Download Lecture 2

Lecture 2
Astro 1001
6/1/07
Group Work
• When people say “day”, “month”, or “year”
which kind of day, month, and year do they
mean?
• You have a sundial that indicates that the
solar time is 3pm. The longitude of
Minneapolis is about -90 degrees. What
time is it (solar time) in Greenwich,
England (hint: 360/24 = 15)
Stars and Constellations
• About 2000 stars are visible
to the naked eye at night
– Probably ~20 or so in the city
• The constellations that
astronomers used were
determined by the IAU
– Based on European myths
– Asterisms are parts of
constellations
The Celestial Sphere
• No depth perception in space, so it appears
that all of the stars (and planets) lie on a
sphere centered around Earth
– Celestial Poles
– Celestial Equator
– Ecliptic
The Local Sky
• This is the sky that
you personally
observe
– Horizon
– Zenith
– Meridian
• We use these features
to define various
coordinate systems
Celestial Coordinates
• Several different ways
– Altitude and direction (azimuth)
– Right Ascension and Declination
• Declination is how high something is in the
sky
– Can be positive or negative
• RA is how far something is from Meridian
– Measured in units of time
RA and Dec
Angular Sizes
• Angular size is the angle that appears to
separate two objects
– Depends on the distance to the objects
• We use degrees, arcminutes, and arcseconds
– Can write angles as 35°27’35”
More Backyard Astronomy
• Constellations you see
vary with latitude
• Rise and set times of
constellations vary
with longitude
• Circumpolar stars are
those that never rise or
set but simply make
circles
• Constellations along
the ecliptic are called
constellations of the
zodiac
Seasons
• Seasons are related to the tilt of the Earth
• Light is more concentrated when Earth is tilted
towards the Sun
• Important parts of the year
– Solstices
– Equinoxes
Precession
• The Earth is not a perfect sphere
• Things that are not perfect spheres wobble
as they spin
• Earth wobbles on 26,000 year time scales
• Tropic of Cancer is where the Sun is
directly overhead on the summer solstice
Precession cont
Solar Eclipses
• Moon is directly in between Earth and Sun
– Moon must be a new moon
• Umbra is where the Sun is completely blocked
– Usually about 300 km in diameter
• Penumbra is where the sun is partially blocked
– Usually around 7000 km in diameter
Lunar Eclipses
• Must occur when the moon is full
• Lasts longer, more common than solar
eclipses
• Moon often becomes red
Planets
• Planets wander through the zodiacal
constellations (unlike stars)
• Planets exhibit retrograde motion
– Not easily explained with a celestial sphere
Group Work
• Page 50 of your text suggests an experiment
you can do with a friend that demonstrates
retrograde motion. Perform this
demonstration.
Why Nobody Figured This Out
• Greeks (correctly)
thought that a
heliocentric model
should cause stellar
parallax
Heliocentricity cont
• Greeks (correctly) determined that either:
– Earth orbits the Sun but the stars are so far
away that you can’t detect stellar parallax
– There is no parallax because Earth is stationary
• Stars are really far away
– 1 parsec = a parallax arcsecond
Ancient Astronomy
• Many cultures developed astronomy
• One important thing was to identify
equinoxes/solstices
• Some cultures paid particular attention to
the moon
– Metonic cycle allows calendars to be somewhat
synchronized
The Greeks
• We pay particular attention to the Greeks
because they were the first (that we know
of) to rely on natural models
• Greeks usually relied on geocentric models
Famous Greek Philosophers
• Thales
– Successfully predicted
solar eclipse
– Created first scientific
model
• Anaximander
– Invented celestial sphere
– Knew that the Earth had to
be curved
Philosophers cont.
• Pythagoras
– Argued that Earth is a
sphere
– Largely
mystical/aesthetic basis
• Plato/Eudoxus
– Heavenly objects move
in perfect spheres
– Nested spheres account
for different motions
Philosophers cont.
• Aristotle
– Earth was at the center
of everything
• Ptolemy
– Synthesized previous
ideas into a single
model
– Could account for
retrograde motion very
well
Copernicus
• Proposed
heliocentric model
• Primarily due to
aesthetic reasons
– His model was less
accurate than
Ptolemy’s
Tycho Brahe
• Made very high quality naked eye
observations
– Telescope not invented until after his death
• Observed the heavens changing
• He hired Kepler
• Never very successful in creating a model
Johannes Kepler
• At first was still stuck on circles
• Could handle east-west predictions with
circles
– Couldn’t handle north-west variations
• Eventually realized the orbits could be
ellipses
– Lucky!
Ellipses
Kepler’s Three Laws
• Planets orbit the Sun in ellipses
– Sun is at one focus
– Perihelion is the closest point, aphelion is the
most distant point
• Planets sweep out equal areas in equal times
– Planets move a great distance at perihelion, less
quickly at aphelion
The
nd
2
Law
Kepler’s Laws cont
• Distant planets orbit the Sun at slower
average speed, obeying a precise
mathematical relationship
– p2 ά a3
– p2 = k * a3
• Kepler was incorrect about why planets
obeyed his laws
Galileo
• Increased the important of experiments
– Rolling balls demonstrated that Aristotle was wrong
• Observed things that countered astronomical beliefs
at the time
– Moon is imperfect
– Moons orbiting Jupiter
Scientific Method
• Relies on
hypothesis and
predictions
• Hypothesis and
theories are NOT
the same in science
• Observations often
refute hypothesis
Hallmarks of Science
• Science only addressed questions that can
be disproven (in principle)
• Modern science relies only on natural
explanations
• Science progresses through the creation and
testing of models
– Simple models are preferred over complex ones
Verifiable Observations
• Scientific data must be repeatable
• Eyewitness accounts notoriously unreliable
• Objectivity is important
– Individual scientists try to be objective
– Science as a whole is more objective than
individuals
Astrology
• Astrology seeks to explain
human events by the
position of the Sun,
planets, Earth
• Astrology and astronomy
used to be closely related
• Astrological thinking is
really reliant on Earth
centered reasoning
The Basis of Astrology
• Constellations are special
– No, they aren’t
• Positions of planets in constellations are important
– Entirely based on appearances
• Astrologers insist that all planets are important
– Why didn’t ancient astrologers predict other planets
– Why is Pluto important if “Xena” isn’t?
Is Astrology Scientific?
• Nowadays astrologers often don’t make testable
predictions
• Testable predictions turn out to be wrong as often
as chance dictates
– Hundreds of scientific tests have never shown
horoscopes to be more accurate than chance would
allow
• End result: astrology is useless for predicting past,
present, or future events
Group Work
• UFOlogy is the “study” of
UFOs. Almost all people
who practice UFOlogy
believe that UFOs are
explained in part by alien
spacecraft visiting Earth. Is
the basis of such a field
scientific? Why or why not?
For some information, see
page 80 of your textbook.