/3 h.; (iii.) 7 d. 4 h.; (iv.) 16 d. 18 h.; he also published configurations of the satellites calculated for March, April, and a part of May 1613. The periodic times of the satellites, as corrected by later observers, are, (i.) 1 d. 18 h. 28 m.; (ii.) 3 d. 13 h. 15 m.; (iii.) 7 d. 3 h. 43 m.; (iv.) 16 d. 16 h. 32 m.

[Sidenote: Explanation of the variations in brightness of Jupiter’s satellites.]

Lastly, I must not pass over the consideration of the reason why it happens that the Medicean stars, in performing very small revolutions about Jupiter, seem sometimes more than twice as large as at other times. We can by no means look for the explanation in the mists of the Earth’s atmosphere, for they appear increased or diminished, while the discs of Jupiter and neighbouring fixed stars are seen quite unaltered. That they approach and recede from the Earth at the points of their revolutions nearest to and furthest from the Earth to such an extent as to account for so great changes seems altogether untenable, for a strict circular motion can by no means show those phenomena; and an elliptical motion (which in this case would be nearly rectilinear) seems to be both untenable and by no means in harmony with the phenomena observed. But I gladly publish the explanation which has occurred to me upon this subject, and submit it to the judgment and criticism of all true philosophers. It is certain that when atmospheric mists intervene the Sun and Moon appear larger, but the fixed stars and planets less than they really are; hence the former luminaries, when near the horizon, are larger than at other times, but stars appear smaller, and are frequently scarcely visible; also they are still more diminished if those mists are bathed in light; so stars appear very small by day and in the twilight, but the Moon does not appear so, as I have previously remarked. Moreover, it is certain that not only the Earth, but also the Moon, has its own vaporous sphere enveloping it, for the reasons which I have previously mentioned, and especially for those which shall be stated more fully in my _System_; and we may consistently decide that the same is true with regard to the rest of the planets; so that it seems to be by no means an untenable opinion to place round Jupiter also an atmosphere denser than the rest of the ether,[18] about which, like the Moon about the sphere of the elements, the Medicean planets (Jupiter’s satellites) revolve; and that by the intervention of this atmosphere they appear smaller when they are in apogee; but when in perigee, through the absence or attenuation of that atmosphere, they appear larger. Want of time prevents my going further into these matters; my readers may expect further remarks upon these subjects in a short time.

[18] Modern astronomers agree in assigning an atmosphere to Jupiter, but consider it not extensive enough to affect the brightness of the satellites.—(WEBB, _Celestial Objects for Common Telescopes_.) Their absolute magnitudes are different, and their surfaces have been observed to be obscured by spots, which may account for the variations of their brightness. These spots, like the lunar spots, are probably due to variations of reflective power at different parts of their surfaces, for as they always turn the same face to Jupiter, they present different portions of their surfaces to us periodically, and it has been ascertained by observation that “these fluctuations in their brightness are periodical, depending on their position with respect to the Sun.”—(HERSCHEL, _Outlines of Astronomy_; ARAGO, _Astronomie Populaire_, 1854.)

_Original Configurations of Jupiter’s Satellites observed by Galileo in the months of January, February, and March 1610, and published with the 1st edition of his book_ Sidereus Nuncius, _Venice, 1610._

