Download Integrative Studies 410 Our Place in the Universe

Naked-Eye Astronomy
The most important graph:
2
1/r
• Force of gravity (and EM) falls off like that
• Brightness of stars goes down like that
• Simple reason:
– things spread out over a sphere
– dilutes over the surface of the sphere
– surface scales like r2
Example: Two 100W light bulbs
• One APPEARS nine time brighter than the
other. How far away is it?
• We can’t compute how far away it is in
absolute terms, but it must be √9=3 times
farther than the other one.
• Which one is brighter?
Appearances
• Don’t be fooled by appearances!
– The sun and the moon appear to be the same
size in the sky (0.5 degrees)
– Alpha Centauri appears to be much dimmer
than the Sun
– Alpha Centauri and Vega appear to be equally
bright
Think!
• The moon and the sun COULD be at
different distances
• Alpha Centauri and Vega COULD be
different types of stars
• Find out  Measure!
• That’s where science starts
Homework
• Typing in formulae in WebAssign:
• Area of circle is pi*r^2
• Webpage:
– http://faculty.otterbein.edu/utrittmann/is2403-02
• WebAssign: www.webassign.net
Peer Instruction: How it works
• Peer instruction is learning by instructing
your fellow students and being instructed by
them
• The process involves 6 steps:
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–
–
–
–
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Mini-lecture by course instructor
Conceptual multiple-choice question is put up
Flash-cards are used to “poll the audience”
A few minutes of discussion between students
“Final answer” via flash-cards
The instructor explains the correct answer
Concept Questions
• Concept questions maybe easy to answer,
but are not simple
• You need background knowledge to answer
them
• They teach you how to use facts and
knowledge to find the answer to a problem
• They test if you got the concept rather than
just knowing facts
Why it works
• Carefully chosen questions
• It is easier to be convinced and to convince
if the reasoning is sound and hence the
answer correct
How
answers are
revised in a
typical
question
Right to right
Wrong to right
Right to wrong
No 2nd answer
wrong to wrong
1
2
3
4
5
What is Astronomy?
• The science dealing with all the celestial bodies in
the Universe
– Cosmology is the branch of astronomy that deals with
the cosmos, or Universe as a whole
• The medieval list of the Liberal Arts: grammar,
rhetoric, logic (trivium); arithmetic, music,
geometry and astronomy (quadrivium)
• Is an “exact science” for ~5000 yrs
– Most rapid advancements in astronomy have occurred
during the Renaissance and the 20th century
– Success has been a result of development and
exploitation of the scientific
method
Why study Astronomy?
• Practical reasons: seasons, tides, navigation, space
technology, satellite communication
• Idealistic reasons: cosmological questions
(“Where do we come from?”), aesthetics, curiosity
“Two things fill the mind with ever new and increasing
admiration and reverence, the more frequently and
enduringly the reasoning mind is occupied with them:
the star spangled sky over me and the moral law in
me.” (I. Kant)
Astronomy and Culture
• Astronomy had and has an enormous
influence on human culture and the way we
organize our lives
• For example:
– The year is the rotation period of the Earth
around the Sun
– The year is subdivided into months, the period
of the Moon around the Earth
– The weeks seven days are named after the
seven bodies in the solar system known in
antiquity: Sunday, Monday, Saturday (obv.),
Tuesday=Mardi, Wednesday = Mercredi,
Thursday=Jeudi, Friday=Vendredi
What’s in the Universe?
Stars
nebulae
molecular clouds
star clusters
THE UNIVERSE
clusters
and
superclusters
galaxies
like the
Milky Way
quasars
voids
Sun
planets
Solar System
black holes
pulsars
moons
comets
meteors
asteroids
dust
terrestrial
jovian
Basic Observations in Astronomy
• Positions of objects (sun, moon, planets, stars …)
• Motion of objects
– with respect to you, the observer
- with respect to other objects in the sky
• Changes (day/night, seasons, etc.)
• Appearance of objects (phases of the moon, etc.)
• Special events (eclipses, transitions, etc.)
 All “in the sky”, i.e. on the Celestial
Sphere
Position: Angles vs. Distances
• Locations in the sky are easy to measure: 2
angles
• Distances from observer are hard (one
length)
 Together they give the location of an
object in three-dimensional space
Angles and Angular Size
• Angles measured in degrees
–
–
–
•
full circle = 360; right angle = 90
1 = 60' (minutes of arc or arc minutes)
1' = 60" (seconds of arc or arc seconds)
Typical angular sizes:
– Moon 0.5, Sun 0.5, Jupiter 20”, Betelgeuse (α Ori) 0.05”
The Trouble with Angles
• Angular size of an object cannot tell us its actual
size – depends on how far away it is
• Sun and Moon have very nearly the same angular
size (30' = ½) when viewed from Earth
Without Distances …
• We do not know the size of an object
• This makes it hard to figure out the “inner
workings” of an object
• We can’t picture the structure of the solar
system, galaxy, cosmos
The most important measurement
in Astronomy: Distance!
• The distances are astronomical!
• The distance scales are very different
–
–
–
–
Solar system: light minutes
Stars: light years
Galaxies: 100,000 ly
Universe: billions of ly
• Need different “yardsticks”
Yardsticks and the Expanding
Universe
• Realizing (measuring) the distances to
objects means realizing how big the
universe is:
– We realized that the solar system is not the
universe
– We realized that our galaxy is not the universe
– We realized that the universe is not static
What’s up in the night sky?
The Celestial Sphere
• An imaginary sphere
surrounding the earth,
on which we picture the
stars attached
• Axis through earth’s
north and south pole
goes through celestial
north and south pole
• Earth’s equator
Celestial equator
Celestial Coordinates
Earth: latitude, longitude
Sky:
• declination (dec)
[from equator,+/-90°]
• right ascension (RA)
[from vernal equinox,
0-24h; 6h=90°]
Examples:
• Westerville, OH
40.1°N, 88°W
• Betelgeuse (α Orionis)
dec = 7° 24’
RA = 5h 52m
What’s up for you?
Observer
Coordinates
• Horizon – the
plane you stand on
• Zenith – the point
right above you
• Meridian – the
line from North to
Zenith to south
…depends where you are!
• Your local sky –
your view depends on your location on earth