Transcript class 2, F10

Preceptors and Times
We will discuss this in class, these are the
volunteers so far
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Richard Avis
Jillian Capano
Brendan Forte
Samantha Hunt
Christian Madonna
Shaina Singer
End of Ch. 1
Looking back in time
• Light, although fast, travels at a finite speed.
• It takes:
– 8 minutes to reach us from the Sun
– 8 years to reach us from Sirius (8 light-years away)
– 1,500 years to reach us from the Orion Nebula
• The farther out we look into the Universe, the farther
back in time we see!
1.1 Our Modern View of the Universe
Our goals for learning:
•
What is our place in the universe?
We are on a planet, orbiting a star, in a galaxy (which is a member of the Local Group of galaxies in the Local
Supercluster).
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How did we come to be?
— The matter in our bodies came from the Big Bang, which produced hydrogen and helium.
— All other elements were constructed from H and He in stars and then recycled into new star systems,
including our solar system.
• How can we know what the universe was like in
the past?
—When we look to great distances we are seeing events that
happened long ago because light travels at a finite speed.
• Can we see the entire universe?
– Nope!
1.1 Our Modern View of the Universe
Our goals for learning:
•
What is our place in the universe?
We are on a planet, orbiting a star, in a galaxy (which is a member of the Local Group of galaxies in the Local
Supercluster).
•
How did we come to be?
— The matter in our bodies came from the Big Bang, which produced hydrogen and helium.
— All other elements were constructed from H and He in stars and then recycled into new star systems,
including our solar system.
• How can we know what the universe was like in the past?
— When we look to great distances we are seeing events that happened long ago because light
travels at a finite speed.
• Can we see the entire universe?
—No, the observable portion of the universe is about 14 billion
light-years in radius because the universe is about 14 billion
years old. ALSO (not in Ch. 1 of the book), we can “see”
only about 4% of the universe, 96% is made of “dark matter”
and “dark energy”.
What have we learned?
• How is Earth moving in our solar system?
— It rotates on its axis once a day and orbits the
Sun at a distance of 1 AU = 150 million km
• How is our solar system moving in the Milky
Way Galaxy?
— Stars in the Local Neighborhood move
randomly relative to one another and orbit
the center of the Milky Way in about 230
million years
What have we learned?
• How do galaxies move within the universe?
— All galaxies beyond the Local Group are
moving away from us with expansion of the
universe: the more distant they are, the faster
they’re moving
• Are we ever sitting still?
— No!
1.2 The Scale of the Universe
Our goals for learning:
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How big is Earth compared with our solar system?
How far away are the stars?
How big is the Milky Way Galaxy?
How big is the universe?
How do our lifetimes compare to the age of the
universe?
The scale of the solar system
• On a 1-to-10 billion
scale:
— Sun is the size
of a large
grapefruit (14
cm)
— Earth is the size
of a tip of a ball
point pen, 15
meters away.
How far away are the stars?
On our 1-to-10 billion scale, it’s just a few minutes
walk to Pluto.
How far would you have to walk to reach the nearest
star, Alpha Centauri?
A.
B.
C.
D.
1 mile
10 miles
100 miles
the distance across the United States
(2500 miles)
How far away are the stars?
On our 1-to-10 billion scale, it’s just a few minutes
walk to Pluto.
How far would you have to walk to reach the nearest
star, Alpha Centauri?
A.
B.
C.
D.
1 mile
10 miles
100 miles
the distance across the United States
(2500 miles)
Answer: D, the distance across the
United States
How do our lifetimes compare to the age
of the Universe?
• The Cosmic Calendar (p.14 and 15): A scale on
which we compress the history of the universe into
1 year.
What have we learned?
• How big is the Earth compared to our solar system?
— On a scale of 1-to-10 billion, the Sun is about the size of a
grapefruit. The Earth is the size of a tip of a ball point pen
about 15 m away. The distances between planets are very
large compared with their sizes.
• How far away are the stars?
— On the same scale, the stars are thousands of kilometers
away.
• How big is the Milky Way Galaxy?
— 100 billion stars; it would take more than 3,000 years to
count the stars in the Milky Way Galaxy at a rate of one per
second. The Milky Way Galaxy is about 100,000 light-years
across.
What have we learned?
• How big is the universe?
— 100 billion galaxies in the observable universe
— 14 billion light-years
— As many stars as grains of sand on Earth’s beaches
• How do our lifetimes compare to the age of the
universe?
— On a cosmic calendar that compresses the history of
the universe into one year, human civilization is just
a few seconds old, and a human lifetime is a fraction
of a second.
1.3 Spaceship Earth
Our goals for learning:
• How is Earth moving in our solar system?
• How is our solar system moving in the Milky
Way Galaxy?
• How do galaxies move within the universe?
• Are we ever sitting still?
Are we ever sitting still?
Earth rotates on axis: > 1,000 km/hr
Earth orbits Sun: > 100,000 km/hr
Solar system moves among stars: ~ 70,000 km/hr
Milky Way rotates: ~ 800,000 km/hr
Milky Way moves
in Local Group
Universe
expands
How do galaxies move within the universe?
Galaxies are carried along with the expansion of the universe.
But how did Hubble figure out that the universe is expanding?
Hubble discovered that…
• all galaxies outside our Local Group are
moving away from us.
• the more distant the galaxy, the faster it is
racing away.
Conclusion: We live in an expanding universe.
