Transcript Wednesday, February 19

Motion of the Planets
Science Lecture Series 2014, Otterbein University
Dr. Robert Grubbs, 2005 Nobel Laureate in Chemistry
Professor of Chemistry, California Institute of Technology
-General Lecture (Thursday, 2/20/14, 4 p.m., Riley Auditorium in Battelle
Fine Arts Center): "Green Chemistry: Lessons from Catalysis"
-The general lecture will be followed by a reception in the Science
Building atrium. Both events are free and open to the public.
-Technical Lecture (Friday, 2/21/14, 11:45 a.m., Roush 114):
"Design and Applications of Selective Olefin Metathesis Catalysts"
PLEASE TRY TO ATTEND!
Seasons Debriefing
• If the Northern Hemisphere were tilted 90
degrees towards the Sun, which location
would be warmer in the summer: the Arctic
Circle or Florida?
“Strange” motion of the Planets
Planets usually move from W to E relative to the stars,
but sometimes strangely turn around in a loop, the so
called retrograde motion
MARS 2005
Simulation of Mars 2003 is on this Webpage by C. Seligman
Retrograde Motion
What can we conclude from
observing patterns in the sky?
• Earth OR Celestial Sphere rotates
• Earth rotates around the Sun OR Sun moves
about Earth
• Moon rotates around the Earth or v.v.?
– Must be former, due to moon phases observed!
• Size of the earth from two observers at
different locations
• Size of moon & moon’s orbit from eclipses
Simple observations – profound
Questions
• Just using eyes & brain can provoke
“cosmological” questions:
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Is the Earth the center of the Universe?
How far away are Sun and Moon?
How big are they?
How big is the Earth?
How heavy is the Earth?
Earth or Sun the Center?
• Aristotle (384–322 BC)
– Argued that the planets move on spheres around the
Earth (“geocentric” model)
– Argues that the earth is spherical based on the shape of
its shadow on the moon during lunar eclipses
• Aristarchus (310–230 BC)
– Attempts to measure relative distance and sizes of sun
and moon
– Proposes, nearly 2000 years before Copernicus, that all
planets orbit the Sun, including the Earth
(“heliocentric” model)
Counter Argument
or not?
• Objection to
Aristarchus’s model:
parallax of stars is not
observed (back then)
• Aristarchus argued that
this means the stars must
be very far away
Measuring the Size of the Earth
• Eratosthenes (ca. 276 BC)
– Measures the radius of the earth to about 20%
Documentation discerns subtle Effects
Hipparchus (~190 BC)
– His star catalog a
standard reference
for sixteen centuries!
– Introduces
coordinates for the
celestial sphere
– Also discovers
precession of the
equinoxes
How far away is the Moon?
• The Greeks used a special configuration of
Earth, Moon and Sun (link) in a lunar eclipse
• Can measure EF in units of Moon’s diameter,
then use geometry and same angular size of
Earth and Moon to determine Earth-Moon
distance
That means we can size it up!
• We can then take distance (384,000 km)
and angular size (1/2 degree) to get the
Moon’s size
• D = 0.5/360*2π*384,000km = 3,350 km
How far away is the Sun?
• This is much harder to measure!
• The Greeks came up with a lower limit,
showing that the Sun is much further away
than the Moon
• Consequence: it is much bigger than the
Moon
• We know from eclipses: if the Sun is X times
bigger, it must be X times farther away
Simple, ingenious idea – hard
measurement