Satellite and Ring systems

Galileo Galilei (1564-1642) and The Medicean Stars

Galileo, born in Pisa and raised in Florence, Italy, lived a long, turbulent, but highly productive life. He challenged the scientific and religious authority of his time and paved the way for a new observational astronomy as well as a new physics. His telescopic discovery of Jupiter’s moons convinced him that the Copernican system was correct and eventually led to his 1633 trial before the Inquisition. As a result of the trial, Galileo was made to denounce his beliefs and was sentenced to house arrest in his villa outside of Florence for the rest of his life. Ironically, this far-sighted scientist, who had enlarged the universe a thousandfold, died in 1642 totally blind and confined to the space of his villa. As fate would have it, Isaac Newton was born Christmas day of the same year.

On September 25, 1608, many years before Galileo’s tragic end, a Dutch eyeglass maker by the name of Hans Lippershey applied for a patent on "a certain device by means of which all things at a very great distance can be seen as if they were nearby." But Lippershey was not given a patent because the government officials in the newly formed Dutch Republic felt that the new device could be too easily copied. Indeed, the telescope was already being sold by a Dutch peddler at a yearly autumn fair in Frankfurt, Germany.

Galileo first heard the rumor of the new Dutch invention in May 1609. He immediately obtained some lenses and made his own telescope having a magnification of three times. Over the next few months, by trial and error, he tripled the magnification and donated the instrument to the Venetian Republic. By giving the Venetian Senate the sole right to manufacture this telescope, Galileo hoped to improve his position at the state-supported University of Padua where he had been teaching mathematics since 1592. His contract at the university was, in fact, renewed for life and his salary doubled. However, much to his disappointment he later found that his new contract excluded further salary increases.

Perhaps hoping to improve his position by improving the telescope, Galileo increased the magnification to 20 times and began observing the heavens. He first directed the new instrument at the Moon, writing in January 7, 1610,

. . . it is seen that the Moon is most evidently not at all of an even, smooth, and regular surface, as a great many people believe of it and of other heavenly bodies, but on the contrary it is rough and unequal. In short it is shown to be such that sane reasoning cannot conclude otherwise than that it is full of prominences and cavities similar, but much larger, to the mountains and valleys spread over the Earth’s surface.

Note: All quotes as well as specifics concerning dates are taken from Albert Van Helden’s excellent translation of Galileo’s book Siderius Nuncius, The University of Chicago Press, 1989. Used by permission.

Not all planets were favorably placed for viewing at the beginning of January 1610. Saturn and Mars were located in the sky close to the Sun and at their greatest distance from the Earth while Venus was in the morning sky. But Galileo still noticed that these planets looked like small disks through his telescope. The situation was much better for Jupiter, which had just passed opposition. Rising around sunset, it was the brightest object, except for the Moon, in the evening sky. In his January 7 letter, Galileo mentioned for the first time that

. . . only this evening I have seen Jupiter accompanied by three fixed stars, totally invisible because of their smallness: and the configuration was in this form:

East * * O * West

By about January 11 Galileo realized that the "stars" were not fixed at all but were revolving around Jupiter. He also recognized the great significance of this fact and knew that it would not be long before others made the same discovery. He therefore decided to write a book describing his observations. But being politically astute, he first wrote a letter describing his discoveries to Cosimo de Medici, an ex-pupil, who had become the grand duke Cosimo II of Tuscany. Galileo soon heard from the Grand Duke and his three brothers who all were "astonished by this new proof of [his] almost supernatural intelligence."

On February 13 Galileo wrote to the grand duke’s secretary,

. . . since it is up to me, the first discoverer, to name these new planets, I wish, in imitation of the ancient sages who placed the most excellent heroes of that age among the stars, to inscribe these with the name of the Most Serene Grand Duke. There only remains in me a little indecision whether I should dedicate all four to the Grand Duke only, calling them Cosmian after his name or, rather, since they are exactly four in number, dedicate them to all four brothers with the name Medicean Stars.

Much to Galileo’s surprise he was informed that the grand duke liked the latter choice. On March 13, 1610, he sent the grand duke an unbound copy of his new book and six days later a bound copy along with the telescope that he had used to discover the Medicean Stars. For the book Galileo chose the name Sidereus Nuncius or The Starry Messenger.

