Transcript Wednesday, August 27 - Otterbein University

Patterns in the Sky & Constellations
Don’t be fooled by what you see!
• 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
• Are you upside down?
Think!
• The moon and the sun COULD be at different distances
• Alpha Centauri and Vega COULD be different types of
stars
• YOU could be upside down!
• The apple COULD attract the Earth with the same force
 Expand your universe!
Top Down
• The Universe is accelerating its Expansion
– How do we know?
• Supernovae are dimmer than they should be
in a standard expanding universe
– What is a supernova? What is a standard
universe?
• SN are massive stars exploding at the end of
their lives
– How do we know?
Top Down
• Stars are hot gas balls that fuse H to He;
they run out of fuel
– How do we know?
• Can measure spectra, compare to the sun
– What is a spectrum? How does the sun “work”?
• The sun is 300,000 more massive than the
earth, consists of H & He, produces a lot of
energy  must be fusion
– How do we know?
Top Down
• Measure distance to sun, use Newton
gravity to obtain mass, measure H & He
spectra in lab
– How do we measure distance to sun?
– What is Newton gravity?
– What are spectra in the lab?
• Use Kepler’s laws, observe special
configuration of planets from different
positions on earth
– What are Kepler’s laws? How big is Earth?
Top Down
• Planets go around the sun in ellipses
– How do we know?
• The observer’s view is different for
different places on Earth  radius
– How do we know
Look at the sky!
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
Our vantage point: Earth
Basic Observations in Astronomy
• We see (on clear days!):
– A very bright disk that is up about 12 hours. It comes up in
a specific direction, rises higher until it reaches a maximal
altitude in a second direction, then sinks lower until
disappearing in a direction opposite of the direction where
is came up
– A less bright object that changes its appearance and is also
up for a (different) duration of 12 hours. Same rise/set
pattern as very bright object.
– When the very bright disk is not visible, we see many tiny
specs of light of different brightness and color
Basic Observations in Astronomy
• We see further:
– The tiny specs move across the sky as the hours go by. One
group moves across the sky in 12 hours. Same rise/set
pattern as bright object.
– The position of the specs wrt other specs is fixed, but they
move wrt to the ground
– Careful observation reveals a handful of exceptions from
this rule:
• some bright specs move slowly wrt to the other fixed specs, and also
are visible 12 hours. Same rise/set pattern as bright object.
• One spec sits at the center of this motion and does not move
Conventions
• These patterns repeat every day, let’s name
them
–
–
–
–
–
–
Sun
East, South, West
Moon
Daytime + Nighttime = Day (needs to be revisited later!)
Planets
Polaris, the North Star
More names, now that we’ve seen
Observer
Coordinates
• Horizon – the
plane you stand on
• Zenith – the point
right above you
• Meridian – the
line from North to
Zenith to south
Hypothesis
• During a day, it looks like all “lights in the
sky” travel around us, like the are fixed to
an (invisible) sphere that turns around us.
• Call it The Celestial Sphere
Further Observation
• If we move to a new observing place on
Earth, the pattern remains the same (bright
light rises & sets, etc.), but:
– Position of North Star changes
– Maximal altitude of Sun, special stars changes
Conclusion: Earth’s coordinates
projected onto 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
Confusing! Let’s go with Patterns
in the Sky!
• We can group specs of light together to form
triangles, squares, etc.
• This allows us to find them the next night and
follow their motion
• Talk to other observers, and give them
names: Bear, Bull, Lion, Hunter, Queen, etc.
 The Constellations
Constellations of Stars
• About 5000 stars visible with naked eye
• About 3500 of them from the northern hemisphere
• Stars that appear to be close are grouped together
into constellations since antiquity
• Officially 88 constellations
(with strict boundaries for classification of objects)
• Names range from mythological (Perseus,
Cassiopeia) to technical (Air Pump, Compass)
Constellation 1: Orion
Orion as seen at night
Orion as imagined by men
Orion “from the side”
Stars in a constellation are not connected in
any real way; they aren’t even close
together!
Constellation 1: Orion
• “the Hunter”
• Bright Stars:
D) Betelgeuze
E) Rigel
• Deep Sky Object:
i) Orion Nebula
Constellation: Gemini
• “the Twins”
• zodiacal sign
• Brightest Stars:
I) Castor
J=K) Pollux