The Essential Galileo (11 page)

[§1.6] I have now finished my brief account of the observations which I have thus far made with regard to the moon, the fixed stars, and the galaxy. There remains the matter that seems to me to deserve to be considered the most important in this work. That is, I should disclose and publish to the world the occasion of discovering and observing four planets never seen from the beginning of the world up to our own times, their positions, and the observations made during [80] the last two months about their movements and their changes of magnitude. And I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day owing to the restriction of my time. However, I give them warning again that they will need a very accurate spyglass, such as I have described at the beginning of this account, so that they may not approach such an inquiry to no purpose.

On the seventh day of January of the present year, 1610, at the first hour of the following night, when I was viewing the constellations of the heavens through a spyglass, the planet Jupiter presented itself to my view. As I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, owing to want of power in my other spyglass. That is, three little stars, small but very bright, were near the planet. Although I believed them to belong to the number of fixed stars, yet they made me wonder somewhat, because they seemed to be arranged exactly in a straight line parallel to the ecliptic, and to be brighter than the rest of the stars equal to them in magnitude. Their position with reference to one another and to Jupiter was as follows:

On the east side there were two stars, and a single one towards the west. The star that was furthest towards the east, and the western star, appeared rather larger than the third. I scarcely troubled at all about the distance between them and Jupiter, for, as I have already said, at first I believed them to be fixed stars.

However, when on 8 January, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things. There were three little stars all west of Jupiter, and nearer together than on the previous night; and they were separated from one another by equal intervals as the following illustration shows:

At this point, although I gave no thought at all to the fact that the stars appeared closer to one another, yet I began to wonder how Jupiter could one day be found to the east of all the aforesaid fixed stars when the day before it had been west of two of them. And forthwith I wondered whether the planet might have been moving with direct motion, contrary to the calculation of astronomers, and so might have passed those stars by its own proper motion. I therefore waited for the next night with the most intense longing, but I was disappointed in my hope, for the sky was covered with clouds in every direction.

[81] But on 10 January the stars appeared in the following position with regard to Jupiter:

There were only two, and both on the east side of Jupiter, the third, as I thought, being hidden by the planet. They were situated, just as before, exactly in the same straight line with Jupiter, and along the zodiac. After seeing this, I understood that the corresponding changes of position could not by any means belong to Jupiter. Moreover, I knew that the stars I saw had always been the same, for there were no others either in front or behind, within a great distance along the zodiac. Finally, changing from perplexity to amazement, I became certain that the observed interchange of position was due not to Jupiter but to the said stars. Thus, I thought that henceforth they ought to be observed with more attention and precision.

Accordingly, on 11 January I saw an arrangement of the following kind:

That is, there were only two stars to the east of Jupiter, the nearer of which was three times as far from it as from the star further to the east; and the star furthest to the east was nearly twice as large as the other one. But on the previous night they had appeared nearly of equal magnitude. I therefore concluded, and decided unhesitatingly, that there were three stars in the heavens moving around Jupiter, like Venus and Mercury around the sun. This was finally established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, wandering sidereal bodies performing their revolutions around Jupiter. The following account will report on the observations of these changes of position made with more exactness on succeeding nights. I have also measured the intervals between them with the spyglass, in the manner already explained. Besides this, I have given the times of observation, especially when several were made in the same night, for the revolutions of these planets are so swift that an observer may generally get differences of position every hour.

On 12 January at the first hour of the next night, I saw these heavenly bodies arranged in this manner:

The star furthest to the east was greater than the one furthest to the west, but both were very conspicuous and bright. The distance of each one from Jupiter was two minutes. [82] A third star, certainly not in view before, began to appear at the third hour; it nearly touched Jupiter on the east side and was exceedingly small. They were all arranged in the same straight line, along the ecliptic.

On 13 January, for the first time, four stars were in view in the following position with regard to Jupiter:

There were three to the west, and one to the east. They made almost a straight line, but the middle star of those to the west deviated a little from the straight line towards the north. The star furthest to the east was at a distance of two minutes from Jupiter. There were intervals of only one minute between Jupiter and the nearest star, and between the stars themselves, west of Jupiter. All the stars appeared of the same size, and though small they were very brilliant and far outshone the fixed stars of the same magnitude.

On 14 January the weather was cloudy.

On 15 January at the third hour of the night, the four stars were in a state with reference to Jupiter depicted in the next diagram:

All were to the west and arranged almost in the same straight line; but the star that counted third from Jupiter was raised a little to the north.

The nearest to Jupiter was the smallest of all; the rest appeared successively larger. The intervals between Jupiter and the three nearest stars were all successively equal and of the magnitude of two minutes each; but the star furthest to the west was four minutes distant from the star nearest to it. They were very brilliant, and not at all twinkling, and such they have always appeared both before and since. But at the seventh hour there were only three stars, presenting with Jupiter an appearance of the following kind: