Motion of the Planets

PLEASE TRY TO ATTEND!

Science Lecture Series 2014, Otterbein UniversityDr. 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"

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: – 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

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