1.1 Our Modern View of the Universe
• 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?• How can we know what the universe was like in the past?• Can we see the entire universe?
Our goals for learning:
A moderately large object which orbits a star; it shines by reflected light. Planets may be
rocky, icy, or gaseous in composition.
PLANET
What about Pluto?What about objects larger than Pluto that
have been discovered? What about asteroids?
The definition of a planet is not so clear(it was much easier for the ancient
Greeks….for them a planet was a star that moved and there were 5 of them)
PLANET
The sum total of all matter and energy; that is, everything within and between
all galaxies
THE UNIVERSE
1.1 Our Modern View of the Universe
• 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?• Can we see the entire universe?
Our goals for learning:
Where do we come from?• The first (and simplest) atoms were created during
the Big Bang.• More complex atoms were created in stars.• When the star dies, they are expelled into space….
to form new stars and planets!
Most of the atoms in our bodies were created in the core of a star!
SPEED OF LIGHT
• The speed of light in the vacuum of space is constant! All light travels the same speed!
c = speed of light
= 290,000,000 m/sec
= 2.9 x 108 m/sec
= 290,000 km/sec
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
• 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?
Our goals for learning:
1.1 Our Modern View of the Universe
• 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?– Nope!
Our goals for learning:
1.1 Our Modern View of the Universe
• 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”.
Our goals for learning:
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
• 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?
Our goals for learning:
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. 1 mileB. 10 milesC. 100 milesD. the distance across the United States
(2500 miles)
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
• 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?
Our goals for learning:
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
Universeexpands
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.
1. Patterns in The Sky: Stars and constellations
Celestial coordinates: Celestial sphere, poles, equator, ecliptic, right ascension*, declination* (*not in book)
Seasons: Tilt in Earth’s axis (23.5 degrees), Equinoxes and soltices, precession
2. The Moon and Eclipses Lunar and Solar Eclipses
3. Ancient Mystery of the Planets: Apparent Retrograde motion of planets
Outline of lecture 2 (Ch 2)
2.1 Patterns in the Night Sky
• 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?
Our goals for learning:
• 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.
• What is the celestial sphere?• An
imaginary sphere surrounding the Earth upon which the stars, Sun, Moon, and planets appear to reside.
The Celestial SphereNorth & South celestial poles
the points in the sky directly above the Earth’s North and South poles
celestial equatorthe extension of the Earth’s equator onto the celestial sphere
eclipticthe annual path of the Sun through the celestial sphere, which is a projection of ecliptic plane
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
The Local Sky
zeniththe point directly above you
horizonall points 90° from the zenith
altitudethe angle above the horizon
meridiandue 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.
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.
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
eclipticthe apparent path of the Sun through the sky
equinoxwhere the ecliptic (the Sun) intersects the celestial equator
solsticewhere the ecliptic (the Sun) is farthest from the celestial equator
zodiac
the constellations which lie along the ecliptic
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)
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 12 hours and a Declination of zero degrees?
Question 2
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 minus 23.5 degrees (i.e. 23.5 degrees South)?
Question 5
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.