Galileo's The Starry Messenger - Key terms and Ideas - ISLS - Autumn 2003 - Chabot College

Interdisciplinary Studies in Letters & Science

Chabot College

Galileo's The Starry Messenger & Other Readings

Discussion Led by Scott Hildreth

"I do not feel obliged to believe that the same god who has endowed us with sense, reason, and intellect has intended us to forgo their use." -Galileo

Key Terms & Ideas to Discuss

Page Quote/Concept Explanation

11
Astronomy Prior to Galileo
Geocentrism - that the sun, moon, planets, "fixed stars", and everything else, orbited Earth; the Earth did NOT move nor rotate, but was fixed at the center of everything.

Early geocentric theories had the Earth as a flat plate or cylinder; later models had the Earth as a sphere. The planets moved about earth on perfect circular orbits, but the centers of those orbits were not exactly aligned on earth - instead they were placed a distance away and called "eccentric".

Ptolemy (120 AD) was the final and most effective supporter of the geocentric theory, and his model became the standard adopted by the Christian Church, and remained unchallenged for more than 1200 years.

A key element to the geocentric theory was how it tried to model and explain the observed motion of the five wandering planets (Mercury, Venus, Mars, Jupiter, and Saturn), which occasionally seemed to stop in the sky, move backwards for days or weeks, and then move forwards again in front of the background stars. This retrograde motion of the planets does occur. Geocentric models used "epicycles", or little loop-the-loops, to model the motion of planets.

The competing theory to geocentrism was heliocentrism, holding that the Sun, not Earth, was the center of the solar system. This theory put the planets in their proper places outwards from the Sun, and required that the Earth must rotate on its axis (creating day and night) and revolve about the sun in an oribt (creating the seasons and year).

KEY QUESTIONS:

Why was the geocentric theory so popular with the Church? With people?

How would the geocentric theory explain day and night? The moon's phases? The Seasons? The changing constellations we see over the year?

Why was a model needed at all? What could you do with a model of the heavens? (cf. geocentric concept map)

12 Copernicus
Nicolai Copernik (Nicholas Copernicus was his "latinized" name used in his publications) was born the nephew of a powerful bishop in Poland, and grew up with a church-sponsored university education as well as access to the first printed volumes of rediscovered greek works. This renaissance led to the idea of Aristarchus - that the Sun, not Earth, was the center. Copernicus' heliocentric theory accounted for retrograde motion, but posed some difficult problems.

Copernicus ended up keeping epicycles in his model, which meant it wasn't that much easier to use than the Ptolemaic models used by the church.

KEY QUESTIONS:

Why was the heliocentric theory discounted by scientists prior to Galileo? By the Church?

Why was the heliocentric theory more effective in accounting for the observed motions of the planets.

13 Supernova of 1572
A supernova is the explosion of a star that releases so much energy in so short a time that the object is able to be seen - in the daytime - even in the bright sunlit skies. Supernova seen in the night-time sky could be bright enough to cast shadows on the ground; they can be brighter than any of the planets. They brighten quickly and "suddenly" appear, and fade away after a few weeks or a month. About one supernova is seen every 100 years or so within the Milky Way Galaxy, our "home".

The remnant of this explosion is visible today, best seen with X-ray telescopes orbiting the Earth above our atmosphere. Compared with other SNR, the Tycho SN of 1572 indicates the progenitor star was completely destroyed in the blast.

KEY QUESTIONS:

Why would a supernova be interesting or even noticed? What would the sudden appearance of a new star mean to theology?

13 Comet of 1577
A comet is a small (perhaps 10 km in diameter) icy body that typically orbits the sun at great distances, remaining frozen. If its orbit is perturbed by a chance collision or influenced by the pull of a passing neighbor star, the comet can slowly move into the inner solar system, gaining speed as it "falls" and - after crossing the orbit of Mars - heating up until its outer edges begin to turn to gas. The gas and dust released stream away from the nucleus and form the comet's "coma" and "tail" - the latter can stretch up to 10 million miles.

KEY QUESTIONS:

Comets were more common than supernova - Halley's comet returns every 76 years. Why were they important to astronomy, and to our conception of the distances to the planets and stars?

13 Tycho Brahe
A Danish astronomer and instrument builder extraordinaire, responsible for gathering the data used by Johannes Kepler to determine the actual orbits of the planets and the mathematical relationships of those orbits. Tycho was an astute political analyst, and recognized the value in placating church leaders who sought an Earth-centered cosmology in line with their interpretation of the Bible.

His "Tychonic system" placed the Earth at the center, orbited by the moon and sun, but then had the planets orbit the sun.

KEY QUESTIONS:

Tycho is, in many ways, similar to Galileo - outspoken, controversial, and egotistical. He survived by patronage, as did Galileo. But he apparently didn't care much for the WHY questions about planetary orbits and motions; rather, he cared deeply about getting accurate data. Does he offer us another view of what it is to be a "scientist"?

17-18 Johannes Kepler
A German mathematician and theoretical astronomer driven to define the orbits of the planets with single equations; he worked for Brahe in Prague and after Brahe's death, was finally able to define his "laws of planetary orbits."

