Astroblog

Comparing Galileo's drawing of Jupiters Moon's (top) with a modern prediction for the Moon's appearance at that time (Below). For a drawing using a fairly ordinary telescope , drawn by hand and then made into a woodcut, it's pretty accurate (scaling between the images is different, sorry).

The small stars in Galileo's diagram represent Jupiter's Moons.


Continuing my reading of Galileo's Sidereus Nuncius (The Starry Messenger).

Galileo's audience have been shocked by the revelation that the Moon was not a perfect object. They were stunned when the Milky was was revealed to be not an etheric glow in the celestial spheres, but myriads of stars to small to be resolved to the unaided eye. But the next revelation was the most amazing.

Galileo was the first to discover completely new worlds.

On the seventh day of January in this present years 1610, at the first hour of night, when I was viewing the heavenly bodies with a telescope, Jupiter presented itself to me. And because I had prepared a very excellent instrument for myself, I perceived (as I had not done before on account of the weakness of my previous instrument) that there were three bright starlets beside the planet, small indeed, but very bright.

The " very excellent instrument" was probably no more than 20x in magnification (you can get a replica yourself). Now, Galileo was not expecting new planets, indeed nothing would have lead anyone to expect there could be more than the 7 classical planets. But he did note that the three starlets to the east of Jupiter were aligned with the ecliptic. Unusual and interesting, but not the harbinger of new worlds. Imagine yourself looking at three tiny stars near Jupiter, how would your realise that these dots were Moons, not stars?

The next night though the three stars were on the western side of Jupiter.

Now, as Jupiter was travelling westward at the time, there was no way that fixed stars could turn up on the western side of Jupiter. The penny still hadn't dropped, but Galileo now sets watch on Jupiter and the starlets. After two further observations, it was clear that the starlets never moved far from Jupiter, and were travelling in the ecliptic line. Galileo writes:

I now decided beyond all doubt that there existed in the heavens three stars wandering about Jupiter as do Venus and Mercury around the Sun, ... [at this time Galileo had not seen all four Moons]

Note how cheekily Galileo inserts a heliocentric idea in the text, true it could be referring to Tycho Brahes system, but throughout the Starry Messenger there is more than a hint of heliocentrism. However, his discoveries were so astonishing that most people ignored the heliocentrism and attacked the existence of mountains on the Moon, and Moons around Jupiter.

Most of the rest of the book is list after list of Jovian Moon positions. Given that the man had just discovered new worlds, the dry list may come as some surprise. However, it had an important point, and people scoured this section very attentively. Galileo had to establish beyond doubt that the starlets really did circle Jupiter. One way was to establish orbital periods for the starlets, which require two things, accurate measurement of the starlet positions, and some way to identify individual Moons. The furthest starlet was easy, no other Moon went so far from Jupiter, but distinguish three near identical points of light as they merged and separated was an enormous task. Galileo did not achieve it in the Starry Messenger, but did later on. He did get an approximate orbital period for the outermost starlet (IV, which we now call Callisto), and this approximate period got him in trouble later. Also, he was able to show by comparing the motion of Jupiter and the starlets to the fixed stars, that the starlets moved in latitude and longitude exactly with Jupiter. The Moons orbited Jupiter.

But now we have not one planet revolving around another while both run through a great orbit around the sun; our own eyes show us four stars that wander around Jupiter as does the Moon around the earth, while all together trace out a grand revolution about the sun in the space of twelve years.

Naughty Galileo again! More heliocentrisim snuck in. He also resorted to atmospheres to explain the apparent dimming of the Moons as they approached Jupiter (this is really an optical effect due to the brightness of Jupiter).

The existence of the Moons was attacked as soon the the Starry Messenger was published. There was some reason to be sceptical. Galileo's telescopes were far from perfect instruments, and there were know artefacts in them. But Galileo's careful observations put paid to those sorts of arguments. The orbital features of the starlets could not be explained by mere artefact.

Many attacks were supremely foolish, even by the standard of the times. Francesco Sizzi argued that for metaphysical reasons, there could be no more than seven planets, therefore the Medician stars could not be real. Sizzi had even seen the Medician stars himself. But Sizzi also had a more subtle argument up his sleve. If the Mediciean stars were Moons, he said, then they should show a consistent periodic orbit like our Moon. But the orbits he derived from Galileo's observations did not agree with Galileo's stated orbital period. Which was because Sizzi had based his orbits on an incorrect figure. There were several observational errors in the printed work, not surprising given the difficulties of making measurements with the primitive telescope, and Sizzi started his calculations from an erroneous position for Callisto, and them compounded his problem by taking Galileo's approximate orbital period as an exact one.

At this remove, it is hard to understand these attacks, but in those days mere measurement was not match for metaphysics. People were supposed to start from metaphysical reasoning and argue to a conclusion. Observations and measurements came a poor second. However, eventually people like Sizzi were won around by just these measurements. After extended conversations with Galileo, Sizzi derived reasonable orbits and became his supporter.

Gallileo's Starry Messenger is a slim book, most of it is taken up with dry recitations of the positions of the Medician stars; Jupiter's Moons. But it held twin revolutions. It began the shattering of the Ptolemaic astronomy, and heralded the heliocentric solar system. And in shattering the Aristotelian version of the cosmos, and emphasising precise and careful measurement, it ushered in the modern era of science, where metaphysics was discarded and observation and careful testing of hypotheses became the standard.

Twin revolutions, started by a tube that today we would consider a toy.

Blogging the Starry Messenger - Introduction
Blogging the Starry Messenger - The Telescope
Blogging the Starry Messenger - The Moon
Blogging the Starry Messenger - The Stars

Labels: blogs, Galileo, Science Blogging