Outline of lecture 2 (Ch 2)
1.

Patterns in The Sky: Stars and constellations
Celestial coordinates:
Celestial sphere, poles, equator, ecliptic, right ascension*,
declination* (*not in book)
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Seasons: Tilt in Earth’s axis (23.5 degrees),
Equinoxes and soltices, precession
2.
The Moon and Eclipses
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3.
Lunar and Solar Eclipses
Ancient Mystery of the Planets:
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Apparent Retrograde motion of planets
2.1 Patterns in the Night Sky
Our goals for learning:
• What does the universe look like from
Earth?
• Why do stars rise and set?
• Why do the constellations we see depend on
latitude and time of year?
• What is a constellation?
• A constellation is a region of the sky. The sky is
divided into 88 official constellations.
Constellations
• Most official constellation names come from
antiquity.
• The patterns of stars have no physical significance!
Stars that appear close together may lie at very
different distances.
• Modern astronomers use
them as landmarks.
Constellations: Orion
• What is the
celestial
sphere?
• An
imaginary
sphere
surrounding
the Earth
upon which
the stars,
Sun, Moon,
and planets
appear to
reside.
The Celestial Sphere
North & South celestial poles
the points in the sky directly above the Earth’s
North and South poles
celestial equator
the extension of the Earth’s equator onto the
celestial sphere
ecliptic
the annual path of the Sun through the celestial sphere,
which is a projection of ecliptic plane
The Celestial Sphere
The Milky Way
You’ve probably seen this band of light across the sky.
What are we actually seeing?
The Milky Way
•Our Galaxy is shaped like a disk.
•Our solar system is in that disk.
•When we look at the Milky Way in the sky,
we are looking along that disk.
Measuring the Sky
We measure the sky in angles, not distances.
• Full circle = 360º
• 1º = 60 arcmin
• 1 arcmin = 60 arcsec
Measuring Angles in the Sky
The Local Sky
zenith
the point directly above you
horizon
all points 90° from the zenith
altitude
the angle above the horizon
meridian
due north horizon zenith due south horizon
Review: Coordinates on the Earth
• Latitude: position north or south of equator
• Longitude: position east or west of prime
meridian (runs through Greenwich,
England)
The Daily Motion
• As the Earth rotates, the sky
appears to us to rotate in the
opposite direction.
• The sky appears to rotate around
the N (or S) celestial poles.
• If you are standing at the poles,
nothing rises or sets.
• If you are standing at the equator,
everything rises & sets 90 to the
horizon.
Time Exposure
Photograph:
Star Trails
The Daily Motion
daily circles --- CCW looking north, CW looking south
Annual Motion
• As the Earth orbits the Sun, the Sun appears to move
eastward with respect to the stars.
• The Sun circles the celestial sphere once every year.
2.3 Seasons
• What is the cause of the seasons on Earth?
Annual Motion
• The Earth’s axis is tilted 23.5° from being
perpendicular to the ecliptic plane.
• Therefore, the celestial equator is tilted 23.5°
to the ecliptic.
• As seen from Earth, the Sun spends 6 months
north of the celestial equator and 6 months
south of the celestial equator.
• Seasons are caused by the Earth’s axis tilt,
not the distance from the Earth to the Sun!
Annual Motion
ecliptic
the apparent path of the Sun through the sky
equinox
where the ecliptic (the Sun) intersects the celestial equator
solstice
where the ecliptic (the Sun) is farthest from the celestial equator
zodiac
the constellations which lie along the ecliptic
The Cause of the Seasons
Coordinates on the Celestial
Sphere (not in book)
• Latitude: position north or south of equator
• Longitude: position east or west of prime
meridian (runs through Greenwich,
England)
• Declination: position north or south of celestial
equator (in degrees)
• Right Ascension: distance (in hours, 0 to 23h 59
min.) East of vernal equinox (where the sun
crosses the celestial equator going North)
Question 1
What point on the celestial sphere is defined by a Right
Ascension of zero hours and a Declination of zero
degrees?
Question 1
What point on the celestial sphere is defined by a Right
Ascension of zero hours and a Declination of zero
degrees?
The vernal (spring) equinox
Question 2
What point on the celestial sphere is defined by a Right
Ascension of 12 hours and a Declination of zero
degrees?
Question 2
What point on the celestial sphere is defined by a Right
Ascension of 12 hours and a Declination of zero
degrees?
The autumnal (fall) equinox
Question 3
What is an equinox?
Question 4
What point on the celestial sphere is defined by being
on the ecliptic at a Declination of +23.5 degrees?
Question 4
What point on the celestial sphere is defined by being
on the ecliptic at a Declination of +23.5 degrees?
The Summer Solstice
Question 5
What point on the celestial sphere is defined by being
on the ecliptic at a Declination of minus 23.5
degrees (i.e. 23.5 degrees South)?
Question 5
What point on the celestial sphere is defined by being
on the ecliptic at a Declination of minus 23.5
degrees (i.e. 23.5 degrees South)?
The Winter Solstice
Axis tilt causes uneven heating by
sunlight throughout the year.
When is summer?
(in the Northern Hemisphere)
• The solstice which occurs around June 21 is
considered the first day of summer.
• However, it takes time for the more direct
sunlight to heat up the land and water.
• Therefore, July & August are typically hotter
than June.
2.4 Precession
• What is the Earth’s cycle of precession?