Interdisciplinary Studies in Letters & Science

Chabot College

Galileo's The Starry Messenger

Pages 50 - 58

Pages
Quote
Interpretation/Comments

50 - 51 "...the discovery of four PLANETS never seen from the creation of the world up to our time, together with the occasion of my having discovered and studied them..."
These are moons, not planets (although Ganymede and Callisto, the two largest, are bigger than the planets Mercury and Pluto).

What does it mean to the church for new moons, never before seen, to be discovered?

51 "I invite all astronomers to apply themselves to examine them and determine their periodic times..." The nature of science demands that any - and every - discovery be independently verified by experienced observers. Note Galileo's warning - that a very accurate telescope is required.

51 "Though I believed them to be among the host of fixed stars, they aroused my curiosity somewhat by appearing to lie in an exact straight line parallel to the ecliptic ..."
The ecliptic refers to the path of the sun and planets in our sky over the year. It represents the plane of our solar system - think of it like a large pancake, which we and the rest of the planets are within. From our vantage point, we see all the other planets in the same disk, which on our sky, appears as a line.

For Jupiter's moon's to lie on the same plane as all of the planets would have definitely provoked interest. The reason they do gives us an idea that the formation of the solar system and the formation of Jupiter and its moons followed similar physical processes.

51 - 56 Diagrams of Jupiter's Moons Note the attempt to catalog what is observed; this is really no different than what Darwin did in the Galapagos.

54 "This night for the first time I wanted to observe the progress of Jupiter and its accompanying planets along the line of the zodiac in relation to some fixed star..." Galileo wants to check that Jupiter AND its moons continue to move together against the background stars. This observation is key to show that the Earth could move and keep its moon.

55 "They marked out an exactly straight line along the course of the ecliptic. The progress of these planets toward the east is seen quite clearly by reference to the fixed star mentioned..."
Again the line-up of the moons offers a clue to their importance and clear relation to our solar system, rather than to the realm of fixed stars.

Numbers mentioned here refer to angles, not times; on the sky, astronomers measure the positions and separations of planets and stars using degrees, minutes (one sixtieth of a degree), and seconds (one sixtieth of a minute). These are the same as our latitude or longitude subdivisions.

56 - 57 "Above all, since they sometimes follow and sometimes precede Jupiter by the same intervals, and they remain within very limited distances either to east or west of Jupiter... no one can doubt that they complete their revolutions about Jupiter and at the same time effect all together a twelve-year period about the center of the Universe." This is the key conclusion from his observations - Earth is not the center of all motion, and Jupiter's moons stay with Jupiter as it moves.

57 "Here we have a fine and elegant argument for quieting the doubts of those who, while accepting with tranquil mind the revolutions of the planets about the sun in the Copernican system, are mightily distrubed tohave the mooon alone revolve about the earth and accompany it in an annual rotation about the sun." Galileo refutes the misconception that eath would lose its moon if we orbited the sun.

http://www.galaxypix.freeserve.co.uk/solarsys/Jupiter/jupiter1.htm

http://www.news.cornell.edu/releases/April00/Simonelli.moons.deb.html

http://www.astrovid.com/mtwilimg.htm

Pages	Quote	Interpretation/Comments
50 - 51	"...the discovery of four PLANETS never seen from the creation of the world up to our time, together with the occasion of my having discovered and studied them..."	These are moons, not planets (although Ganymede and Callisto, the two largest, are bigger than the planets Mercury and Pluto). What does it mean to the church for new moons, never before seen, to be discovered?
51	"I invite all astronomers to apply themselves to examine them and determine their periodic times..."	The nature of science demands that any - and every - discovery be independently verified by experienced observers. Note Galileo's warning - that a very accurate telescope is required.
51	"Though I believed them to be among the host of fixed stars, they aroused my curiosity somewhat by appearing to lie in an exact straight line parallel to the ecliptic ..."	The ecliptic refers to the path of the sun and planets in our sky over the year. It represents the plane of our solar system - think of it like a large pancake, which we and the rest of the planets are within. From our vantage point, we see all the other planets in the same disk, which on our sky, appears as a line. For Jupiter's moon's to lie on the same plane as all of the planets would have definitely provoked interest. The reason they do gives us an idea that the formation of the solar system and the formation of Jupiter and its moons followed similar physical processes.
51 - 56	Diagrams of Jupiter's Moons	Note the attempt to catalog what is observed; this is really no different than what Darwin did in the Galapagos.
54	"This night for the first time I wanted to observe the progress of Jupiter and its accompanying planets along the line of the zodiac in relation to some fixed star..."	Galileo wants to check that Jupiter AND its moons continue to move together against the background stars. This observation is key to show that the Earth could move and keep its moon.
55	"They marked out an exactly straight line along the course of the ecliptic. The progress of these planets toward the east is seen quite clearly by reference to the fixed star mentioned..."	Again the line-up of the moons offers a clue to their importance and clear relation to our solar system, rather than to the realm of fixed stars. Numbers mentioned here refer to angles, not times; on the sky, astronomers measure the positions and separations of planets and stars using degrees, minutes (one sixtieth of a degree), and seconds (one sixtieth of a minute). These are the same as our latitude or longitude subdivisions.
56 - 57	"Above all, since they sometimes follow and sometimes precede Jupiter by the same intervals, and they remain within very limited distances either to east or west of Jupiter... no one can doubt that they complete their revolutions about Jupiter and at the same time effect all together a twelve-year period about the center of the Universe."	This is the key conclusion from his observations - Earth is not the center of all motion, and Jupiter's moons stay with Jupiter as it moves.
57	"Here we have a fine and elegant argument for quieting the doubts of those who, while accepting with tranquil mind the revolutions of the planets about the sun in the Copernican system, are mightily distrubed tohave the mooon alone revolve about the earth and accompany it in an annual rotation about the sun."	Galileo refutes the misconception that eath would lose its moon if we orbited the sun.