The Essential Galileo (17 page)

The star furthest to the east was ten minutes from Jupiter; the next was near Jupiter, being at a distance of thirty seconds from the planet; the next star was on the western side, at a distance of two minutes and thirty seconds from Jupiter; and the star further to the west was one minute from this. The two stars near to Jupiter appeared small, especially the one on the east; the outer stars were very bright, particularly the one on the west. They made a straight line in the direction of the ecliptic exactly. The progression of these planets towards the east was plainly seen by reference to the aforesaid fixed star, for Jupiter and its attendant planets were nearer to it, as may be seen in the figure. At the fifth hour, the star on the east near to Jupiter was one minute from the planet.

On 28 February at the first hour, only two stars were visible, one on the east at a distance of nine minutes from Jupiter, and another on the west at a distance of two minutes. They were both rather bright, and in the same straight line with Jupiter. Moreover, a straight line drawn from the fixed star perpendicular to this straight line fell upon the planet on the east, as in this figure:

But at the fifth hour a third star was seen at a distance of two minutes from Jupiter on the east, in the position shown in the figure:

On 1 March at forty minutes, four stars, all on the east, were seen. The one nearest to Jupiter was two minutes from it; the next was one minute from this; the third was twenty seconds from the second, and was brighter than the others; [94] and the one still further to the east was four minutes from the third, and was smaller than the others. They formed a line that was almost straight; only the third from Jupiter was slightly above the line. The fixed star formed an equilateral triangle with Jupiter and the most easterly planet, as in the following figure:

On 2 March at forty minutes, three planets were in attendance, two on the east and one on the west, in the configuration shown in this diagram:

The one furthest to the east was seven minutes from Jupiter and thirty seconds from the next; the one on the west was separated from Jupiter by an interval of two minutes. The outer ones were brighter and larger than the middle one, which appeared very small. The one furthest to the east seemed to be raised a little towards the north, out of the straight line drawn through the others and Jupiter. The fixed star already mentioned was at a distance of eight minutes from the western planet, along the perpendicular drawn from the same planet to the straight line passing through all the planets, as shown in the figure given.

I have wanted to report these comparisons of the position of Jupiter and its adjacent planets to a fixed star so that anyone may be able to understand from them that the movements of these planets both in longitude and in latitude agree exactly with the motions derived from tables.

These are my observations of the four Medicean Planets, recently discovered for the first time by me. Although I am not yet able to deduce by calculation from these observations the orbits of these bodies, I may be allowed to make some statements based upon them, well worthy of attention. In the first place, since they are sometimes behind and sometimes before Jupiter at like distances and deviate from this planet towards the east and towards the west only within very narrow limits of divergence, and since they accompany this planet when its motion is retrograde as well as when it is direct, no one can doubt that they perform their revolutions around this planet while at the same time they all together accomplish orbits of twelve years' duration around the center of the world. Moreover, they revolve in unequal circles, which is evidently the conclusion [95] from the fact that I never saw two planets in conjunction when their distance from Jupiter was great, whereas near Jupiter two, three, and sometimes all four have been found closely packed together. Furthermore, it may be deduced that the revolutions of the planets that describe smaller circles around Jupiter are more rapid, for the satellites nearer to Jupiter are often seen in the east when the day before they have appeared in the west, and vice versa; also the satellite moving in the greatest orbit seems to me, after carefully weighing the timing of its returning to positions previously noticed, to have a periodic time of half a month.

Additionally, we have a notable and splendid argument to remove the scruple of those who can tolerate the revolution of the planets around the sun in the Copernican system, but are so disturbed by the motion of one moon around the earth (while both accomplish an orbit of a year's length around the sun) that they think this constitution of the universe must be rejected as impossible. For now we have not just one planet revolving around another while both traverse a vast orbit around the sun, but four planets which our sense of sight presents to us circling around Jupiter (like the moon around the earth) while the whole system travels over a mighty orbit around the sun in the period of twelve years.

Lastly, I must not pass over the consideration of the reason why it happens that the Medicean Stars, in performing very small revolutions around Jupiter, seem sometimes more than twice as large as at other times. We can by no means look for an explanation in the mists of the earth's atmosphere, for they appear increased or diminished while the discs of Jupiter and the neighboring fixed stars are seen quite unaltered. It seems altogether untenable 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 such great changes, for a strict circular motion can by no means produce those phenomena; and an elliptical motion (which in this case would be almost rectilinear) seems to be both unthinkable and by no means in harmony with the observed phenomena. But I gladly offer the explanation that has occurred to me upon this subject, and I submit it to the judgment and criticism of all true philosophers. It is known that when atmospheric mists intervene, the sun and moon appear larger, but the fixed stars and planets smaller; hence the former luminaries, when near the horizon, are larger than at other times, but stars appear smaller and are frequently scarcely visible; and 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, [96] for the reasons which I have previously mentioned, and especially for those that shall be stated more fully in my
System;
and we may accordingly decide that the same is true with regard to the rest of the planets; so it seems to be by no means an untenable opinion to place also around Jupiter an atmosphere denser than the rest of the aether, around which, like the moon around the sphere of the elements, the Medicean Planets revolve; then by the interposition of this atmosphere, they appear smaller when they are at apogee; but when in perigee, through the absence or attenuation of that atmosphere, they appear larger. Lack of time prevents me from going further into these matters; my readers may expect further remarks upon these subjects in a short time.