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This is a "compare and contrast" question, so we need to focus on similarities and
differences. 3.5 marks represent about 15 minutes' work, so a significant amount of
detail is required.
First, plan: review the careers of the gentlemen in question.
- Johannes Kepler
- used Tycho Brahe's Mars data to develop his three laws of planetary motion;
- tried to develop an explanation for planetary motion using magnetism;
- increased the scale of the solar system by recognising that his calculated orbit
of Mars implied that Mars' parallax was <1';
- invented, but did not actually build, the improved "Keplerian" design of refracting
telescope;
- observed SN1604 ("Kepler's supernova")
- Galileo Galilei
- improved early telescope designs, and was the first person to use the telescope
extensively for astronomical observations;
- discovered mountains and craters on the Moon, the phases of Venus, and the
"Galilean" satellites of Jupiter;
- did some carefully designed experiments on bodies sliding down inclined planes,
which did much to establish what later became Newton's first law;
- got into trouble with the Church as a result of writing a popular book which
was rather too strongly pro-Copernican.
From this list it becomes fairly obvious that the principal difference
between them is that Kepler was primarily a theoretician, whereas Galileo
was primarily an observer. The principal similarity is that both
made major contributions to the establishment of the heliocentric model of the
solar system - Galileo by making discoveries which were much easier to understand in
a heliocentric context, and Kepler by demonstrating that heliocentric
(well, heliofocal) elliptical orbits described planetary motions much better than
any other model.
Another similarity is that both made very important contributions to the use
of the telescope in astronomy - Galileo by demonstrating its superiority to
naked-eye observation, and Kepler by greatly improving the design (the Keplerian
convex eyepiece gives a much better field of view and allows to development of
focal-plane instrumentation such as cross-hairs and micrometers).
Finally, we may note another similarity: both show signs of a modern scientific
outlook, in contrast to most of their predecessors: Galileo by developing the
concept of the properly designed and controlled laboratory experiment, and Kepler
by trying to apply terrestrial physics to planetary motions (the fact that it was the
wrong physics shouldn't obscure the correctness of the concept!).
Note that the broad similarities also involve detailed contrasts: they both
contributed to the telescope, but in different ways; they both supported heliocentric
models, but in different ways; they both introduced modern scientific concepts, but in
different ways.
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This is a "contrast" question, so we focus on the differences. First, summarise
the two states of play:
- Start of 17th century (~1600)
- Three extant models: Ptolemaic (geocentric), Copernican (heliocentric), Tychonic
(planets go round Sun, Sun and Moon go round Earth).
- All three are descriptive; all assume that terrestrial physics does not apply to
celestial bodies; all use only circular motion (with epicycles).
- End of 17th century (~1700)
- Planetary motion governed by Newton's law of gravity, which also describes
falling bodies and projectile motion on Earth and the orbits of the various planetary
satellites (five of Saturn's satellites had been discovered by 1700).
We are asked to contrast, not the models themselves, but their scientific status.
The essential contrasts are
- descriptive vs explanatory:
the early models are purely descriptive: they make no attempt to explain why
the planets move as they do (except by appealing to some assumed "natural tendency"),
whereas the Newtonian model explains all orbits (including those with different
shapes, like comets, and those around different primaries, like satellites) as natural
consequences of an inverse square law of force.
- celestial vs universal:
the early models all assume that the physics of the "sublunary realm" does not apply
to celestial bodies, so they do not use results from terrestrial physics to help in
understanding planetary motions, whereas Newtonian gravity is very explicitly "universal",
applying to all relevant situations, e.g. falling bodies, projectiles, tides,
the Moon's orbit, comets, etc.
It's worth noting that these really are distinct conditions: it is perfectly possible to
imagine an explanatory model which preserved the distinction between celestial and terrestrial
physics (i.e. developing an inverse square law to explain elliptical orbits, without
realising that this also explained the motion of falling bodies). It is a bit more
difficult, but not in principle impossible, to imagine a universal model which is
still descriptive (i.e. asserting that all unconstrained objects describe orbits
around larger masses which have the form of conic sections, but not recognising that this
is a consequence of an inverse square law of force).
Note that in order to make a good job of answering this question, it is essential
to identify the relevant dates correctly! When this question was asked, a surprising
number of candidates contrasted the Ptolemaic and Copernican models, which corresponds
to considering the start and end of the 16th century. The moral is that
you need to go into the examination with a clear mental timeline of who
did what, when, in the history of astronomy.
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