The Starry Messenger

In Sidereus Nuncius Galileo reported his observations of Jupiter’s moons between January 7 and March 2, 1610. He made sketches and estimated the angular separations between Jupiter and the moons. He wrote,

That is, two stars were near him on the east and one on the west; the more eastern one and the western one appeared a bit larger than the remaining one. I was not in the least concerned with their distances from Jupiter, for, as we said above, at first I believed them to be fixed stars. But when, on the eighth, I returned to the same observation, guided by I know not what fate, I found a very different arrangement. For all three little stars were to the west of Jupiter and closer to each other than the previous night, and separated by equal intervals, as shown in the adjoining sketch. Even though at this point

East O * * * West

I had by no means turned my thought to the mutual motions of these stars, yet I was aroused by the question of how Jupiter could be to the east of all the said fixed stars when the day before he had been to the west of two of them. I was afraid, therefore, that perhaps, contrary to the astronomical computations, his motion was direct and that, by his proper motion, he had bypassed those stars. For this reason I waited eagerly for the next night. But I was disappointed in my hope, for the sky was everywhere covered with clouds.

 

               Then, on the tenth, the stars appeared in this position with regard to Jupiter.  Only two stars were near him,                 both to the east. The third, as I thought, was  hidden behind Jupiter.

East * * O West

As before, they were in the same straight line with Jupiter and exactly aligned along the zodiac. When I saw this, and since I knew that such changes could in no way be assigned to Jupiter, and since I knew, moreover, that the observed stars were always the same ones (for no others, either preceding or following Jupiter, were present along the zodiac for a great distance), now, moving from doubt to astonishment, I found that the observed change was not in Jupiter but in the said stars. And so, on the eleventh, I saw the following arrangement:

East * * O West

There were only two stars on the east, of which the middle one was three times as far from Jupiter than from the more eastern one, and the more eastern one was about twice as large as the other, although the previous night they had appeared about equal. I therefore arrived at the conclusion, entirely beyond doubt, that in the heavens there are three stars wandering around Jupiter like Venus and Mercury around the Sun. This was at length seen clear as day in many subsequent observations, and also that there are not only three, but four wandering stars making their revolutions about Jupiter. The following is an account of the changes in their positions, accurately determined from then on. I also measured the distances between them with the glass, by the procedure explained above. I have added the times of the observations, especially when more than one were made on the same night, for the revolutions of these planets are so swift that the hourly differences can often be perceived as well.

Thus, on the twelfth, at the first hour of the following night, I saw the stars arranged in this manner.

East * * O * West

The more eastern star was larger than the western one, but both were very conspicuous and bright. Both were two minutes distant from Jupiter.

 

For a view of the sky as Galileo saw it click   Evening of January 7, 1510.

 

Lords of the Rings

In January of 1610, Saturn was close to the Sun and setting in the early evening. It was not well placed for observation. With his newly improved telescope, Galileo was able to see little about this planet except that it looked like a small disk. But in July, Saturn was visible throughout the evening and Galileo discovered that it seemed to have a peculiar appearance. He wrote to the secretary of the Grand Duke Cosimo II of Tuscany:

 

                [T]he star Saturn is not a single one, but an arrangement of three that almost touch each other and never                 move or change with respect to each other; and they are placed on a line along the zodiac, the one in the                 middle being about three times larger than the other two on the sides; and they are situated in this form 

  oOo  

Galileo’s best telescopes were unable to show Saturn’s rings, which appeared as he drew them in his letter. The strange appearance of Saturn was not resolved until 1655 when Christian Huygens established that a ring system encircled the planet.

	StarryNight Movie:  Saturn's Rings
	Project 12: The Moon's of Jupiter

Ground-based telescopes reveal three obvious rings with what appear to be dark gaps between some of the rings. The Voyager spacecraft sent back to Earth some of the most detailed pictures ever obtained of the rings of Saturn. These pictures show that the three major rings are themselves made up of hundreds of thousands of "ringlets." The rings are composed of smallish particles orbiting about Saturn in a very thin disk.