Transcript Document

Reminder: HW2 due next
Essay Topic Due: Next
Question: When should we
hold the final?
A total lunar eclipse will be visible over
Arizona skies on February 20, 2008. It
will take place just before sunset in
North America and much of the western
hemisphere. The full moon will pass
into the shadow of the earth and be
visible over Tucson skies at 6:48
p.m. Sky gazers should look to the
eastern horizon.
l = Distance from Alexandria to
D = Circumference of the Earth
l = (7.2/360)*D
l = 5000 stades
D=250,000 stades
1 stade = 157 meters (a Greek
stadium at, for example, Olympia)
D=157*250000 meters
D=39,250,000 meters
D = 39,250 kilometers
Modern Value = 40,070 kilometers
Planets known to the Ancients
• Mercury (Hermes), dim and close to the horizon,
Mercury is difficult to see. He moves quickly
across the sky. Messenger of the gods.
• Venus (Aphrodite), the morning and evening
star, very bright but variable, goddess of love.
• Mars (Ares), the red planet, god of war.
• Jupiter (Zeus), very bright, king of the gods.
• Saturn (Kronos), bright and pale yellow, first of
the Titan’s, father of Zeus.
Motion of the Planets
• The most difficult challenge facing ancient
astronomers was explaining the motion of
the planets. The word “planet” derives
from the Greek “planetes,” which means
wanderer. Planets move in the sky
relative to stars. They also vary in
brightness and, on occasion, even change
direction. This is known as retrograde
Image of the Sky
QuickTime™ and a
are needed to see this picture.
Planets, Sun, Moon are close to
a Great Circle - the Ecliptic
QuickTime™ and a
are needed to see this picture.
Sun (just set)
The Celestial Sphere
Figure from Astronomy: from the Earth to the Universe by Pasachoff
Apparent motion of the Sun
The seasons are caused by the tilt
of the Earth.
From Astronomy: from the Earth to the Universe by Pasachoff
Earth is tilted 23.5º from the plane of the solar system
Retrograde Motion
The Movie
How do we explain all this?
Ptolemy (85-150 AD) lived in Alexandria, Egypt, where he
wrote The Almagest , which recorded a sophisticated
system for describing the motion of celestial bodies.
Learning in the Greek/Roman world
declined in the centuries after Ptolemy,
but was kept alive in the Muslim world.
Almagest means “greatest” in Arabic.
Ptolemy’s Geocentric System, codified
in the Almagest
Figures from Astronomy Today by Chaisson and McMillan
This is getting complicated.
Arguments for a Geocentric Universe
• If the Earth were moving, we would have a
sense of motion. There is no sensation of
motion; therefore, the Earth is not moving.
• We don’t fall off as the Earth speeds ahead.
• The apparent positions of the stars do not
change, i.e. we see no parallax.
• The Earth is an important place and it is natural
that it is at the center of the Universe.
Ptolemy’s Own Defense
• Ptolemy’s model can be criticized on the
grounds that it is cumbersome and lacking
in beauty and symmetry.
• “Let no one, seeing the difficulty of our
devices, find troublesome such
hypotheses. For it is not proper to apply
human things to divine things from such
dissimilar examples.”
• Translation: What seems complicated to
humans may seem simple to the gods.
Summary and a Look Ahead
• The concept of Celestial Spheres allows us to
predict the motion of the stars.
• If we add epicycles and several other “fixes” it is
possible to predict the motion of the planets.
• Ptolemy’s geocentric model provided the most
accurate predictions of celestial motion. It
allowed people to navigate to distant parts.
• The geocentric model is cumbersome and
inelegant. The symmetry that so drove the
ancient Greeks has been lost. Is there an
alternate, simpler model, that can explain the
Layout of the Solar System
From Astronomy: from the Earth to the Universe by Pasachoff