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 2nd 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.