—————————+———————————+———————————————+——————————————— FIG. | DATE. | EAST. | WEST. —————————+———————————+———————————————+——————————————— 1 | Jan. 7 | | | • • ◯ • —————————+———————————+——————————————————————————————— 2 | 8 | | | ◯ • • • —————————+———————————+——————————————————————————————— 3 | 10 | | | • • ◯ —————————+———————————+——————————————————————————————— 4 | 11 | | | • • ◯ —————————+———————————+——————————————————————————————— 5 | 12 | | | • •◯ • —————————+———————————+—————————-————————————————————— 6 | 13 | | | • ◯ • ⠁ • —————————+———————————+——————————————————————————————— 7 | 15 | • | | ◯ • • • —————————+———————————+——————————————————————————————— 8 | 15 | | | ◯ • • • —————————+———————————+——————————————————————————————— 9 | 16 | | | • ◯ • • —————————+———————————+——————————————————————————————— 10 | 17 | | | • ◯ • —————————+———————————+——————————————————————————————— 11 | Jan. 17 | • • | | • ◯ —————————+———————————+——————————————————————————————— 12 | 18 | | | • ◯ • —————————+———————————+——————————————————————————————— 13 | 19 | | | • ◯ • • —————————+———————————+——————————————————————————————— 14 | 19 | | | • • ◯ • • —————————+———————————+——————————————————————————————— 15 | 20 | | | • ◯ • • —————————+———————————+——————————————————————————————— 16 | 20 | | | • ◯ • • —————————+———————————+——————————————————————————————— 17 | 20 | • ◯ • • | | • —————————+———————————+——————————————————————————————— 18 | 21 | | | • • ◯ • —————————+———————————+——————————————————————————————— 19 | 22 | • ◯• • | | • —————————+———————————+——————————————————————————————— 20 | 22 | | | • ◯ • • • —————————+———————————+——————————————————————————————— 21 | 23 | | | • • ◯ • —————————+———————————+——————————————————————————————— 22 | 23 | | | • ◯ —————————+———————————+——————————————————————————————— 23 | 24 | • • ◯ | | • —————————+———————————+——————————————————————————————— 24 | 25 | | | • • ◯ —————————+———————————+——————————————————————————————— 25 | 26 | | | • • ◯ • —————————+———————————+——————————————————————————————— 26 | 26 | • | | • • ◯ • —————————+———————————+——————————————————————————————— 27 | 27 | | | • ◯ —————————+———————————+——————————————————————————————— 28 | 30 | • | | • ◯ • —————————+———————————+——————————————————————————————— 29 | 31 | • | | • ◯ • —————————+———————————+——————————————————————————————— 30 | Jan. 31 | • | | • ◯ • —————————+———————————+——————————————————————————————— 31 | Feb. 1 | | | • • ◯ • —————————+———————————+——————————————————————————————— 32 | 2 | | | • ◯ • • —————————+———————————+——————————————————————————————— 33 | 2 | | | • • ◯ • • —————————+———————————+——————————————————————————————— 34 | 3 | | | • ◯ • • —————————+———————————+——————————————————————————————— 35 | 4 | | | • • ◯ • • —————————+———————————+——————————————————————————————— 36 | 4 | | | • • ◯ • • —————————+———————————+——————————————————————————————— 37 | 6 | | | • ◯ • —————————+———————————+——————————————————————————————— 38 | 7 | | | • • ◯ —————————+———————————+——————————————————————————————— 39 | 8 | | | • • • ◯ —————————+———————————+——————————————————————————————— 40 | 9 | | | • • ◯ • —————————+———————————+——————————————————————————————— 41 | 10 | | | • • ◯ —————————+———————————+——————————————————————————————— 42 | 11 | | | • • ◯ • —————————+———————————+——————————————————————————————— 43 | 11 | • | | • • ◯ • —————————+———————————+——————————————————————————————— 44 | 11 | | | • • • ◯ • —————————+———————————+——————————————————————————————— 45 | 12 | | | • • ◯ • • —————————+———————————+——————————————————————————————— 46 | 13 | | | • • ◯ • • —————————+———————————+——————————————————————————————— 47 | 15 | | | • • • ◯ —————————+———————————+——————————————————————————————— 48 | 15 | | | • • ◯ —————————+———————————+——————————————————————————————— 49 | Feb. 15 | | | • • ◯ • —————————+———————————+——————————————————————————————— 50 | 16 | | | • ◯ • • —————————+———————————+——————————————————————————————— 51 | 17 | | | • ◯ • —————————+———————————+——————————————————————————————— 52 | 18 | | | • ◯ • • —————————+———————————+——————————————————————————————— 53 | 18 | | | • • ◯ • • —————————+———————————+——————————————————————————————— 54 | 19 | | | ◯ • • —————————+———————————+——————————————————————————————— 55 | 21 | | | • ◯ • • —————————+———————————+——————————————————————————————— 56 | 25 | | | • • ◯ • —————————+———————————+——————————————————————————————— 57 | 26 | | | • ◯ • —————————+———————————+——————————————————————————————— 58 | 26 | | | • ◯ • • —————————+———————————+——————————————————————————————— 59 | 26 | • ◯ • • | | ⨀ Star. —————————+———————————+——————————————————————————————— 60 | 27 | • • ◯ • • | | Star ⨀ —————————+———————————+——————————————————————————————— 61 | 28 | • ◯ • | | Star ⨀ —————————+———————————+——————————————————————————————— 62 | 28 | | | • • ◯ • —————————+———————————+——————————————————————————————— 63 | Mar. 1 | • | | • • • ◯ | | Star ⨀ —————————+———————————+——————————————————————————————— 64 | 2 | • | | • ◯ • | | Star ⨀ —————————+———————————+———————————————————————————————

A PART OF THE PREFACE TO KEPLER’S DIOPTRICS

FORMING

_A CONTINUATION OF GALILEO’S SIDEREAL MESSENGER._

In the preface to Kepler’s _Dioptrics_ there are introduced letters of Galileo about the new and astonishing discoveries which he had made in the heavens by the aid of the telescope since the publication of his work, _The Sidereal Messenger_. The portion of the preface which refers to Galileo, containing these letters and Kepler’s remarks upon them, is added here, as continuing the original account of Galileo’s astronomical discoveries.

_Extract from the Preface of Kepler’s Dioptrics. Augsburg, 1611._

[Sidenote: Kepler remarks on the importance of the application of the telescope to astronomical investigations as indicated by Galileo’s discoveries, published in his _Sidereal Messenger_.]