KEY QUESTIONS:

Kepler, on the other hand is VERY different than Galileo or Brahe - externally. Scientifically, though, his mathematical abilities and perseverance were second to none. What did Galileo do "better", and how did that result in our view that Galileo, and not Kepler, is the "father" of science?

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Chabot College

Page	Quote/Concept	Explanation
11	Astronomy Prior to Galileo	Geocentrism - that the sun, moon, planets, "fixed stars", and everything else, orbited Earth; the Earth did NOT move nor rotate, but was fixed at the center of everything. Early geocentric theories had the Earth as a flat plate or cylinder; later models had the Earth as a sphere. The planets moved about earth on perfect circular orbits, but the centers of those orbits were not exactly aligned on earth - instead they were placed a distance away and called "eccentric". Ptolemy (120 AD) was the final and most effective supporter of the geocentric theory, and his model became the standard adopted by the Christian Church, and remained unchallenged for more than 1200 years. A key element to the geocentric theory was how it tried to model and explain the observed motion of the five wandering planets (Mercury, Venus, Mars, Jupiter, and Saturn), which occasionally seemed to stop in the sky, move backwards for days or weeks, and then move forwards again in front of the background stars. This retrograde motion of the planets does occur. Geocentric models used "epicycles", or little loop-the-loops, to model the motion of planets. The competing theory to geocentrism was heliocentrism, holding that the Sun, not Earth, was the center of the solar system. This theory put the planets in their proper places outwards from the Sun, and required that the Earth must *rotate* on its axis (creating day and night) and revolve about the sun in an oribt (creating the seasons and year). *KEY QUESTIONS:* Why was the geocentric theory so popular with the Church? With people? How would the geocentric theory explain day and night? The moon's phases? The Seasons? The changing constellations we see over the year? Why was a model needed at all? What could you do with a model of the heavens? (cf. geocentric concept map)
12	Copernicus	Nicolai Copernik (Nicholas Copernicus was his "latinized" name used in his publications) was born the nephew of a powerful bishop in Poland, and grew up with a church-sponsored university education as well as access to the first printed volumes of rediscovered greek works. This renaissance led to the idea of Aristarchus - that the Sun, not Earth, was the center. Copernicus' heliocentric theory accounted for retrograde motion, but posed some difficult problems. Copernicus ended up keeping epicycles in his model, which meant it wasn't that much easier to use than the Ptolemaic models used by the church. *KEY QUESTIONS:* Why was the heliocentric theory discounted by scientists prior to Galileo? By the Church? Why was the heliocentric theory more effective in accounting for the observed motions of the planets.
13	Supernova of 1572	A supernova is the explosion of a star that releases so much energy in so short a time that the object is able to be seen - in the daytime - even in the bright sunlit skies. Supernova seen in the night-time sky could be bright enough to cast shadows on the ground; they can be brighter than any of the planets. They brighten quickly and "suddenly" appear, and fade away after a few weeks or a month. About one supernova is seen every 100 years or so within the Milky Way Galaxy, our "home". The remnant of this explosion is visible today, best seen with X-ray telescopes orbiting the Earth above our atmosphere. Compared with other SNR, the Tycho SN of 1572 indicates the progenitor star was completely destroyed in the blast. *KEY QUESTIONS:* Why would a supernova be interesting or even noticed? What would the sudden appearance of a new star mean to theology?
13	Comet of 1577	A comet is a small (perhaps 10 km in diameter) icy body that typically orbits the sun at great distances, remaining frozen. If its orbit is perturbed by a chance collision or influenced by the pull of a passing neighbor star, the comet can slowly move into the inner solar system, gaining speed as it "falls" and - after crossing the orbit of Mars - heating up until its outer edges begin to turn to gas. The gas and dust released stream away from the nucleus and form the comet's "coma" and "tail" - the latter can stretch up to 10 million miles. *KEY QUESTIONS:* Comets were more common than supernova - Halley's comet returns every 76 years. Why were they important to astronomy, and to our conception of the distances to the planets and stars?
13	Tycho Brahe	A Danish astronomer and instrument builder extraordinaire, responsible for gathering the data used by Johannes Kepler to determine the actual orbits of the planets and the mathematical relationships of those orbits. Tycho was an astute political analyst, and recognized the value in placating church leaders who sought an Earth-centered cosmology in line with their interpretation of the Bible. His "Tychonic system" placed the Earth at the center, orbited by the moon and sun, but then had the planets orbit the sun. *KEY QUESTIONS:* Tycho is, in many ways, similar to Galileo - outspoken, controversial, and egotistical. He survived by patronage, as did Galileo. But he apparently didn't care much for the WHY questions about planetary orbits and motions; rather, he cared deeply about getting accurate data. Does he offer us another view of what it is to be a "scientist"?
17-18	Johannes Kepler	A German mathematician and theoretical astronomer driven to define the orbits of the planets with single equations; he worked for Brahe in Prague and after Brahe's death, was finally able to define his "laws of planetary orbits." *KEY QUESTIONS:* Kepler, on the other hand is VERY different than Galileo or Brahe - externally. Scientifically, though, his mathematical abilities and perseverance were second to none. What did Galileo do "better", and how did that result in our view that Galileo, and not Kepler, is the "father" of science?