Transcript GeometryAstronomy

Geometry in Astronomy
By Dr. Harold Williams
of Montgomery College Planetarium
http://montgomerycollege.edu/Departments/planet/
Lunar Eclipse for Lunatics
Why do we see phases of the Moon?
• Lunar phases are a
consequence of the
Moon’s 27.3-day orbit
around Earth, relative to
the fixed stars, the
siders, so the sidereal
month is 27.3 days. But
the synodic month a
cycle of phases is
29.53 days.
Phases of the Moon: 29.53-day cycle
new
crescent
first quarter
gibbous
full
gibbous
last quarter
crescent
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waxing
• Moon visible in afternoon/evening.
• Gets “fuller” and rises later each day.
waning
• Moon visible in late night/morning.
• Gets “less” and sets later each day.
What causes eclipses?
• The Earth and Moon cast shadows.
• When either passes through the other’s shadow, we
have an eclipse.
When can eclipses occur?
• Lunar eclipses can
occur only at full
moon.
• Lunar eclipses can
be penumbral,
partial, or total.
How come we don’t we have an eclipse at every new and
full moon?
– The Moon’s orbit is tilted 5° to ecliptic plane…
– So we have about two eclipse seasons each year, with a lunar
eclipse at new moon and solar eclipse at full moon.
What is an ellipse?
An ellipse looks like an elongated circle
What are Kepler’s three laws of planetary motion?
Kepler’s First Law: The orbit of each planet around
the Sun is an ellipse with the Sun at one focus.
Kepler’s Second Law: As a planet moves around its
orbit, it sweeps out equal areas in equal times.
 means that a planet travels faster when it is nearer to the Sun and
slower when it is farther from the Sun.
Kepler’s Third Law
More distant planets orbit the Sun at slower
average speeds, obeying the relationship
p2 = a3
p = orbital period in years
a = avg. distance from Sun in AU
How did Kepler state his third
Law?
2 log(p) = 3log(a)
p = orbital period in years
a = avg. distance from Sun in AU
Colors of Light
• White light is made up of many different colors
I know noting about the stars without looking
at their spectral lines.
Light and Matter:
Reading Messages from the Cosmos
Three Types of Spectra
Special Topic: Polarized Sunglasses
• Polarization describes the direction in
which a light wave is vibrating
• Reflection can change the polarization of
light
• Polarized sunglasses block light that reflects
off of horizontal surfaces
• Demonstrate Polarization and optical
activity to rotate polarization!