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 CoordinatesEarth: 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